Search

A search for nonresonant new phenomena, originating from either contact interactions or large extra spatial dimensions, has been carried out using events with two isolated electrons or muons. These events, produced at the LHC in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$, were recorded by the ATLAS detector. The data sample, collected throughout 2011, corresponds to an integrated luminosity of 4.9 and $5.0{\mathrm{fb}}^{-1}$ in the $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ channels, respectively. No significant deviations from the Standard Model expectation are observed. Using a Bayesian approach, 95% confidence level lower limits ranging from 9.0 to 13.9 TeV are placed on the energy scale of $\ell \ell qq$ contact interactions in the left-left isoscalar model. Lower limits ranging from 2.4 to 3.9 TeV are also set on the string scale in large extra dimension models. After combining these limits with results from a similar search in the diphoton channel, slightly more stringent limits are obtained.

The cross section and transverse single-spin asymmetries of ${\mu }^{-}$ and ${\mu }^{+}$ from open heavy-flavor decays in polarized $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$ were measured by the PHENIX experiment during 2012 at the Relativistic Heavy Ion Collider. Because heavy-flavor production is dominated by gluon-gluon interactions at $\sqrt{s}=200\mathrm{GeV}$, these measurements offer a unique opportunity to obtain information on the trigluon correlation functions. The measurements are performed at forward and backward rapidity ($1.4<|y|<2.0$) over the transverse momentum range of $1.25<p_T<7\mathrm{GeV}/c$ for the cross section and $1.25<p_T<5\mathrm{GeV}/c$ for the asymmetry measurements. The obtained cross section is compared to a fixed-order-plus-next-to-leading-log perturbative-quantum-chromodynamics calculation. The asymmetry results are consistent with zero within uncertainties, and a model calculation based on twist-3 three-gluon correlations agrees with the data.

A search for the standard model Higgs boson is performed in the diphoton decay channel. The data used correspond to an integrated luminosity of $4.9{\mathrm{fb}}^{-1}$ collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=7\mathrm{TeV}$. In the diphoton mass range 110–150 GeV, the largest excess with respect to the background-only hypothesis is observed at 126.5 GeV, with a local significance of 2.8 standard deviations. Taking the look-elsewhere effect into account in the range 110–150 GeV, this significance becomes 1.5 standard deviations. The standard model Higgs boson is excluded at 95% confidence level in the mass ranges of 113–115 GeV and 134.5–136 GeV.

A search for a Higgs boson has been performed in the $H\to WW\to \ell \nu jj$ channel in $1.04{\mathrm{fb}}^{-1}$ of $pp$ collision data at $\sqrt{s}=7\mathrm{TeV}$ recorded with the ATLAS detector at the Large Hadron Collider. No significant excess of events is observed over the expected background and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range $240\mathrm{GeV}<m_H<600\mathrm{GeV}$. The best sensitivity is reached for $m_H=400\mathrm{GeV}$, where the 95% confidence level upper bound on the cross section for $H\to WW$ production is 3.1 pb, or 2.7 times the standard model prediction.

A search for new phenomena in $t\overline{t}$ events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on $1.04{\mathrm{fb}}^{-1}$ of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, top-partner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.

The results of a search for pair production of the scalar partners of bottom quarks in $2.05{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ using the ATLAS experiment are reported. Scalar bottom quarks are searched for in events with large missing transverse momentum and two jets in the final state, where both jets are identified as originating from a bottom quark. In an $R$-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom quark decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits are obtained in the ${\tilde{b}}_1-{\tilde{\chi }}_1^0$ mass plane such that for neutralino masses below 60 GeV scalar bottom masses up to 390 GeV are excluded.

A search for contact interactions has been performed using dimuon events recorded with the ATLAS detector in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The data sample corresponds to an integrated luminosity of $42{\mathrm{pb}}^{-1}$. No significant deviation from the standard model is observed in the dimuon mass spectrum, allowing the following 95% C.L. limits to be set on the energy scale of contact interactions: $\Lambda >4.9\mathrm{TeV}$ (4.5 TeV) for constructive (destructive) interference in the left-left isoscalar compositeness model. These limits are the most stringent to date for $\mu \mu qq$ contact interactions.

The measurement of charged-particle event shape variables is presented in inclusive inelastic $pp$ collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The observables studied are the transverse thrust, thrust minor, and transverse sphericity, each defined using the final-state charged particles’ momentum components perpendicular to the beam direction. Events with at least six charged particles are selected by a minimum-bias trigger. In addition to the differential distributions, the evolution of each event shape variable as a function of the leading charged-particle transverse momentum, charged-particle multiplicity, and summed transverse momentum is presented. Predictions from several Monte Carlo models show significant deviations from data.

A search is presented for microscopic black holes in a like-sign dimuon final state in proton–proton collisions at $\sqrt{s}=8\mathrm{TeV}$. The data were collected with the ATLAS detector at the Large Hadron Collider in 2012 and correspond to an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$. Using a high track multiplicity requirement, $0.6\pm 0.2$ background events from Standard Model processes are predicted and none observed. This result is interpreted in the context of low-scale gravity models and 95% C.L. lower limits on microscopic black hole masses are set for different model assumptions.

We present azimuthal angle correlations of intermediate transverse momentum () hadrons from dijets in and collisions at and 200 GeV. The away-side dijet induced azimuthal correlation is broadened, non-Gaussian, and peaked away from in central and semicentral collisions in all the systems. The broadening and peak location are found to depend upon the number of participants in the collision, but not on the collision energy or beam nuclei. These results are consistent with sound or shock wave models, but pose challenges to Cherenkov gluon radiation models.

The invariant yields, $dN/dy$, for $J/\psi$ production at forward rapidity $(1.2<|y|<2.2)$ in $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=193\mathrm{GeV}$ have been measured as a function of collision centrality. The invariant yields and nuclear-modification factor $R_{AA}$ are presented and compared with those from $\mathrm{Au}+\mathrm{Au}$ collisions in the same rapidity range. Additionally, the direct ratio of the invariant yields from $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ collisions within the same centrality class is presented, and used to investigate the role of $c\overline{c}$ coalescence. Two different parametrizations of the deformed Woods-Saxon distribution were used in Glauber calculations to determine the values of the number of nucleon-nucleon collisions in each centrality class, $N_{\mathrm{coll}}$, and these were found to give significantly different $N_{\mathrm{coll}}$ values. Results using $N_{\mathrm{coll}}$ values from both deformed Woods-Saxon distributions are presented. The measured ratios show that the $J/\psi$ suppression, relative to binary collision scaling, is similar in $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ for peripheral and midcentral collisions, but that $J/\psi$ show less suppression for the most central $\mathrm{U}+\mathrm{U}$ collisions. The results are consistent with a picture in which, for central collisions, increase in the $J/\psi$ yield due to $c\overline{c}$ coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between $c\overline{c}$ coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine $N_{\mathrm{coll}}$.

A search is performed for top-quark pairs ($t\overline{t}$) produced together with a photon ($\gamma$) with transverse energy greater than 20 GeV using a sample of $t\overline{t}$ candidate events in final states with jets, missing transverse momentum, and one isolated electron or muon. The data set used corresponds to an integrated luminosity of $4.59{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. In total, 140 and 222 $t\overline{t}\gamma$ candidate events are observed in the electron and muon channels, to be compared to the expectation of $79\pm 26$ and $120\pm 39$ non-$t\overline{t}\gamma$ background events, respectively. The production of $t\overline{t}\gamma$ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The $t\overline{t}\gamma$ production cross section times the branching ratio (BR) of the single-lepton decay channel is measured in a fiducial kinematic region within the ATLAS acceptance. The measured value is ${\sigma }_{t\overline{t}\gamma }^{\mathrm{fid}}\times \mathrm{BR}=63\pm 8{(\text{stat})}_{-13}^{+17}(\text{syst})\pm 1(\text{lumi})\mathrm{fb}$ per lepton flavor, in good agreement with the leading-order theoretical calculation normalized to the next-to-leading-order theoretical prediction of $48\pm 10\mathrm{fb}$.

We present measurements of electrons and positrons from the semileptonic decays of heavy-flavor hadrons at midrapidity ($\left|y\right|<$ 0.35) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=62.4$ GeV. The data were collected in 2010 by the PHENIX experiment that included the new hadron-blind detector. The invariant yield of electrons from heavy-flavor decays is measured as a function of transverse momentum in the range $1<p_T^e<5$ GeV/$c$. The invariant yield per binary collision is slightly enhanced above the $p+p$ reference in $\mathrm{Au}+\mathrm{Au}$ 0%–20%, 20%–40%, and 40%–60% centralities at a comparable level. At this low beam energy this may be a result of the interplay between initial-state Cronin effects, final-state flow, and energy loss in medium. The $v_2$ of electrons from heavy-flavor decays is nonzero when averaged between $1.3<p_T^e<2.5$ GeV/$c$ for 0%–40% centrality collisions at $\sqrt{s_{NN}}=62.4$ GeV. For 20%–40% centrality collisions, the $v_2$ at $\sqrt{s_{NN}}=62.4$ GeV is smaller than that for heavy-flavor decays at $\sqrt{s_{NN}}=200$ GeV. The $v_2$ of the electrons from heavy-flavor decay at the lower beam energy is also smaller than $v_2$ for pions. Both results indicate that the heavy quarks interact with the medium formed in these collisions, but they may not be at the same level of thermalization with the medium as observed at $\sqrt{s_{NN}}=200$ GeV.

Dijet events produced in LHC proton-proton collisions at a center-of-mass energy $\sqrt{s}=8\mathrm{TeV}$ are studied with the ATLAS detector using the full 2012 data set, with an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$. Dijet masses up to about 4.5 TeV are probed. No resonancelike features are observed in the dijet mass spectrum. Limits on the cross section times acceptance are set at the 95% credibility level for various hypotheses of new phenomena in terms of mass or energy scale, as appropriate. This analysis excludes excited quarks with a mass below 4.06 TeV, color-octet scalars with a mass below 2.70 TeV, heavy $W^{\prime }$ bosons with a mass below 2.45 TeV, chiral $W^{*}$ bosons with a mass below 1.75 TeV, and quantum black holes with six extra space-time dimensions with threshold mass below 5.66 TeV.

This Letter presents measurements of the polarization of the top quark in top-antitop quark pair events, using $4.7{\mathrm{fb}}^{-1}$ of proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at $\sqrt{s}=7\mathrm{TeV}$. Final states containing one or two isolated leptons (electrons or muons) and jets are considered. Two measurements of ${\alpha }_{\ell }P$, the product of the leptonic spin-analyzing power and the top quark polarization, are performed assuming that the polarization is introduced by either a $CP$ conserving or a maximally $CP$ violating production process. The measurements obtained, ${\alpha }_{\ell }{P}_{\mathrm{CPC}}=-0.035\pm 0.014(\mathrm{stat})\pm 0.037(\mathrm{syst})$ and ${\alpha }_{\ell }{P}_{\mathrm{CPV}}=0.020\pm 0.016(\mathrm{stat}{)}_{-0.017}^{+0.013}(\mathrm{syst})$, are in good agreement with the standard model prediction of negligible top quark polarization.

The ATLAS experiment has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The full data set acquired in 2010 is used, corresponding to an integrated luminosity of $37{\mathrm{pb}}^{-1}$. The background, consisting of hadronic jets and isolated electrons, is estimated with fully data-driven techniques and subtracted. The differential cross sections, as functions of the di-photon mass ($m_{\gamma \gamma }$), total transverse momentum ($p_{\mathrm{T},\gamma \gamma }$), and azimuthal separation ($\Delta {\varphi }_{\gamma \gamma }$), are presented and compared to the predictions of next-to-leading-order QCD.

This Letter presents the first search for a heavy particle decaying into an $e^{\pm }{\mu }^{\mp }$ final state in $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions at the LHC. The data were recorded by the ATLAS detector during 2010 and correspond to a total integrated luminosity of $35{\mathrm{pb}}^{-1}$. No excess above the standard model background expectation is observed. Exclusions at 95% confidence level are placed on two representative models. In an $R$-parity violating supersymmetric model, tau sneutrinos with a mass below 0.75 TeV are excluded, assuming all $R$-parity violating couplings are zero except ${\lambda }_{311}^{\prime }=0.11$ and ${\lambda }_{312}=0.07$. In a lepton flavor violating model, a $Z^{\prime }$-like vector boson with masses of 0.70–1.00 TeV and corresponding cross sections times branching ratios of 0.175–0.183 pb is excluded. These results extend to higher mass $R$-parity violating sneutrinos and lepton flavor violating $Z$’s than previous constraints from the Tevatron.

A search is conducted for the electroweak pair production of a chargino and a neutralino $pp\to {\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^0$, where the chargino decays into the lightest neutralino and a $W$ boson, ${\tilde{\chi }}_1^{\pm }\to {\tilde{\chi }}_1^0{W}^{\pm }$, while the neutralino decays into the lightest neutralino and a Standard Model-like 125 GeV Higgs boson, ${\tilde{\chi }}_2^0\to {\tilde{\chi }}_1^{0}h$. Fully hadronic, semileptonic, diphoton, and multilepton (electrons, muons) final states with missing transverse momentum are considered in this search. Higgs bosons in the final state are identified by either two jets originating from bottom quarks ($h\to b\overline{b}$), two photons ($h\to \gamma \gamma$), or leptons from the decay modes $h\to WW$, $h\to ZZ$ or $h\to \tau \tau$. The analysis is based on $36.1{\mathrm{fb}}^{-1}$ of $\sqrt{s}=13\mathrm{TeV}$ proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. Observations are consistent with the Standard Model expectations, and 95% confidence-level limits of up to 680 GeV in ${\tilde{\chi }}_1^{\pm }/{\tilde{\chi }}_2^0$ mass are set in the context of a simplified supersymmetric model.

A measurement of the inclusive $pp\to t\overline{t}+X$ production cross section in the $\tau +\text{jets}$ final state using only the hadronic decays of the $\tau$ lepton is presented. The measurement is performed using $20.2{\mathrm{fb}}^{-1}$ of proton-proton collision data recorded at a center-of-mass energy of $\sqrt{s}=8\mathrm{TeV}$ with the ATLAS detector at the Large Hadron Collider. The cross section is measured via a counting experiment by imposing a set of selection criteria on the identification and kinematic variables of the reconstructed particles and jets, and on event kinematic variables and characteristics. The production cross section is measured to be ${\sigma }_{t\overline{t}}=239\pm 29\mathrm{pb}$, which is in agreement with the measurements in other final states and the theoretical predictions at this center-of-mass energy.

A search is presented for the pair production of heavy vectorlike quarks, $T\overline{T}$ or $B\overline{B}$, that decay into final states with jets and no reconstructed leptons. Jets in the final state are classified using a deep neural network as arising from hadronically decaying $W/Z$ bosons, Higgs bosons, top quarks, or background. The analysis uses data from the ATLAS experiment corresponding to $36.1{\mathrm{fb}}^{-1}$ of proton-proton collisions with a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ delivered by the Large Hadron Collider in 2015 and 2016. No significant deviation from the Standard Model expectation is observed. Results are interpreted assuming the vectorlike quarks decay into a Standard Model boson and a third-generation-quark, $T\to Wb,Ht,Zt$ or $B\to Wt,Hb,Zb$, for a variety of branching ratios. At 95% confidence level, the observed (expected) lower limit on the vectorlike $B$-quark mass for a weak-isospin doublet ($B$, $Y$) is 950 (890) GeV, and the lower limits on the masses for the pure decays $B\to Hb$ and $T\to Ht$, where these results are strongest, are 1010 (970) GeV and 1010 (1010) GeV, respectively.