Search

The results of a search for supersymmetry in events with large missing transverse momentum and heavy-flavor jets using an integrated luminosity corresponding to $2.05{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ recorded with the ATLAS detector at the Large Hadron Collider are reported. No significant excess is observed with respect to the prediction for standard model processes. Results are interpreted in a variety of $R$-parity conserving models in which scalar bottoms and tops are the only scalar quarks to appear in the gluino decay cascade, and in an SO(10) model framework. Gluino masses up to 600–900 GeV are excluded, depending on the model considered.

A generic search for anomalous production of events with at least three charged leptons is presented. The search uses a $pp$-collision data sample at a center-of-mass energy of $\sqrt{s}=7\mathrm{TeV}$ corresponding to $4.6{\mathrm{fb}}^{-1}$ of integrated luminosity collected in 2011 by the ATLAS detector at the CERN Large Hadron Collider. Events are required to contain at least two electrons or muons, while the third lepton may either be an additional electron or muon, or a hadronically decaying tau lepton. Events are categorized by the presence or absence of a reconstructed tau-lepton or $Z$-boson candidate decaying to leptons. No significant excess above backgrounds expected from Standard Model processes is observed. Results are presented as upper limits on event yields from non-Standard-Model processes producing at least three prompt, isolated leptons, given as functions of lower bounds on several kinematic variables. Fiducial efficiencies for model testing are also provided. The use of the results is illustrated by setting upper limits on the production of doubly charged Higgs bosons decaying to same-sign lepton pairs.

A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks to the lightest neutralino (${\tilde{\chi }}_1^0$) is reported. It uses an LHC proton-proton data set at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ with an integrated luminosity of $3.2{\mathrm{fb}}^{-1}$ collected with the ATLAS detector in 2015. The signal is searched for in events containing several energetic jets, of which at least three must be identified as $b$ jets, large missing transverse momentum, and, potentially, isolated electrons or muons. Large-radius jets with a high mass are also used to identify highly boosted top quarks. No excess is found above the predicted background. For ${\tilde{\chi }}_1^0$ masses below approximately 700 GeV, gluino masses of less than 1.78 TeV and 1.76 TeV are excluded at the 95% C.L. in simplified models of the pair production of gluinos decaying via sbottom and stop, respectively. These results significantly extend the exclusion limits obtained with the $\sqrt{s}=8\mathrm{TeV}$ data set.

Results are presented on the production of jets of particles in association with a $Z/{\gamma }^{*}$ boson, in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector. The analysis includes the full 2010 data set, collected with a low rate of multiple proton-proton collisions in the accelerator, corresponding to an integrated luminosity of $36{\mathrm{pb}}^{-1}$. Inclusive jet cross sections in $Z/{\gamma }^{*}$ events, with $Z/{\gamma }^{*}$ decaying into electron or muon pairs, are measured for jets with transverse momentum $p_T>30\mathrm{GeV}$ 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.

A search for the standard model Higgs boson produced in association with a vector boson with the decay $H\to \tau \tau$ is presented. The data correspond to $20.3{\mathrm{fb}}^{-1}$ of integrated luminosity from proton-proton collisions at $\sqrt{s}=8\mathrm{TeV}$ recorded by the ATLAS experiment at the LHC during 2012. The data agree with the background expectation, and 95% confidence-level upper limits are placed on the cross section of this process. The observed (expected) limit, expressed in terms of the signal strength $\mu =\sigma /{\sigma }_{\mathrm{SM}}$ for $m_{\mathrm{H}}=125\mathrm{GeV}$, is $\mu <5.6$ (3.7). The measured value of the signal strength is $\mu =2.3\pm 1.6$.

A search for dark matter in association with a Higgs boson decaying to two photons is presented. This study is based on data collected with the ATLAS detector, corresponding to an integrated luminosity of $36.1{\mathrm{fb}}^{-1}$ of proton-proton collisions at the LHC at a center-of-mass energy of 13 TeV in 2015 and 2016. No significant excess over the expected background is observed. Upper limits at 95% confidence level are set on the visible cross section for beyond the Standard Model physics processes, and the production cross section times branching fraction of the Standard Model Higgs boson decaying into two photons in association with missing transverse momentum in three different benchmark models. Limits at 95% confidence level are also set on the observed signal in two-dimensional mass planes. Additionally, the results are interpreted in terms of 90% confidence-level limits on the dark-matter–nucleon scattering cross section, as a function of the dark-matter particle mass, for a spin-independent scenario.

This Letter presents a search for pair production of heavy down-type quarks decaying via $b^{\prime }\to Wt$ in the $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ channel, as $b^{\prime }{\overline{b}}^{\prime }\to W^{-}t{W}^{+}\overline{t}\to b\overline{b}{W}^{+}{W}^{-}{W}^{+}{W}^{-}\to l^{\pm }\nu b\overline{b}q\overline{q}q\overline{q}q\overline{q}$. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum $W$ bosons. In data corresponding to an integrated luminosity of $1.04{\mathrm{fb}}^{-1}$ from $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV can be excluded at the 95% confidence level.

The ATLAS detector is used to search for excited leptons in the electromagnetic radiative decay channel ${\ell }^{*}\to \ell \gamma$. Results are presented based on the analysis of $pp$ collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of $2.05\mathrm{f}{\mathrm{b}}^{-1}$. No evidence for excited leptons is found, and limits are set on the compositeness scale $\Lambda$ as a function of the excited lepton mass $m_{{\ell }^{*}}$. In the special case where $\Lambda =m_{{\ell }^{*}}$, excited electron and muon masses below 1.87 TeV and 1.75 TeV are excluded at 95% C.L., respectively.

This Letter presents a normalized differential cross-section measurement in a fiducial phase-space region where interference effects between top-quark pair production and associated production of a single top quark with a $W$ boson and a $b$-quark are significant. Events with exactly two leptons ($ee$, $\mu \mu$, or $e\mu$) and two $b$-tagged jets that satisfy a multiparticle invariant mass requirement are selected from $36.1{\mathrm{fb}}^{-1}$ of proton-proton collision data taken at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector at the LHC in 2015 and 2016. The results are compared with predictions from simulations using various strategies for the interference. The standard prescriptions for interference modeling are significantly different from each other but are within $2\sigma$ of the data. State-of-the-art predictions that naturally incorporate interference effects provide the best description of the data in the measured region of phase space most sensitive to these effects. These results provide an important constraint on interference models and will guide future model development and tuning.

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 regions $m_{{\ell }^{+}{\ell }^{-}}>5\mathrm{GeV}$ and $80<m_{4\ell }<100\mathrm{GeV}$, the measured cross sections are $76\pm 18\text{(stat)}\pm 4\text{(syst)}\pm 1.4\text{(lumi)}\mathrm{fb}$ and $107\pm 9\text{(stat)}\pm 4\text{(syst)}\pm 3.0\text{(lumi)}\mathrm{fb}$ at $\sqrt{s}=7$ and 8 TeV, respectively. By subtracting the nonresonant $4\ell$ production contributions and normalizing with $Z\to {\mu }^{+}{\mu }^{-}$ events, the branching fraction for the $Z$ boson decay to $4\ell$ is determined to be $(3.20\pm 0.25\text{(stat)}\pm 0.13\text{(syst)})\times 10^{-6}$, consistent with the standard model prediction.

The ATLAS detector at the Large Hadron Collider is used to search for the lepton flavor violating process $Z\to e\mu$ in $pp$ collisions using $20.3{\mathrm{fb}}^{-1}$ of data collected at $\sqrt{s}=8\mathrm{TeV}$. An enhancement in the $e\mu$ invariant mass spectrum is searched for at the $Z$-boson mass. The number of $Z$ bosons produced in the data sample is estimated using events of similar topology, $Z\to ee$ and $\mu \mu$, significantly reducing the systematic uncertainty in the measurement. There is no evidence of an enhancement at the $Z$-boson mass, resulting in an upper limit on the branching fraction, $\mathcal{B}(Z\to e\mu )<7.5\times 10^{-7}$ at the 95% confidence level.

Azimuthal decorrelations between the two central jets with the largest transverse momenta are sensitive to the dynamics of events with multiple jets. We present a measurement of the normalized differential cross section based on the full data set ($\int \mathcal{L}dt=36{\mathrm{pb}}^{-1}$) acquired by the ATLAS detector during the 2010 $\sqrt{s}=7\mathrm{TeV}$ proton-proton run of the LHC. The measured distributions include jets with transverse momenta up to 1.3 TeV, probing perturbative QCD in a high-energy regime.

A measurement is presented of the $t\overline{t}$ inclusive production cross section in $pp$ collisions at a center-of-mass energy of $\sqrt{s}=8\mathrm{TeV}$ using data collected by the ATLAS detector at the CERN Large Hadron Collider. The measurement was performed in the $\text{lepton}+\text{jets}$ final state using a data set corresponding to an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$. The cross section was obtained using a likelihood discriminant fit and $b$-jet identification was used to improve the signal-to-background ratio. The inclusive $t\overline{t}$ production cross section was measured to be $260\pm 1{(\text{stat})}_{-23}^{+22}(\text{stat})\pm 8(\text{lumi})\pm 4(\text{beam})\mathrm{pb}$ assuming a top-quark mass of 172.5 GeV, in good agreement with the theoretical prediction of ${253}_{-15}^{+13}\mathrm{pb}$. The $t\overline{t}\to (e,\mu )+\text{jets}$ production cross section in the fiducial region determined by the detector acceptance is also reported.

Results of a search for the pair production of photon-jets—collimated groupings of photons—in the ATLAS detector at the Large Hadron Collider are reported. Highly collimated photon-jets can arise from the decay of new, highly boosted particles that can decay to multiple photons collimated enough to be identified in the electromagnetic calorimeter as a single, photonlike energy cluster. Data from proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $36.7{\mathrm{fb}}^{-1}$, were collected in 2015 and 2016. Candidate photon-jet pair production events are selected from those containing two reconstructed photons using a set of identification criteria much less stringent than that typically used for the selection of photons, with additional criteria applied to provide improved sensitivity to photon-jets. Narrow excesses in the reconstructed diphoton mass spectra are searched for. The observed mass spectra are consistent with the Standard Model background expectation. The results are interpreted in the context of a model containing a new, high-mass scalar particle with narrow width, $X$, that decays into pairs of photon-jets via new, light particles, $a$. Upper limits are placed on the cross section times the product of branching ratios $\sigma \times \mathcal{B}(X\to aa)\times \mathcal{B}(a\to \gamma \gamma {)}^2$ for $200\mathrm{GeV}<m_X<2\mathrm{TeV}$ and for ranges of $m_a$ from a lower mass of 100 MeV up to between 2 and 10 GeV, depending upon $m_X$. Upper limits are also placed on $\sigma \times \mathcal{B}(X\to aa)\times \mathcal{B}(a\to 3{\pi }^0{)}^2$ for the same range of $m_X$ and for ranges of $m_a$ from a lower mass of 500 MeV up to between 2 and 10 GeV.

This Letter presents a measurement of the $W^{+}{W}^{-}$ production cross section in $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions by the ATLAS experiment, using $34{\mathrm{pb}}^{-1}$ of integrated luminosity produced by the Large Hadron Collider at CERN. Selecting events with two isolated leptons, each either an electron or a muon, 8 candidate events are observed with an expected background of $1.7\pm 0.6$ events. The measured cross section is ${41}_{-16}^{+20}(\mathrm{stat})\pm 5(\mathrm{syst})\pm 1(\mathrm{lumi})\mathrm{pb}$, which is consistent with the standard model prediction of $44\pm 3\mathrm{pb}$ calculated at next-to-leading order in QCD.

A search for the charged-lepton-flavor-violating process $Z\to e\mu$ is presented, using $139{\mathrm{fb}}^{-1}$ of $\sqrt{s}=13\mathrm{TeV}$ $pp$ collision data collected by the ATLAS experiment at the LHC. An excess in the $e\mu$ invariant mass spectrum near the $Z$ boson mass would be a striking signature of new physics. No excess is observed, and an upper limit $\mathcal{B}(Z\to e\mu )<2.62\times {10}^{-7}$ is placed on the branching fraction at 95% confidence level, which is the most stringent limit to date.

A search is performed for delayed and nonpointing photons originating from the displaced decay of a neutral long-lived particle (LLP). The analysis uses the full run 2 dataset of proton-proton collisions delivered by the LHC at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of $139{\mathrm{fb}}^{-1}$. The capabilities of the ATLAS electromagnetic calorimeter are exploited to precisely measure the arrival times and trajectories of photons. The results are interpreted in a scenario where the LLPs are pair produced in exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into a photon and a particle that escapes direct detection, giving rise to missing transverse momentum. No significant excess is observed above the expectation due to Standard Model background processes. The results are used to set upper limits on the branching ratio of the exotic decay of the Higgs boson. A model-independent limit is also set on the production of photons with large values of displacement and time delay.

This Letter presents a measurement of the inelastic proton-proton cross section using $60{\mu \mathrm{b}}^{-1}$ of $pp$ collisions at a center-of-mass energy $\sqrt{s}$ of 13 TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using rings of plastic scintillators in the forward region ($2.07<|\eta |<3.86$) of the detector. A cross section of $68.1\pm 1.4\mathrm{mb}$ is measured in the fiducial region $\xi =M_X^2/s>{10}^{-6}$, where $M_X$ is the larger invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this $\xi$ range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with $M_X>13\mathrm{GeV}$. The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross section of $78.1\pm 2.9\mathrm{mb}$ is measured, consistent with the inelastic cross section increasing with center-of-mass energy.

A search for new resonances decaying into jets containing $b$-hadrons in $pp$ collisions with the ATLAS detector at the LHC is presented in the dijet mass range from 0.57 to 7 TeV. The data set corresponds to an integrated luminosity of up to $36.1{\mathrm{fb}}^{-1}$ collected in 2015 and 2016 at $\sqrt{s}=13\mathrm{TeV}$. No evidence of a significant excess of events above the smooth background shape is found. Upper cross-section limits and lower limits on the corresponding signal mass parameters for several types of signal hypotheses are provided at 95% C.L. In addition, 95% C.L. upper limits are set on the cross sections for new processes that would produce Gaussian-shaped signals in the di-$b$-jet mass distributions.

This Letter presents a measurement of $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ production in $\mathrm{Pb}+\mathrm{Pb}$ collisions recorded by the ATLAS detector at the Large Hadron Collider at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ with an integrated luminosity of $0.49{\mathrm{nb}}^{-1}$. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from $\gamma \gamma$ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.