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.
The ratio of the production cross sections for W and Z bosons in association with jets has been measured in proton-proton collisions at $\sqrt{s}$ = 7 TeV with the ATLAS experiment at the Large Hadron Collider. The measurement is based on the entire 2011 dataset, corresponding to an integrated luminosity of 4.6 fb$^{-1}$. Inclusive and differential cross-section ratios for massive vector bosons decaying to electrons and muons are measured in association with jets with transverse momentum $p_T$ > 30 GeV and jet rapidity $|y|$ < 4.4. The measurements are compared to next-to-leading-order perturbative QCD calculations and to predictions from different Monte Carlo generators implementing leading-order matrix elements supplemented by parton showers.
Distribution of inclusive jet multiplicity.
Breakdown of systematic uncertainties in percent in inclusive jet multiplicity in the electron channel.Uncertainties have been symmetrised and the sign denotes the sign of the original up-variation.
Breakdown of systematic uncertainties in percent in inclusive jet multiplicity in the muon channel.Uncertainties have been symmetrised and the sign denotes the sign of the original up-variation.
Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3 fb$^{-1}$ of $pp$ collisions produced by the Large Hadron Collider at a center-of-mass energy of $\sqrt{s} = 8$ TeV and recorded by the ATLAS detector. Cross sections are obtained from measured $H \rightarrow \gamma \gamma$ and $H \rightarrow ZZ ^{*}\rightarrow 4\ell$ event yields, which are combined accounting for detector efficiencies, fiducial acceptances and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be $\sigma_{pp \to H} = 33.0 \pm 5.3 \, ({\rm stat}) \pm 1.6 \, ({\rm sys}) \mathrm{pb}$. The measurements are compared to state-of-the-art predictions.
Measured cross section in bins of $p_{\rm{T}}^{\rm{H}}$.
Measured cross section in bins of $|y^{\rm{H}}|$.
Measured cross section in bins of $N_{\rm{jets}}$.
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.
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.
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.
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.
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 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.
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.
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