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.

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.

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.

This paper describes a measurement of the $W$ boson transverse momentum distribution using ATLAS $pp$ collision data from the 2010 run of the LHC at $\sqrt{s}=7\mathrm{TeV}$, corresponding to an integrated luminosity of about $31{\mathrm{pb}}^{-1}$. Events form both $W\to e\nu$ and $W\to \mu \nu$ are used, and the transverse momentum of the $W$ candidates is measured through the energy deposition in the calorimeter from the recoil of the $W$. The resulting distributions are unfolded to obtain the normalized differential cross sections as a function of the $W$ boson transverse momentum. We present results for $p_T^W<300\mathrm{GeV}$ in the electron and muon channels as well as for their combination, and compare the combined results to the predictions of perturbative QCD and a selection of event generators.

A measurement of the cross section for the inclusive production of isolated prompt photons in $pp$ collisions at a center-of-mass energy $\sqrt{s}=7\mathrm{TeV}$ is presented. The measurement covers the pseudorapidity ranges $|{\eta }^{\gamma }|<1.37$ and $1.52\le |{\eta }^{\gamma }|<1.81$ in the transverse energy range $15\le E_{\mathrm{T}}^{\gamma }<100\mathrm{GeV}$. The results are based on an integrated luminosity of $880{\mathrm{nb}}^{-1}$, collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading-order perturbative QCD calculations.

A search is presented for direct chargino production based on a disappearing-track signature using $20.3{\mathrm{fb}}^{-1}$ of proton-proton collisions at $\sqrt{s}=8\mathrm{TeV}$ 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 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 production cross sections of the inclusive Drell-Yan processes $W^{\pm }\to \ell \nu$ and $Z/{\gamma }^{*}\to \ell \ell$ ($\ell =e$, $\mu$) are measured in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector. The cross sections are reported integrated over a fiducial kinematic range, extrapolated to the full range, and also evaluated differentially as a function of the $W$ decay lepton pseudorapidity and the $Z$ boson rapidity, respectively. Based on an integrated luminosity of about $35{\mathrm{pb}}^{-1}$ collected in 2010, the precision of these measurements reaches a few percent. The integrated and the differential $W^{\pm }$ and $Z/{\gamma }^{*}$ cross sections in the $e$ and $\mu$ channels are combined, and compared with perturbative QCD calculations, based on a number of different parton distribution sets available at next-to-next-to-leading order.

The ATLAS experiment has observed 1995 $Z$ boson candidates in data corresponding to $0.15{\mathrm{nb}}^{-1}$ of integrated luminosity obtained in the 2011 LHC $\mathrm{Pb}+\mathrm{Pb}$ run at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. The $Z$ bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event $Z$ boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the $Z$ boson with respect to the event plane is found to be consistent with zero.

This Letter presents the first search for supersymmetry in final states containing one isolated electron or muon, jets, and missing transverse momentum from $\sqrt{s}=7\mathrm{TeV}$ proton-proton collisions at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of $35{\mathrm{pb}}^{-1}$. No excess above the standard model background expectation is observed. Limits are set on the parameters of the minimal supergravity framework, extending previous limits. Within this framework, for $A_0=0\mathrm{GeV}$, $\tan \beta =3$, and $\mu >0$ and for equal squark and gluino masses, gluino masses below 700 GeV are excluded at 95% confidence level.

Distributions sensitive to the underlying event are studied in events containing one or more charged-particle jets produced in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector at the Large Hadron Collider (LHC). These measurements reflect $800\mu {\mathrm{b}}^{-1}$ of data taken during 2010. Jets are reconstructed using the anti-$k_t$ algorithm with radius parameter $R$ varying between 0.2 and 1.0. Distributions of the charged-particle multiplicity, the scalar sum of the transverse momentum of charged particles, and the average charged-particle $p_{\mathrm{T}}$ are measured as functions of $p_{\mathrm{T}}^{\mathrm{jet}}$ in regions transverse to and opposite the leading jet for $4\mathrm{GeV}<p_{\mathrm{T}}^{\mathrm{jet}}<100\mathrm{GeV}$. In addition, the $R$ dependence of the mean values of these observables is studied. In the transverse region, both the multiplicity and the scalar sum of the transverse momentum at fixed $p_{\mathrm{T}}^{\mathrm{jet}}$ vary significantly with $R$, while the average charged-particle transverse momentum has a minimal dependence on $R$. Predictions from several Monte Carlo tunes have been compared to the data; the predictions from Pythia 6, based on tunes that have been determined using LHC data, show reasonable agreement with the data, including the dependence on $R$. Comparisons with other generators indicate that additional tuning of soft-QCD parameters is necessary for these generators. The measurements presented here provide a testing ground for further development of the Monte Carlo models.

A search for the Higgs boson has been performed in the $H\to W{W}^{(*)}\to {\ell }^{+}\nu {\ell }^{-}\overline{\nu }$ channel ($\ell =e/\mu$) with an integrated luminosity of $2.05{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range $110\mathrm{GeV}<m_{\mathrm{H}}<300\mathrm{GeV}$. The observations exclude the presence of a standard model Higgs boson with a mass $145<m_{\mathrm{H}}<206\mathrm{GeV}$ at 95% confidence level.

This paper describes searches for the pair production of first or second generation scalar leptoquarks using $35{\mathrm{pb}}^{-1}$ of proton-proton collision data recorded by the ATLAS detector at $\sqrt{s}=7\mathrm{TeV}$. Leptoquarks are searched in events with two oppositely-charged muons or electrons and at least two jets, and in events with one muon or electron, missing transverse momentum and at least two jets. After event selection, the observed yields are consistent with the predicted backgrounds. Leptoquark production is excluded at the 95% CL for masses $M_{\mathrm{LQ}}<376$ (319) GeV and $M_{\mathrm{LQ}}<422$ (362) GeV for first and second generation scalar leptoquarks, respectively, when assuming the branching fraction of a leptoquark to a charged lepton is equal to 1.0 (0.5).