A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3fb-1 of pp collision data at sqrt(s)=8TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino.
Distribution of mT2 for events passing all the signal candidate selection requirements, except that on mT2 of the L90 and L120 selections, for SF events.
Distribution of mT2 for events passing all the signal candidate selection requirements, except that on mT2 of the L90 and L120 selections, for DF events.
Distribution of mT2 for events passing all the signal candidate selection requirements, except that on mT2 of the L100 selection, for SF events.
Searches are performed for resonant and non-resonant Higgs boson pair production in the $\gamma\gamma b\bar{b}$ final state using 20 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the CERN Large Hadron Collider. A 95% confidence level upper limit on the cross section times branching ratio of non-resonant production is set at 2.2 pb, while the expected limit is 1.0 pb. The difference derives from a modest excess of events, corresponding to 2.4 standard deviations from the background-only hypothesis. The limit observed in the search for a narrow $X \to hh$ resonance ranges between 0.7 and 3.5 pb as a function of the resonance mass.
The observed and expected limit on non-resonant Higgs boson pair production, measured in the GAMMA GAMMA B BBAR final state.
A 95% CL upper limit on the cross section times branching ratio of a narrow resonance decaying to pairs of Higgs bosons as a function of MX (see text for more details). The measurement is made in the GAMMA GAMMA B BBAR final state.
A search has been performed, using the full 20.3 fb$^{-1}$ data sample of 8 TeV proton--proton collisions collected in 2012 with the ATLAS detector at the LHC, for photons originating from a displaced vertex due to the decay of a neutral long-lived particle into a photon and an invisible particle. The analysis investigates the diphoton plus missing transverse momentum final state, and is therefore most sensitive to pair-production of long-lived particles. The analysis technique exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction, as well as the time of flight, of photons. No excess is observed over the Standard Model predictions for background. Exclusion limits are set within the context of Gauge Mediated Supersymmetry Breaking models, with the lightest neutralino being the next-to-lightest supersymmetric particle and decaying into a photon and gravitino with a lifetime in the range from 250 ps to about 100 ns.
Observed CLs contour in the GMSB (LAMBDA, TAU(NEUTRALINO) ) signal space.
Observed CLs contour with minus 1-sigma in the GMSB (LAMBDA, TAU(NEUTRALINO) ) signal space.
Observed CLs contour with plus 1-sigma in the GMSB (LAMBDA, TAU(NEUTRALINO) ) signal space.
A search for scalar particles decaying via narrow resonances into two photons in the mass range $65-600$ GeV is performed using 20.3 fb$^{-1}$ of $\sqrt{s}$ = 8 TeV $pp$ collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95 % confidence level on the production cross-section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches.
The measured fiducial cross section limit.
The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of $20.1 \rm{fb}^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ or $\tilde{t}\rightarrow b\tilde{\chi}_{1}^{\pm} \rightarrow b W^{\left(\ast\right)} \tilde{\chi}_{1}^{0}$, where $\tilde{\chi}_{1}^{0}$ ($\tilde{\chi}_{1}^{\pm}$) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$. For a branching fraction of 100%, top squark masses in the range 270-645 GeV are excluded for $\tilde{\chi}_{1}^{0}$ masses below 30 GeV. For a branching fraction of 50% to either $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ or $\tilde{t}\rightarrow b\tilde{\chi}_{1}^{\pm}$, and assuming the $\tilde{\chi}_{1}^{\pm}$ mass to be twice the $\tilde{\chi}_{1}^{0}$ mass, top squark masses in the range 250-550 GeV are excluded for $\tilde{\chi}_{1}^{0}$ masses below 60 GeV.
Etmiss distribution for SRA1 and SRA2 after all selection requirements except those on Etmiss.
Etmiss distribution for SRA3 and SRA4 after all selection requirements except those on Etmiss.
Etmiss distribution for SRB after all selection requirements except those on Etmiss.
A low-background inclusive search for new physics in events with same-sign dileptons is presented. The search uses proton-proton collisions corresponding to 20.3 fb$^{-1}$ of integrated luminosity taken in 2012 at a centre-of-mass energy of 8 TeV with the ATLAS detector at the LHC. Pairs of isolated leptons with the same electric charge and large transverse momenta of the type $e^{\pm}e^{\pm}, e^{\pm}\mu^{\pm}$, and $\mu^{\pm}\mu^{\pm}$ are selected and their invariant mass distribution is examined. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the cross-sections for processes beyond the Standard Model. Limits are placed as a function of the dilepton invariant mass within a fiducial region corresponding to the signal event selection criteria. Exclusion limits are also derived for a specific model of doubly charged Higgs boson production.
Expected and observed numbers of isolated same-sign lepton pairs in the $e^{\pm}e^{\pm}, e^{\pm}\mu^{\pm}$ and $\mu^{\pm}\mu^{\pm}$ channel for various cuts on the dilepton invariant mass, m($\ell^{\pm}\ell^{\pm}$). The uncertainties shown are the systematic uncertainties.
Expected and observed numbers of positively or negatively charged lepton pairs for various cuts on the dilepton invariant mass, $m(\ell \ell)$. The uncertainties shown are the systematic uncertainties.
Upper limit at 95% CL on the fiducial cross section for $\ell^{\pm} \ell^{\pm}$ pairs from non-SM signals. The expected limits and their $1 \sigma$ uncertainties are given together with the observed limits derived from the data. Limits are given separately for the $e^{\pm}e^{\pm}, e^{\pm}\mu^{\pm}$ and $\mu^{\pm}\mu^{\pm}$ channel inclusively and separated by charge.
The inclusive top quark pair ($t\bar{t}$) production cross-section $\sigma_{t\bar{t}}$ has been measured in $pp$ collisions at $\sqrt{s}=7$ TeV and $\sqrt{s}=8$ TeV with the ATLAS experiment at the LHC, using $t\bar{t}$ events with an opposite-charge $e\mu$ pair in the final state. The measurement was performed with the 2011 7 TeV dataset corresponding to an integrated luminosity of 4.6 fb$^{-1}$ and the 2012 8 TeV dataset of 20.3 fb$^{-1}$. The cross-section was measured to be: $\sigma_{t\bar{t}}=182.9\pm 3.1\pm 4.2\pm 3.6 \pm 3.3$ pb ($\sqrt{s}=7$ TeV) and $\sigma_{t\bar{t}}=242.9\pm 1.7\pm 5.5\pm 5.1\pm 4.2$ pb ($\sqrt{s}=8$ TeV, updated as described in the Addendum), where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the knowledge of the integrated luminosity and of the LHC beam energy. The results are consistent with recent theoretical QCD calculations at next-to-next-to-leading order. Fiducial measurements corresponding to the experimental acceptance of the leptons are also reported, together with the ratio of cross-sections measured at the two centre-of-mass energies. The inclusive cross-section results were used to determine the top quark pole mass via the dependence of the theoretically-predicted cross-section on $m_t^{\rm pole}$, giving a result of $m_t^{\rm pole}=172.9^{+2.5}_{-2.6}$ GeV. By looking for an excess of $t\bar{t}$ production with respect to the QCD prediction, the results were also used to place limits on the pair-production of supersymmetric top squarks $\tilde{t}_1$ with masses close to the top quark mass decaying via $\tilde{t}_1\rightarrow t\tilde{\chi}^0_1$ to predominantly right-handed top quarks and a light neutralino $\tilde{\chi}_0^1$, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 177 GeV are excluded at the 95% confidence level.
95% CL exclusion limit on signal strength.
95% CL exclusion limit on signal cross section for the 7 TeV dataset.
95% CL exclusion limit on signal cross section for the 8 TeV dataset.
Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the LHC are reported. Data were collected in proton--proton collisions at a center-of-mass energy of 8 TeV and correspond to an integrated luminosity of 20.3 fb$^{-1}$. The observed data are well described by the expected Standard Model backgrounds. The expected (observed) upper limit on the fiducial cross section for the production of such events is 6.1 (5.3) fb at 95% confidence level. Exclusion limits are presented on models of new phenomena with large extra spatial dimensions, supersymmetric quarks, and direct pair production of dark-matter candidates.
Distribution of ETmiss in the data and for the expected background in the single-muon control region. The total background expectation has been normalized to the observed number of events in this control region. Overflow is included in the final bin.
Distribution of ETmiss in the data and for the expected background in the two-muon control region. The total background expectation is normalized to the observed number of events in this control region. Overflows are included in the final bin.
Distribution of ETmiss in the data and for the expected background in the two-electron control region. The total background expectation is normalized to the observed number of events in this control region. Overflows are included in the final bin.
We search for evidence of physics beyond the Standard Model in the production of final states with multiple high transverse momentum jets, using 20.3 fb$^{-1}$ of proton-proton collision data recorded by the ATLAS detector at $\sqrt{s} = 8$ TeV. No excess of events beyond Standard Model expectations is observed, and upper limits on the visible cross-section for non-Standard Model production of multi-jet final states are set. Using a wide variety of models for black hole and string ball production and decay, the limit on the cross-section times acceptance is as low as 0.16 fb at the 95% CL for a minimum scalar sum of jet transverse momentum in the event of about 4.3 TeV. Using models for black hole and string ball production and decay, exclusion contours are determined as a function of the production mass threshold and the gravity scale. These limits can be interpreted in terms of lower-mass limits on black hole and string ball production that range from 4.6 to 6.2 TeV.
Number of data events (20.3 fb$^{-1}$), number of predicted events from the fit, statistical uncertainty on the fit, systematic uncertainty on the choice of control region, and on the choice of fit function versus inclusive $H_{\textrm{T}}^{\textrm{min}}$ lower bin edge for inclusive jet multiplicity $N_{\textrm{Jet}} \geq 3$. The total uncertainty is obtained by adding the three uncertainties linearly.
Number of data events (20.3 fb$^{-1}$), number of predicted events from the fit, statistical uncertainty on the fit, systematic uncertainty on the choice of control region, and on the choice of fit function versus inclusive $H_{\textrm{T}}^{\textrm{min}}$ lower bin edge for inclusive jet multiplicity $N_{\textrm{Jet}} \geq 4$. The total uncertainty is obtained by adding the three uncertainties linearly.
Number of data events (20.3 fb$^{-1}$), number of predicted events from the fit, statistical uncertainty on the fit, systematic uncertainty on the choice of control region, and on the choice of fit function versus inclusive $H_{\textrm{T}}^{\textrm{min}}$ lower bin edge for inclusive jet multiplicity $N_{\textrm{Jet}} \geq 5$. The total uncertainty is obtained by adding the three uncertainties linearly.
A search is presented for production of dark matter particles recoiling against a leptonically decaying Z boson in 20.3 fb$^{-1}$ of pp collisions at $\sqrt{s}$=8 TeV with the ATLAS detector at the Large Hadron Collider. Events with large missing transverse momentum and two oppositely-charged electrons or muons consistent with the decay of a Z boson are analyzed. No excess above the Standard Model prediction is observed. Limits are set on the mass scale of the contact interaction as a function of the dark matter particle mass using an effective field theory description of the interaction of dark matter with quarks or with Z bosons. Limits are also set on the coupling and mediator mass of a model in which the interaction is mediated by a scalar particle.
Summary of the systematic uncertainties for the largest background process (ZZ->llnunu). Statistical uncertaintes are from MC simulation sample size.
Observed yields and expected SM backgrounds in each signal region. Statistical, systematic, and luminosity uncertainteis are added in quadrature to give the total background estimate and uncertainties.
The observed and expected upper limits on the fiducial cross section at 95% C.L. for each signal region.
The results of a search for top squark (stop) pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The analysis is performed with proton--proton collision data at $\sqrt{s} = 8$ TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of $20$ fb$^{-1}$. The lightest supersymmetric particle (LSP) is taken to be the lightest neutralino which only interacts weakly and is assumed to be stable. The stop decay modes considered are those to a top quark and the LSP as well as to a bottom quark and the lightest chargino, where the chargino decays to the LSP by emitting a $W$ boson. A wide range of scenarios with different mass splittings between the stop, the lightest neutralino and the lightest chargino are considered, including cases where the $W$ bosons or the top quarks are off-shell. Decay modes involving the heavier charginos and neutralinos are addressed using a set of phenomenological models of supersymmetry. No significant excess over the Standard Model prediction is observed. A stop with a mass between $210$ and $640$ GeV decaying directly to a top quark and a massless LSP is excluded at $95$ % confidence level, and in models where the mass of the lightest chargino is twice that of the LSP, stops are excluded at $95$ % confidence level up to a mass of $500$ GeV for an LSP mass in the range of $100$ to $150$ GeV. Stringent exclusion limits are also derived for all other stop decay modes considered, and model-independent upper limits are set on the visible cross-section for processes beyond the Standard Model.
Expected and observed $H_{T,sig}^{miss}$ distribution for tN_med SR, before applying the $H_{T,sig}^{miss}>12$ requirement. The uncertainty includes statistical and all experimental systematic uncertainties. The last bin includes overflows.
Expected and observed large-R jet mass distribution for tN_boost SR, before applying the large-R jet mass$>75$ GeV requirement. The uncertainty includes statistical and all experimental systematic uncertainties. The last bin includes overflows.
Expected and observed b-jet multiplicity distribution for bCc_diag SR, before applying the b-jet multiplicity$=0$ requirement. The uncertainty includes statistical and all experimental systematic uncertainties. The last bin includes overflows.
A search for new phenomena in LHC proton-proton collisions at a center-of-mass energy of $\sqrt{s}=8$ TeV was performed with the ATLAS detector using an integrated luminosity of 17.3 fb$^{-1}$. The angular distributions are studied in events with at least two jets; the highest dijet mass observed is 5.5 TeV. All angular distributions are consistent with the predictions of the Standard Model. In a benchmark model of quark contact interactions, a compositeness scale below 8.1 TeV in a destructive interference scenario and 12.0 TeV in a constructive interference scenario is excluded at 95 % CL; median expected limits are 8.9 TeV for the destructive interference scenario and 14.1 TeV for the constructive interference scenario.
mjj region 600 - 800 GeV. The observed systematic is the experimental uncertainty, while the SM prediction systematic is the theoretical uncertainty.
mjj region 800 - 1200 GeV. The observed systematic is the experimental uncertainty, while the SM prediction systematic is the theoretical uncertainty.
mjj region 1200 - 1600 GeV. The observed systematic is the experimental uncertainty, while the SM prediction systematic is the theoretical uncertainty.
Results of a search for supersymmetry via direct production of third-generation squarks are reported, using $20.3$ fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 8$ TeV recorded by the ATLAS experiment at the LHC in 2012. Two different analysis strategies based on monojet-like and $c$-tagged event selections are carried out to optimize the sensitivity for direct top squark pair production in the decay channel to a charm quark and the lightest neutralino ($\tilde{t}_1 \to c + \tilde{\chi}_{1}^{0}$) across the top squark--neutralino mass parameter space. No excess above the Standard Model background expectation is observed. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the ($m_{\tilde{t}_1}$, $m_{\tilde{\chi}_{1}^{0}}$) parameter space. A top squark of mass up to about 240 GeV is excluded at 95$\%$ confidence level for arbitrary neutralino masses, within the kinematic boundaries. Top squark masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a scenario where the top squark and the lightest neutralino are nearly degenerate in mass, top squark masses up to 260 GeV are excluded. The results from the monojet-like analysis are also interpreted in terms of compressed scenarios for top squark pair production in the decay channel $\tilde{t}_1 \to b + ff^{'} + \tilde{\chi}^{0}_{1}$ and sbottom pair production with $\tilde{b}_1 \to b + \tilde{\chi}^{0}_{1}$, leading to a similar exclusion for nearly mass-degenerate third-generation squarks and the lightest neutralino. The results in this paper significantly extend previous results at colliders.
Distribution of the discriminator against b-jets, log(Pcharm/Pb), for the first-leading jet. For illustration purposes, the distributions of two different SUSY scenarios for stop pair production with the decay mode $\tilde{t}_1 \rightarrow c + \tilde{\chi}^{0}_1$ are included. In the SUSY signal, the first-leading jet mostly originates from ISR.
Distribution of the discriminator against b-jets, log(Pcharm/Pu), for the third-leading jet. For illustration purposes, the distributions of two different SUSY scenarios for stop pair production with the decay mode $\tilde{t}_1 \rightarrow c + \tilde{\chi}^{0}_1$ are included. In the SUSY signal, the third-leading jet is expected to contain a large fraction of c-jets.
The measured $E_T^{miss}$ distribution in the $W \rightarrow \mu \nu$ control region, for the M1 selection, compared to the background predictions. The latter include the global normalization factors extracted from the fit.
This paper reports the results of a search for strong production of supersymmetric particles in 20.1 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 8 TeV using the ATLAS detector at the LHC. The search is performed separately in events with either zero or at least one high $p_\mathrm{T}$ lepton (electron or muon), large missing transverse momentum, high jet multiplicity and at least three jets identified as originated from the fragmentation of a b-quark. No excess is observed with respect to the Standard Model predictions. The results are interpreted in the context of several supersymmetric models involving gluinos and scalar top and bottom quarks, as well as a mSUGRA/CMSSM model. Gluino masses up to 1340 GeV are excluded, depending on the model, significantly extending the previous ATLAS limits.
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A generic search for anomalous production of events with at least three charged leptons is presented. The data sample consists of $pp$ collisions at $\sqrt{s}=8$ TeV collected in 2012 by the ATLAS experiment at the CERN Large Hadron Collider, and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Events are required to have at least three selected lepton candidates, at least two of which must be electrons or muons, while the third may be a hadronically decaying tau. Selected events are categorized based on their lepton flavour content and signal regions are constructed using several kinematic variables of interest. No significant deviations from Standard Model predictions are observed. Model-independent upper limits on contributions from beyond the Standard Model phenomena are provided for each signal region, along with prescription to re-interpret the limits for any model. Constraints are also placed on models predicting doubly charged Higgs bosons and excited leptons. For doubly charged Higgs bosons decaying to $e\tau$ or $\mu\tau$, lower limits on the mass are set at 400 GeV at 95% confidence level. For excited leptons, constraints are provided as functions of both the mass of the excited state and the compositeness scale $\Lambda$, with the strongest mass constraints arising in regions where the mass equals $\Lambda$. In such scenarios, lower mass limits are set at 3.0 TeV for excited electrons and muons, 2.5 TeV for excited taus, and 1.6 TeV for every excited-neutrino flavour.
Expected and observed event yields for the most inclusive signal regions.
Expected and observed limits on $\sigma_{95}^{\mathrm{vis}}$ for inclusive signal regions, along with confidence intervals of one and two standard deviations on the expected limits.
The fiducial efficiency for electrons, muons, and taus in different $p_{\mathrm{T}}$ ranges ($\epsilon_{\mathrm{fid}}(p_{\mathrm{T}})$). For electrons and muons from tau decays, the $p_{\mathrm{T}}$ is that of the electron or muon, not the tau. The uncertainties shown reflect the statistical uncertainties of the simulated samples only. UPDATE (18 MAY 2015): Updated fiducial lepton efficiencies.
Measurements of four-lepton (4$\ell$, $\ell=e,\mu$) production cross sections at the $Z$ resonance in $pp$ collisions at the LHC with the ATLAS detector are presented. For dilepton and four-lepton invariant mass region $m_{\ell^+\ell^-} > 5$ GeV and $80 < m_{4\ell} < 100$ GeV, the measured cross sections are $76 \pm 18 \text { (stat) } \pm 4 \text { (syst) } \pm 1.4 \text { (lumi) }$ fb and $107 \pm 9 \text{ (stat) } \pm 4 \text{ (syst) } \pm 3.0 \text { (lumi) }$ fb at $\sqrt s$ = 7 and 8 TeV, respectively. By subtracting the non-resonant 4$\ell$ production contributions and normalizing with $Z\rightarrow \mu^+\mu^-$ events, the branching fraction for the $Z$ boson decay to $4\ell$ is determined to be $\left( 3.20 \pm 0.25\text{ (stat)} \pm 0.13\text{ (syst)} \right) \times 10^{-6}$, consistent with the Standard Model prediction.
The measured individual cross sections in the fiducial region and the combined cross sections for 4-muon and 4-electron final states at a centre-of-collision energy of 7 TeV.
The measured individual cross sections in the fiducial region and the combined cross sections for 2-muon-2-electron final states at a centre-of-collision energy of 7 TeV.
The measured cross section for four-lepton final states at a centre-of-collision energy of 7 TeV.
Results of a search for the electroweak associated production of charginos and next-to-lightest neutralinos, pairs of charginos or pairs of tau sleptons are presented. These processes are characterised by final states with at least two hadronically decaying tau leptons, missing transverse momentum and low jet activity. The analysis is based on an integrated luminosity of 20.3 fb$^{-1}$ of proton--proton collisions at $\sqrt{s}=8$ TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess is observed with respect to the predictions from Standard Model processes. Limits are set at 95% confidence level on the masses of the lighter chargino and next-to-lightest neutralino for various hypotheses for the lightest neutralino mass in simplified models. In the scenario of direct production of chargino pairs, with each chargino decaying into the lightest neutralino via an intermediate tau slepton, chargino masses up to 345 GeV are excluded for a massless lightest neutralino. For associated production of mass-degenerate charginos and next-to-lightest neutralinos, both decaying into the lightest neutralino via an intermediate tau slepton, masses up to 410 GeV are excluded for a massless lightest neutralino.
The stransverse mass distribution in the multi-jet CR-C1N2.
Mt(tau1)+Mt(tau2) distribution in the multi-jet CR C1C1.
Effective mass distribution in the multi-jet CR DS-highMass.
A search for squarks and gluinos in final states containing high-$p_{\rm T}$ jets, missing transverse momentum and no electrons or muons is presented. The data were recorded in 2012 by the ATLAS experiment in $\sqrt{s}=8$ TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of $20.3 \mathrm{fb}^{-1}$. No significant excess above the Standard Model expectation is observed. Results are interpreted in a variety of simplified and specific supersymmetry-breaking models assuming that R-parity is conserved and that the lightest neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 1330 GeV for a simplified model incorporating only a gluino and the lightest neutralino. For a simplified model involving the strong production of first- and second-generation squarks, squark masses below 850 GeV (440 GeV) are excluded for a massless lightest neutralino, assuming mass degenerate (single light-flavour) squarks. In mSUGRA/CMSSM models with $\tan\beta=30$, $A_0=-2m_0$ and $\mu> 0$, squarks and gluinos of equal mass are excluded for masses below 1700 GeV. Additional limits are set for non-universal Higgs mass models with gaugino mediation and for simplified models involving the pair production of gluinos, each decaying to a top squark and a top quark, with the top squark decaying to a charm quark and a neutralino. These limits extend the region of supersymmetric parameter space excluded by previous searches with the ATLAS detector.
The effective mass distribution in 2-jet loose signal region.
The effective mass distribution in 2-jet medium and tight signal regions.
The effective mass distribution in 2-jet (W) signal region.
The results of a search for charged Higgs bosons decaying to a $\tau$ lepton and a neutrino, $H^{\pm} \rightarrow \tau^{\pm} \nu$, are presented. The analysis is based on 19.5 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark. The final state is characterised by the presence of a hadronic $\tau$ decay, missing transverse momentum, $b$-tagged jets, a hadronically decaying $W$ boson, and the absence of any isolated electrons or muons with high transverse momenta. The data are consistent with the expected background from Standard Model processes. A statistical analysis leads to 95\% confidence-level upper limits on the product of branching ratios $ {\cal B}(t\rightarrow bH^\pm) \times {\cal B} (H^\pm\rightarrow \tau^{\pm}\nu) $, between 0.23% and 1.3% for charged Higgs boson masses in the range 80-160 GeV. It also leads to 95% confidence-level upper limits on the production cross section times branching ratio, $\sigma(pp \rightarrow tH^{\pm} + X) \times {\cal B}(H^{\pm} \rightarrow \tau^{\pm} \nu)$, between 0.76 pb and 4.5 fb, for charged Higgs boson masses ranging from 180 GeV to 1000 GeV. In the context of different scenarios of the Minimal Supersymmetric Standard Model, these results exclude nearly all values of $\tan\beta$ above one for charged Higgs boson masses between 80 GeV and 160 GeV, and exclude a region of parameter space with high $\tan\beta$ for $H^{\pm}$ masses between 200 GeV and 250 GeV.
The measured B(t->H+)xB(H+->tau+ nu) limit.
The measured P P --> t H+ --> t tau+nu limit.
An analysis is presented of events containing jets including at least one $b$-tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced four-top-quark production, pair production of chiral $b^\prime$-quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral $b^\prime$-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.
Observed and expected number of events with statistical (first) and systematic (second) uncertainties for the positively charged top pair signal selection. The p-values for agreement between the observed yield and the expected background in each signal region are reported.
Observed and expected number of events with statistical (first) and systematic (second) uncertainties for five of the signal regions defined for VLQ, chiral bprime-quark and four-top-quark production searches. The p-values for agreement between the observed yield and the expected background in each signal region are reported.
Observed and expected number of events with statistical (first) and systematic (second) uncertainties for three of the signal regions defined for VLQ, chiral bprime-quark and four-top-quark production searches. The p-values for agreement between the observed yield and the expected background in each signal region are reported.
A search is presented for the direct pair production of a chargino and a neutralino $pp\to\tilde{\chi}^\pm_1\tilde{\chi}^0_2$, where the chargino decays to the lightest neutralino and the $W$ boson, $\tilde{\chi}^\pm_1 \to \tilde{\chi}^0_1 (W^{\pm}\to\ell^{\pm}\nu)$, while the neutralino decays to the lightest neutralino and the 125 GeV Higgs boson, $\tilde{\chi}^0_2 \to \tilde{\chi}^0_1 (h\to bb/\gamma\gamma/\ell^{\pm}\nu qq)$. The final states considered for the search have large missing transverse momentum, an isolated electron or muon, and one of the following: either two jets identified as originating from bottom quarks, or two photons, or a second electron or muon with the same electric charge. The analysis is based on 20.3 fb$^{-1}$ of $\sqrt{s}=8$ TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations, and limits are set in the context of a simplified supersymmetric model.
Distribution of contransverse mass $m_{\rm CT}$ in CRlbb-T, central $m_{bb}$ bin. The background histograms are obtained from the background-only fit, and their uncertainty represents the total background uncertainty after the fit. The last bin includes overflow.
Distribution of contransverse mass $m_{\rm CT}$ in SRlbb-1 and SRlbb-2, $m_{bb}$ sideband. The background histograms are obtained from the background-only fit, and their uncertainty represents the total background uncertainty after the fit. The last bin includes overflow.
Distribution of the transverse mass of the $W$-candidate $m_{\rm T}^{W}$ for the one lepton and two $b$-jets channel in VRlbb-2, central $m_{bb}$ bin. The background histograms are obtained from the background-only fit, and their uncertainty represents the total background uncertainty after the fit. The last bin includes overflow.
Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy $\sqrt{s}=8$ TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Two leptonic production mechanisms are considered: decays of squarks and gluinos with $Z$ bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the $Z$-boson mass; and decays of neutralinos (e.g. $\tilde{\chi}^{0}_{2} \rightarrow \ell^{+}\ell^{-}\tilde{\chi}^{0}_{1}$), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of 3 standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.
The observed and expected dielectron invariant mass distribution in SR-Z. The negigible estimated contribution from Z+jets is omitted in these distributions.
The observed and expected dimuon invariant mass distribution in SR-Z. The negigible estimated contribution from Z+jets is omitted in these distributions.
The observed and expected $E_T^{miss}$ distribution in the dielectron SR-Z. The negigible estimated contribution from Z+jets is omitted in these distributions. The last bin contains the overflow.
A search for high-mass resonances decaying into $\tau^{+}\tau^{-}$ final states using proton-proton collisions at $\sqrt{s}= 8$ TeV produced by the Large Hadron Collider is presented. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 19.5-20.3 fb$^{-1}$. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of $Z^{\prime}$ resonances decaying into $\tau^+\tau^-$ pairs as a function of the resonance mass. As a result, $Z^{\prime}$ bosons of the Sequential Standard Model with masses less than 2.02 TeV are excluded at 95% credibility. The impact of the fermionic couplings on the $Z^{\prime}$ acceptance is investigated and limits are also placed on a $Z^{\prime}$ model that exhibits enhanced couplings to third-generation fermions.
Signal acceptance times efficiency (ACC*EFF) for Z'L, Z'R, Z'narrow and Z'wide divided by ACC*EFF for Z'SSM as a function of the Z' mass, separately for the had-had and lep-had channels.
Ratio of the Z'NU to Z'SSM cross section times tau+tau- branching fraction (SIG*BR) as a function of sin^2phi and the Z' mass.
Ratio of the Z'NU to Z'SSM acceptance times efficiency (ACC*EFF) in the had-had channel as a function of sin^2phi and the Z' mass.
The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider (LHC) are reported. The search is based on proton-proton collision data at a centre-of-mass energy $\sqrt{s} = 8$ TeV collected in 2012, corresponding to an integrated luminosity of 20 fb$^{-1}$. No significant excess above the Standard Model expectation is observed. Limits are set on the parameters of a minimal universal extra dimensions model, excluding a compactification radius of $1/R_c=950$ GeV for a cut-off scale times radius ($\Lambda R_c$) of approximately 30, as well as on sparticle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV.
Observed and expected $E_T^{miss}/m_{eff}$ distribution in soft single-lepton 3-jet signal region. The last bin includes the overflow.
Observed and expected $E_T^{miss}/m_{eff}$ distribution in soft single-lepton 5-jet signal region. The last bin includes the overflow.
Observed and expected $E_T^{miss}/m_{eff}$ distribution in soft single-lepton 3-jet inclusive signal region. The last bin includes the overflow.
The results of a dedicated search for pair production of scalar partners of charm quarks are reported. The search is based on an integrated luminosity of 20.3 fb$^{-1}$ of pp collisions at $\sqrt{s}=8$ TeV recorded with the ATLAS detector at the LHC. The search is performed using events with large missing transverse momentum and at least two jets, where the two leading jets are each tagged as originating from c-quarks. Events containing isolated electrons or muons are vetoed. In an R-parity-conserving minimal supersymmetric scenario in which a single scalar-charm state is kinematically accessible, and where it decays exclusively into a charm quark and a neutralino, 95% confidence-level upper limits are obtained in the scalar-charm-neutralino mass plane such that, for neutralino masses below 200 GeV, scalar-charm masses up to 490 GeV are excluded.
$m_{CT}$ distribution in signal region (before $m_{CT}$ cuts).
$m_{cc}$ distribution in the signal region with $m_{CT}>150$ GeV.
95% C.L. expected exclusion contour for all regions combined.
A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2012 at $\sqrt{s}$=8 TeV from $pp$ collisions corresponding to an integrated luminosity of $20.3$ fb$^{-1}$ are examined. Particles producing anomalously high ionisation, consistent with long-lived massive particles with electric charges from $|q|=2e$ to $|q|=6e$ are searched for. No signal candidate events are observed, and 95\% confidence level cross-section upper limits are interpreted as lower mass limits for a Drell--Yan production model. The mass limits range between 660 and 785 GeV.
The observed event yield in data in the B region, the probability $f$ to find a particle above the respective $S$(MDT dE/dx) value before tight selection and the expected background yield in the signal region D with its statistical uncertainty. The last column shows the observed event yield in the D region.
Fractions of signal events (in %) with at least one multi-charged particle, which satisfy the given requirements. The uncertainties quoted are statistical.
Overview of separate contributions (in %) to the systematic uncertainty on the signal. The total uncertainty is given by the quadratic sum of the individual uncertainties.
A search is presented for narrow diboson resonances decaying to $WW$ or $WZ$ in the final state where one $W$ boson decays leptonically (to an electron or a muon plus a neutrino) and the other $W/Z$ boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 8 TeV collected by the ATLAS detector at the LHC. No evidence for resonant diboson production is observed, and resonance masses below 700 GeV and 1490 GeV are excluded at 95% confidence level for the spin-2 Randall-Sundrum bulk graviton $G^*$ with coupling constant of 1.0 and the extended gauge model $W'$ boson respectively.
Reconstructed mass distribution in the low-pt resolved region (LRR).
Reconstructed mass distribution in the high-pt resolved region (HRR).
Reconstructed mass distribution in the merged region (MR).
Searches for heavy long-lived charged particles are performed using a data sample of 19.8 fb$^{-1}$ from proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV collected by the ATLAS detector at the Large Hadron Collider. No excess is observed above the estimated background and limits are placed on the mass of long-lived particles in various supersymmetric models. Long-lived tau sleptons in models with gauge-mediated symmetry breaking are excluded up to masses between 440 and 385 GeV for $\tan\beta$ between 10 and 50, with a 290 GeV limit in the case where only direct tau slepton production is considered. In the context of simplified LeptoSUSY models, where sleptons are stable and have a mass of 300 GeV, squark and gluino masses are excluded up to a mass of 1500 and 1360 GeV, respectively. Directly produced charginos, in simplified models where they are nearly degenerate to the lightest neutralino, are excluded up to a mass of 620 GeV. $R$-hadrons, composites containing a gluino, bottom squark or top squark, are excluded up to a mass of 1270, 845 and 900 GeV, respectively, using the full detector; and up to a mass of 1260, 835 and 870 GeV using an approach disregarding information from the muon spectrometer.
Observed data, background estimate and expected signal in the slepton search for the lower of the two masses ($m$) in the two-candidate signal region (GMSB: $\tilde{\tau}_1$ masses of 344.5 and 437 GeV).
Observed data, background estimate and expected signal in the slepton search for the lower of the two masses ($m$) in the two-candidate signal region (GMSB: $\tilde{\tau}_1$ masses of 344.5 and 437 GeV).
Observed data, background estimate and expected signal in the slepton search for the one-candidate signal region (LeptoSUSY: $m_{\tilde q}$ = 2.0, 1.2 and 0.9 TeV with $m_{\tilde g}$ = 1.2 TeV).
A search is conducted for non-resonant new phenomena in dielectron and dimuon final states, originating from either contact interactions or large extra spatial dimensions. The LHC 2012 proton-proton collision dataset recorded by the ATLAS detector is used, corresponding to 20 fb$^{-1}$ at $\sqrt{s}$ = 8 TeV. The dilepton invariant mass spectrum is a discriminating variable in both searches, with the contact interaction search additionally utilizing the dilepton forward-backward asymmetry. No significant deviations from the Standard Model expectation are observed. Lower limits are set on the $\ell\ell q q$ contact interaction scale $\Lambda$ between 15.4 TeV and 26.3 TeV, at the 95% credibility level. For large extra spatial dimensions, lower limits are set on the string scale $M_{S}$ between 3.2 TeV to 5.0 TeV.
Reconstructed dielectron mass distributions for data and the SM background estimate.
Reconstructed dimuon mass distributions for data and the SM background estimate.
Reconstructed $\cos\theta^*$ distributions for data and the SM background estimate in the dielectron channel.
A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at 95% confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV LEP limit to the top squark mass, lower limits between 490 GeV and 650 GeV are placed on the top squark mass within the model considered.
Distribution of $m_{\rm T}^{\rm sum}(\tau_{\mathrm{had}},\tau_{\mathrm{had}})$ for the events passing all the hadron-hadron signal region requirements, except that on the variable itself. The SM background process have been normalised using a fit to the data observed in CRs.
Distribution of $m_{{\rm T}2}(\tau_{\mathrm{had}},\tau_{\mathrm{had}})$ for the events passing all the hadron-hadron signal region requirements, except that on the variable itself. The SM background process have been normalised using a fit to the data observed in CRs.
Distribution of $m_{{\rm T}2}(b \ell, b \tau_{\mathrm{had}})$ for events passing all the lepton-hadron LM signal region requirements, except that on the variable itself. The SM background process have been normalised using a fit to the data observed in CRs.
This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3 fb$^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.
Model dependent cross section upper limits for the b-FDM model from combined results of SR1 and SR2.
Searches for the electroweak production of charginos, neutralinos and sleptons in final states characterized by the presence of two leptons (electrons and muons) and missing transverse momentum are performed using 20.3 fb-1 of proton-proton collision data at sqrt(s) = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. Limits are set on the masses of the lightest chargino, next-to-lightest neutralino and sleptons for different lightest-neutralino mass hypotheses in simplified models. Results are also interpreted in various scenarios of the phenomenological Minimal Supersymmetric Standard Model.
MT2 in WW CR for SR-WWa.
ET(miss,rel) in Top CR for SR-MT2 and SR-WWb/c.
ET(miss,rel) in ZV CR for SR-MT2 and SR-WWb/c.
Results from a search for supersymmetry in events with four or more leptons including electrons, muons and taus are presented. The analysis uses a data sample corresponding to 20.3 $fb^{-1}$ of proton--proton collisions delivered by the Large Hadron Collider at $\sqrt{s}$ = 8 TeV and recorded by the ATLAS detector. Signal regions are designed to target supersymmetric scenarios that can be either enriched in or depleted of events involving the production of a $Z$ boson. No significant deviations are observed in data from Standard Model predictions and results are used to set upper limits on the event yields from processes beyond the Standard Model. Exclusion limits at the 95% confidence level on the masses of relevant supersymmetric particles are obtained. In R-parity-violating simplified models with decays of the lightest supersymmetric particle to electrons and muons, limits of 1350 GeV and 750 GeV are placed on gluino and chargino masses, respectively. In R-parity-conserving simplified models with heavy neutralinos decaying to a massless lightest supersymmetric particle, heavy neutralino masses up to 620 GeV are excluded. Limits are also placed on other supersymmetric scenarios.
The ETmiss distribution in VR0Z.
The effective mass distribution in VR0Z.
The ETmiss distribution in VR2Z.
This Letter reports the observation of a high transverse momentum $Z\rightarrow b\bar{b}$ signal in proton-proton collisions at $\sqrt{s}=8$ TeV and the measurement of its production cross section. The data analysed were collected in 2012 with the ATLAS detector at the LHC and correspond to an integrated luminosity of 19.5 fb$^{-1}$. The $Z\rightarrow b\bar{b}$ decay is reconstructed from a pair of b-tagged jets, clustered with the anti-$k_t$ jet algorithm with $R = 0.4$, that have low angular separation and form a dijet with $p_T > 200$ GeV. The signal yield is extracted from a fit to the dijet invariant mass distribution, with the dominant, multi-jet background mass shape estimated by employing a fully data-driven technique that reduces the dependence of the analysis on simulation. The fiducial cross section is determined to be \[ \sigma^\mathrm{fid}_{Z\rightarrow b\bar{b}} = 2.02 \pm 0.20 ({\rm stat.}) \pm0.25 ({\rm syst.)}\pm 0.06 ({\rm lumi.}){\rm\,pb} = 2.02 \pm{0.33}{\rm\,pb}, \] in good agreement with next-to-leading-order theoretical predictions.
The cross section for Z0 --> BOTTOM BOTTOMBAR measured in the fiducial region.
A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb-1 of sqrt(s) = 8 TeV proton--proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino ($\tilde\chi^\pm_1$) and next-to-lightest neutralino ($\tilde{\chi}_2^0$) production with decays to lightest neutralino ($\tilde{\chi}_1^0$) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons, $\tilde\chi^\pm_1$ and $\tilde{\chi}_2^0$ masses are excluded up to 700 GeV, 380 GeV, 345 GeV, or 148 GeV respectively, for a massless $\tilde{\chi}_1^0$.
Number of expected and observed events in the validation region VR0taub.
For events in the low-ETmiss validation region, the MT distribution in VR0taunoZa.
For events in the low-ETmiss validation region, the MT distribution in VR0tauZa.
A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the $Z$ boson mass, jets tagged as originating from $b$-quarks and missing transverse momentum. The analysis is performed with proton-proton collision data at $\sqrt{s}=8$ TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb$^{-1}$. No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state ($\tilde{t}_2$) followed by the decay to the lighter top squark state ($\tilde{t}_1$) via $\tilde{t}_2 \to Z\tilde{t}_1$, and for $\tilde{t}_1$ pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino ($\tilde{\chi}^0_1$) is the next-to-lightest supersymmetric particle and decays producing a $Z$ boson and a gravitino ($\tilde{G}$) via the $\tilde{\chi}^0_1 \to Z\tilde{G}$ process.
PT(ll) distribution in SR2A for stop GMSB model (data+/-stat, bkg+/-(stat+syst), signal+/-stat), m(stop1)=500 GeV, m(neutralino1)=400 GeV.
njet_3l (data+/-stat, bkg+/-(stat+syst), signal+/-stat).
Missing ET distribution in SR2A for stop GMSB model (data+/-stat, bkg+/-(stat+syst), signal+/-stat), m(stop1)=500 GeV, m(neutralino1)=400 GeV.
A search is presented for dark matter pair production in association with a W or Z boson in pp collisions representing 20.3 fb$^{-1}$ of integrated luminosity at $\sqrt{s}$=8 TeV using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet-mass consistent with a W or Z boson, and with large missing transverse momentum are analyzed. The data are consistent with the Standard Model expectations, and limits are set on the mass scale in effective field theories that describe the interaction of dark matter and Standard Model particles.
Distribution of M(jet) in the data and for the predicted background in the top control region (CR) with one muon, one large-radius jet, two narrow jets, at least one b tag, and ETmiss > 250 GeV.
Distribution of M(jet) in the data and for the predicted background in the signal region (SR) with ETmiss > 350 GeV.
Distribution of M(jet) in the data and for the predicted background in the signal region (SR) with ETmiss > 500 GeV.
This Letter describes a model-independent search for the production of new resonances in photon + jet events using 20 inverse fb of proton--proton LHC data recorded with the ATLAS detector at a centre-of-mass energy of sqrt(s) = 8 TeV. The photon + jet mass distribution is compared to a background model fit from data; no significant deviation from the background-only hypothesis is found. Limits are set at 95% credibility level on generic Gaussian-shaped signals and two benchmark phenomena beyond the Standard Model: non-thermal quantum black holes and excited quarks. Non-thermal quantum black holes are excluded below masses of 4.6 TeV and excited quarks are excluded below masses of 3.5 TeV.
Invariant mass of the photon+jet pair for events passing the final selections. The number of observed events and the fit background estimates are given in each bin, where the fit estimates are rounded to the nearest integer.
The 95% CL upper limits on SIG*BR*A*EPSILON for a hypothetical signal with a Gaussian-shaped M(GAMMA JET) distribution as a function of the signal mass M(G) for four values of the relative width SIGMA(G) / M(G).
Acceptance (A), efficiency (EPSILON), cross-section (SIG) and limits in number of events for the quantum black hole (QBH) benchmark model, as a function of the threshold mass M(th). Uncertainties on the cross section are on the order of 1%. The limits include statistical uncertainties only. Expected limits include the 68% uncertainty band. Acceptance was calculated using parton-level quantities by imposing criteria that apply directly to kinematic selections (photon/jet |eta|, photon/jet transverse momentum, Delta(eta), Delta(R)). All other selections, which in general correspond to event and object quality criteria, were used to calculate the efficiency based on the events included in the acceptance.
A search is presented for direct chargino production based on a disappearing-track signature using 20.3 fb-1 of proton-proton collisions at sqrt(s) = 8 TeV collected with the ATLAS experiment at the LHC. In anomaly-mediated supersymmetry breaking (AMSB) models, the lightest chargino is nearly mass-degenerate with the lightest neutralino and its lifetime is long enough to be detected in the tracking detectors by identifying decays that result in tracks with no associated hits in the outer region of the tracking system. Some models with supersymmetry also predict charginos with a significant lifetime. This analysis attains sensitivity for charginos with a lifetime between 0.1 ns and 10 ns, and significantly surpasses the reach of the LEP experiments. No significant excess above the background expectation is observed for candidate tracks with large transverse momentum, and constraints on chargino properties are obtained. In the AMSB scenarios, a chargino mass below 270 GeV is excluded at 95% confidence level.
The pT distribution of disappearing-track candidates.
Observed CLs contour in the ( M(CHARGINO), TAU(CHARGINO) ) space for tan(beta) = 5 and mu > 0.
Observed CLs contour with minus 1-sigma signal cross-section uncertainty in the ( M(CHARGINO), TAU(CHARGINO) ) space for tan(beta) = 5 and mu > 0.
This Letter presents a search for quantum black-hole production using 20.3 inverse fb of data collected with the ATLAS detector in pp collisions at the LHC at sqrt(s) = 8 TeV. The quantum black holes are assumed to decay into a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the lepton+jet final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.
The combined 95% CL upper limits on the cross section times branching fraction (SIG*BR) for Quantum Black Holes decaying to a lepton and jet, as a function of the threshold mass, Mth.
Numbers of observed events and expected background events for electron+jet channel, along with acceptance (A), experimental efficiency (EPSILON), cumulative efficiency (A*EPSILON), total cross section (SIG*BR) and 95% CL observed upper limit, for various values of the threshold mass, Mth. The leading order cross sections have a statistical precision of the order of 1%. The uncertainties on the predicted background include both statistical and systematic components. Acceptance is calculated using generator-level quantities by imposing selection criteria that apply directly to phase space (electron/jet eta, electron/jet pT, Delta(eta), Delta(phi), <eta>, and Minv). All other selections, which in general correspond to event and object quality criteria, are used to calculate the efficiency on the events included in the acceptance. The cumulative signal efficiency is the product of the acceptance and experimental efficiency.
Numbers of observed events and expected background events for muon+jet channel, along with acceptance (A), experimental efficiency (EPSILON), cumulative efficiency (A*EPSILON), total cross section (SIG*BR) and 95% CL observed upper limit, for various values of the threshold mass, Mth. The leading order cross sections have a statistical precision of the order of 1%. The uncertainties on the predicted background include both statistical and systematic components. Acceptance is calculated using generator-level quantities by imposing selection criteria that apply directly to phase space (muon/jet eta, muon/jet pT, Delta(eta), Delta(phi), <eta>, and Minv). All other selections, which in general correspond to event and object quality criteria, are used to calculate the efficiency on the events included in the acceptance. The cumulative signal efficiency is the product of the acceptance and experimental efficiency.
The results of a search for pair production of supersymmetric partners of the Standard Model third-generation quarks are reported. This search uses 20.1 fb-1 of pp collisions at sqrt{s}=8 TeV collected by the ATLAS experiment at the Large Hadron Collider. The lightest bottom and top squarks (b1 and t1 respectively) are searched for in a final state with large missing transverse momentum and two jets identified as originating from b-quarks. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the visible cross section for processes beyond the Standard Model. Exclusion limits at the 95% confidence level on the masses of the third-generation squarks are derived in phenomenological supersymmetric R-parity-conserving models in which either the bottom or the top squark is the lightest squark. The b1 is assumed to decay via b1->b chi0 and the t via t1->b chipm, with undetectable products of the subsequent decay of the chipm due to the small mass splitting between the chipm and the chi0.
Observed exclusion limit at 95% CL in the ( M(SBOTTOM), M(NEUTRALINO) ) mass plane for the sbottom pair production scenario.
Observed exclusion limit at 95% CL, when moving the nominal signal cross section up by the 1-sigma theoretical uncertainty, in the ( M(SBOTTOM), M(NEUTRALINO) ) mass plane for the sbottom pair production scenario.
Observed exclusion limit at 95% CL, when moving the nominal signal cross section down by the 1-sigma theoretical uncertainty, in the ( M(SBOTTOM), M(NEUTRALINO) ) mass plane for the sbottom pair production scenario.
Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z-boson are presented. The measurements are performed using 20.3 inverse femtobarns of proton-proton collision data collected at a centre-of-mass energy of sqrt(s)=8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5 sigma level. The electroweak process includes the vector boson fusion production of a Z-boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z-boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the SHERPA and POWHEG event generators.
Unfolded normalised differential Z+2j cross section as a function of dijet invariant mass in the baseline region.
Unfolded normalised differential Z+2j cross section as a function of dijet invariant mass in the search region.
Unfolded normalised differential Z+2j cross section as a function of the rapidity separation between the leading jets in the baseline region.
A search is presented for production of a heavy up-type quark ($t^\prime$) together with its antiparticle, assuming a significant branching ratio for subsequent $t^\prime$ decay into a Standard Model Higgs boson and a top quark, as predicted by vector-like quark models. The search is based on 14.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton+jets final state, characterised by an isolated electron or muon with moderately high transverse momentum, significant missing transverse momentum, and at least six jets. The search exploits the high total transverse momenta of all final state objects and the high multiplicity of $b$ jets characteristic of signal events with at least one Higgs boson decaying into $b\bar{b}$, to discriminate against the dominant background from top quark pair production. No significant excess of events above the Standard Model expectation is observed, and upper limits are derived for vector-like quarks of various masses in the two-dimensional plane of $BR(t^\prime \to Wb)$ versus $BR(t^\prime \to Ht)$, where $H$ is the Standard Model Higgs boson, assumed to have a mass of $125$ GeV. Under the branching ratio assumptions corresponding to a weak-isospin doublet (singlet) scenario, a $t^\prime$ quark with mass lower than 790 (640) GeV is excluded at the 95\% confidence level.
The observed and simulated HT distributions in the combined e+jets and mu+jets channels with >=6 jets and 2 b tagged jets.
The observed and simulated HT distributions in the combined e+jets and mu+jets channels with >=6 jets and 3 b tagged jets.
The observed and simulated HT distributions in the combined e+jets and mu+jets channels with >=6 jets and 4 b tagged jets.
A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons ($e$ or $\mu$) with the same electric charge, or at least three isolated leptons. The search also utilises jets originating from b-quarks, missing transverse momentum and other observables to extend its sensitivity. The analysis uses a data sample corresponding to a total integrated luminosity of 20.3 fb$^{-1}$ of $\sqrt{s} =$ 8 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider in 2012. No deviation from the Standard Model expectation is observed. New or significantly improved exclusion limits are set on a wide variety of supersymmetric models in which the lightest squark can be of the first, second or third generations, and in which R-parity can be conserved or violated.
Numbers of observed and background events for SR0b for each bin of the distribution in Meff. The table corresponds to Fig. 4(b). The statistical and systematic uncertainties are combined for the expected backgrounds.
Numbers of observed and background events for SR1b for each bin of the distribution in Meff. The table corresponds to Fig. 4(c). The statistical and systematic uncertainties are combined for the predicted numbers.
Numbers of observed and background events for SR3b for each bin of the distribution in Meff. The table corresponds to Fig. 4(a). The statistical and systematic uncertainties are combined for the predicted numbers.
This Letter presents the first study of $W^{\pm}W^{\pm}jj$, same-electric-charge diboson production in association with two jets, using 20.3 fb$^{-1}$ of proton--proton collision data at $\sqrt{s}=8$ TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two reconstructed same-charge leptons ($e^\pm e^\pm$, $e^\pm \mu^\pm$, and $\mu^\pm \mu^\pm$) and two or more jets are analyzed. Production cross sections are measured in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. First evidence for $W^{\pm}W^{\pm}jj$ production and electroweak-only $W^{\pm}W^{\pm}jj$ production is observed with a significance of $4.5$ and $3.6$ standard deviations respectively. The measured production cross sections are in agreement with Standard Model predictions. Limits at 95% confidence level are set on anomalous quartic gauge couplings.
The fiducial cross sections for the production of W+- W+- JET JET in the inclusive and VBS regions.
The production of $Z$ bosons with one or two isolated high-energy photons is studied using $pp$ collisions at $\sqrt{s}$ = 8 TeV. The analyses use a data sample with an integrated luminosity of 20.3 fb$^{-1}$ collected by the ATLAS detector during the 2012 LHC data taking. The $Z\gamma$ and $Z\gamma\gamma$ production cross sections are measured with leptonic ($e^{+}e^{-}$, $\mu^{+}\mu^{-}$, $\nu\bar{\nu}$) decays of the $Z$ boson, in extended fiducial regions defined in terms of the lepton and photon acceptance. They are then compared to cross-section predictions from the Standard Model, where the sources of the photons are radiation off initial-state quarks and radiative $Z$-boson decay to charged leptons, and from fragmentation of final-state quarks and gluons into photons. The yields of events with photon transverse energy $E_T >$ 250 GeV from $\ell^{+}\ell^{-}\gamma$ events and with $E_T >$ 400 GeV from $\nu\bar{\nu}\gamma$ events are used to search for anomalous triple gauge-boson couplings $ZZ\gamma$ and $Z\gamma\gamma$. The yields of events with diphoton invariant mass $m_{\gamma\gamma} >$ 200 GeV from $\ell^{+}\ell^{-}\gamma\gamma$ events and with $m_{\gamma\gamma} > $ 300 GeV from $\nu\bar{\nu}\gamma\gamma$ events are used to search for anomalous quartic gauge-boson couplings $ZZ\gamma\gamma$ and $Z\gamma\gamma\gamma$. No deviations from Standard Model predictions are observed and limits are placed on parameters used to describe anomalous triple and quartic gauge-boson couplings.
Measured integrated cross sections for the $Z\gamma$ process for charged lepton final states at $\sqrt{s} = 8$ TeV in the extended fiducial regions defined in the paper, table 5. The parton-to-particle correction factors are also shown, which are defined as the ratio of the cross sections at parton-level to the cross sections at particle-level.
Measured integrated cross sections for the $Z\gamma$ process for neutrino final states at $\sqrt{s} = 8$ TeV in the extended fiducial regions defined in the paper, table 5. The parton-to-particle correction factors are also shown, which are defined as the ratio of the cross sections at parton-level to the cross sections at particle-level.
Measured integrated cross sections for the $Z\gamma\gamma$ process for charged lepton final states at $\sqrt{s} = 8$ TeV in the extended fiducial regions defined in the paper, table 5. The parton-to-particle correction factors are also shown, which are defined as the ratio of the cross sections at parton-level to the cross sections at particle-level.
The production rates of prompt and non-prompt $J/\psi$ and $\psi(2\mathrm{S})$ mesons are measured using 2.1 $fb^{-1}$ and 11.4 $fb^{-1}$ of data collected with the ATLAS experiment at the LHC, in proton-proton collisions at $\sqrt{s}=7$ and 8 TeV respectively. Production cross-sections for both prompt and non-prompt production sources, ratios of $\psi(2\mathrm{S})$ to $J/\psi$ production, and fractions of non-prompt to inclusive production for $J/\psi$ and $\psi(2\mathrm{S})$ are measured double-differentially as a function of meson $p_{T}$ and rapidity. These measurements are made in a restricted fiducial volume and also corrected for geometrical acceptance after which they are compared to a variety of theoretical predictions.
Summary of results for cross-section of prompt $J/\psi$ decaying to a muon pair for 7 TeV data in nb/GeV. Uncertainties are statistical and systematic, respectively.
Summary of results for cross-section of prompt $J/\psi$ decaying to a muon pair for 8 TeV data in nb/GeV. Uncertainties are statistical and systematic, respectively.
Summary of results for cross-section of non-prompt $J/\psi$ decaying to a muon pair for 7 TeV data in nb/GeV. Uncertainties are statistical and systematic, respectively.
A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb$^{-1}$ of proton-proton collision data collected at a centre-of-mass energy of $\sqrt{s}=8$ TeV. The lepton-plus-jets final state is used, where the top pair decays to $W^+bW^-\bar{b}$, with one $W$ boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour $Z'$ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with $\Gamma/m =$ 15% decaying to $t\bar{t}$. These range from 2.5 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.
Selection efficiency x Acceptance for a Z' resonance.
Selection efficiency x Acceptance for a KK gluon resonance.
Selection efficiency x Acceptance for a KK graviton resonance.
The distribution and orientation of energy inside jets is predicted to be an experimental handle on colour connections between the hard--scatter quarks and gluons initiating the jets. This Letter presents a measurement of the distribution of one such variable, the jet pull angle. The pull angle is measured for jets produced in $t\bar{t}$ events with one $W$ boson decaying leptonically and the other decaying to jets using 20.3 fb$^{-1}$ of data recorded with the ATLAS detector at a centre-of-mass energy of $\sqrt{s}=8$ TeV at the LHC. The jet pull angle distribution is corrected for detector resolution and acceptance effects and is compared to various models.
Normalised fiducial ttbar differential cross-section for the jet pull angle distribution constructed using all particles.
Normalised fiducial ttbar differential cross-section for the jet pull angle distribution constructed using charged particles.
Statistical bin-bin correlation matrix.
Distributions of transverse momentum $p_T^{ll}$ and the angular variable $\phi^*_\eta$ of Drell--Yan lepton pairs are measured in 20.3 fb$^{-1}$ of proton--proton collisions at $\sqrt{s}=8$ TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton--proton collisions at $\sqrt{s}=7$ TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of $\phi^*_\eta < 1$ the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of $\phi^*_\eta$ this is not generally the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of $p_T^{ll}$ and the fixed-order prediction of DYNNLO falls below the data at high values of $p_T^{ll}$. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the $\phi^*_\eta$ and $p_T^{ll}$ distributions as a function of lepton-pair mass and rapidity.
Fiducial cross sections at Born level in the electron- and muon-pair channels as well as the combined value. The statistical and systematic uncertainties are given as a percentage of the cross section. An additional uncertainty of 2.8% on the integrated luminosity, which is fully correlated between channels and among all $m_{\ell\ell}$ bins, pertains to these measurements. The individual uncertainty sources after the combination are not necessarily orthogonal and also do not include uncertainties uncorrelated between bins of $m_{\ell\ell}$. Therefore their quadratic sum may not give the total systematic uncertainty.
The values of $(1/\sigma)\,\mathrm{d}\sigma/\mathrm{d}\phi^*_{\eta}$ in each bin of $\phi^*_{\eta}$ for the electron and muon channels separately (for various particle-level definitions) and for the Born-level combination in the kinematic region $46\textrm{ GeV} \leq m_{\ell\ell} < 66\textrm{ GeV},\ 0 \leq |y_{\ell\ell}| < 0.8$. The associated statistical and systematic (both uncorrelated and correlated between bins of $\phi^*_{\eta}$) are provided in percentage form.
The values of $(1/\sigma)\,\mathrm{d}\sigma/\mathrm{d}\phi^*_{\eta}$ in each bin of $\phi^*_{\eta}$ for the electron and muon channels separately (for various particle-level definitions) and for the Born-level combination in the kinematic region $46\textrm{ GeV} \leq m_{\ell\ell} < 66\textrm{ GeV},\ 0.8 \leq |y_{\ell\ell}| < 1.6$. The associated statistical and systematic (both uncorrelated and correlated between bins of $\phi^*_{\eta}$) are provided in percentage form.
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