Results of a search for decays of massive particles to fully hadronic final states are presented. This search uses 20.3 fb$^{-1}$ of data collected by the ATLAS detector in $\sqrt{s} = 8$ TeV proton--proton collisions at the LHC. Signatures based on high jet multiplicities without requirements on the missing transverse momentum are used to search for $R$-parity-violating supersymmetric gluino pair production with subsequent decays to quarks. The analysis is performed using a requirement on the number of jets, in combination with separate requirements on the number of $b$-tagged jets, as well as a topological observable formed from the scalar sum of the mass values of large-radius jets in the event. Results are interpreted in the context of all possible branching ratios of direct gluino decays to various quark flavors. No significant deviation is observed from the expected Standard Model backgrounds estimated using jet-counting as well as data-driven templates of the total-jet-mass spectra. Gluino pair decays to ten or more quarks via intermediate neutralinos are excluded for a gluino with mass $m_{\tilde{g}} < 1$ TeV for a neutralino mass $m_{\tilde{\chi}^0_1} = 500$ GeV. Direct gluino decays to six quarks are excluded for $m_{\tilde{g}} < 917$ GeV for light-flavor final states, and results for various flavor hypotheses are presented.
Expected and observed event yields for signal regions with at least 6 jets as a function of transverse momentum requirements.
Expected and observed event yields for signal regions with at least 6 jets with at least 1 b-tag as a function of transverse momentum requirements.
Expected and observed event yields for signal regions with at least 6 jets with at least 2 b-tags as a function of transverse momentum requirements.
This Letter presents a search for a heavy neutral particle decaying into an opposite-sign different-flavor dilepton pair, $e^\pm \mu^\mp$, $e^\pm \tau^\mp$, or $\mu^\pm \tau^\mp$ using 20.3 fb$^{-1}$ of $pp$ collision data at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. The numbers of observed candidate events are compatible with the Standard Model expectations. Limits are set on the cross section of new phenomena in two scenarios: the production of $\tilde{\nu}_{\tau}$ in $R$-parity-violating supersymmetric models and the production of a lepton-flavor-violating $Z'$ vector boson.
Observed and predicted $e\mu$ invariant mass distributions.
The expected invariant mass distribution of signal Z'@0.75TeV and $\tilde{\nu}_{\tau}$@1TeV. The arbitrary choice of couplings are: $\lambda_{311}^{'}=0.11$ and $\lambda_{i3k}=0.07$ for $\tilde{\nu}_{\tau}$, $Q_{ll^{'}}=1$ for Z' .
Observed and predicted $e\tau$ invariant mass distributions.
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.
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.
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.
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.
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.
Measurements of inclusive jet production are performed in $pp$ and Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV with the ATLAS detector at the LHC, corresponding to integrated luminosities of 4.0 $\mathrm{pb}^{-1}$ and 0.14 $\mathrm{nb}^{-1}$, respectively. The jets are identified with the anti-$k_t$ algorithm with $R=0.4$, and the spectra are measured over the kinematic range of jet transverse momentum $32 < p_{\mathrm{T}} < 500$ GeV, and absolute rapidity $|y| < 2.1$ and as a function of collision centrality. The nuclear modification factor, $R_{\mathrm{AA}}$, is evaluated and jets are found to be suppressed by approximately a factor of two in central collisions compared to $pp$ collisions. The $R_{\mathrm{AA}}$ shows a slight increase with $p_{\mathrm{T}}$ and no significant variation with rapidity.
The $\langle T_{\mathrm{AA}} \rangle $ and $\langle N_{\mathrm{part}} \rangle$ values and their uncertainties in each centrality bin.
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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.
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