Search

A search for resonant diboson production using a data sample corresponding to $4.7{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the ATLAS experiment at the Large Hadron Collider in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ is presented. The search for a narrow resonance in the $WW$ or $WZ$ mass distribution is conducted in a final state with an electron or a muon, missing transverse momentum, and at least two jets. No significant excess is observed and limits are set using three benchmark models: $WW$ resonance masses below 940 and 710 GeV are excluded at 95% confidence level for spin-2 Randall–Sundrum and bulk Randall–Sundrum gravitons, respectively; $WZ$ resonance masses below 950 GeV are excluded at 95% confidence level for a spin-1 extended gauge model $W^{\prime }$ boson.

A generic search is presented for a heavy particle decaying to $WZ\to \ell \nu {\ell }^{\prime }{\ell }^{\prime }$ ($\ell$, ${\ell }^{\prime }=e$, $\mu$) final states. The data were recorded by the ATLAS detector in $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions at the Large Hadron Collider and correspond to an integrated luminosity of $1.02{\mathrm{fb}}^{-1}$. The transverse mass distribution of the selected $WZ$ candidates is found to be consistent with the standard model expectation. Upper limits on the production cross section times branching ratio are derived using two benchmark models predicting a heavy particle decaying to a $WZ$ pair.

This paper presents a search for a new heavy particle produced in association with a top or antitop quark. Two models in which the new heavy particle is a color singlet or a color triplet are considered, decaying, respectively, to $\overline{t}q$ or $tq$, leading to a resonance within the $t\overline{t}+\mathrm{\text{jets}}$ signature. The full 2011 ATLAS $pp$ collision data set from the LHC ($4.7{\mathrm{fb}}^{-1}$) is used to search for $t\overline{t}$ events produced in association with jets, in which one of the $W$ bosons from the top quarks decays leptonically and the other decays hadronically. The data are consistent with the Standard Model expectation, and a new particle with mass below 430 Gev for both $W^{\prime }$ boson and color triplet models is excluded at 95% confidence level, assuming unit right-handed coupling.

A search is presented for gluinos decaying via the supersymmetric partner of the top quark using events with two same-sign leptons, jets, and missing transverse momentum. The analysis is performed with $2.05{\mathrm{fb}}^{-1}$ of integrated luminosity from $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ collected by the ATLAS detector at the LHC. No excess beyond the standard model expectation is observed, and exclusion limits are derived for simplified models where the gluino decays via the supersymmetric partner of the top quark and in the minimal supergravity and constrained minimal supersymmetric standard model framework. In those scenarios, gluino masses below 550 GeV are excluded at 95% C.L. within the parameter space considered, significantly extending the coverage with respect to existing limits. Depending on the model parameters, gluino masses up to 750 GeV can also be excluded at 95% C.L.

A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The measurement is based on $4.7{\mathrm{fb}}^{-1}$ of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with standard model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are excluded for LSP masses up to 125 GeV.

A search for a heavy standard model Higgs boson decaying via $H\to ZZ\to {\ell }^{+}{\ell }^{-}\nu \overline{\nu }$, where $\ell =e$, $\mu$, is presented. It is based on proton-proton collision data at $\sqrt{s}=7\mathrm{TeV}$, collected by the ATLAS experiment at the LHC in the first half of 2011 and corresponding to an integrated luminosity of $1.04{\mathrm{fb}}^{-1}$. The data are compared to the expected standard model backgrounds. The data and the background expectations are found to be in agreement and upper limits are placed on the Higgs boson production cross section over the entire mass window considered; in particular, the production of a standard model Higgs boson is excluded in the region $340<m_H<450\mathrm{GeV}$ at the 95% confidence level.

High-resolution photoelectron spectra of xenon (Xe) were measured in the $4p$ photoionization region. The anisotropy parameters β were obtained for several photoelectron peaks with angle-resolved photoelectron measurements. The various complicated spectral features were observed due to the perturbed $4p^{-1}$ spin-orbit doublet and adjacent satellite structures. An analysis based on a relativistic multiconfiguration calculation has been carried out. The key role of the strong interactions between the $4p^{-1}$ core hole and the $4d^{-2}nl$ configurations has been investigated in detail. Experimental peak energies and intensities are in good agreement with theory. We find that the many of the complicated features are $4d^{-2}nl$ satellite lines, and their intensities are determined mostly by the amount of $4p^{-1}$ configurations that are mixed in. The experimental β values agree quite well with theory, indicating that the photoelectron angular distributions are also strongly influenced by the $4p$ direct-ionization contributions.

This paper presents a measurement of the $W^{+}{W}^{-}$ production cross section in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$. The leptonic decay channels are analyzed using data corresponding to an integrated luminosity of $4.6{\mathrm{fb}}^{-1}$ collected with the ATLAS detector at the Large Hadron Collider. The $W^{+}{W}^{-}$ production cross section $\sigma (pp\to W^{+}{W}^{-}+X)$ is measured to be $51.9\pm 2.0(\mathrm{stat})\pm 3.9(\mathrm{syst})\pm 2.0(\mathrm{lumi})\mathrm{pb}$, compatible with the Standard Model prediction of ${44.7}_{-1.9}^{+2.1}\mathrm{pb}$. A measurement of the normalized fiducial cross section as a function of the leading lepton transverse momentum is also presented. The reconstructed transverse momentum distribution of the leading lepton is used to extract limits on anomalous $WWZ$ and $WW\gamma$ couplings.

We have determined the Fano parameters and oscillator strengths of the triply photoexcited Li ${2s}^{2}2p$ ${}^2{P}^o$ autoionizing resonance in the ${\mathrm{Li}}^{+}$ and ${\mathrm{Li}}^{2+}$ partial photoion yields. Using monochromatized photons we observed an asymmetric resonance profile not only in the ${\mathrm{Li}}^{+}$ but also in the ${\mathrm{Li}}^{2+}$ partial cross section. The interaction between the triply excited state and the double-photoionization continuum takes place via a one-step double autoionization, since a two-step autoionization of the discrete state is energetically not possible. Our theoretical calculations using an $R$-matrix method for both the ${\mathrm{Li}}^{+}$ and ${\mathrm{Li}}^{2+}$ partial cross sections show resonance profiles in good agreement with experimental result.

Using a sample of dilepton top-quark pair ($t\overline{t}$) candidate events, a study is performed of the production of top-quark pairs together with heavy-flavor (HF) quarks, the sum of $t\overline{t}+b+X$ and $t\overline{t}+c+X$, collectively referred to as $t\overline{t}+\mathrm{HF}$. The data set used corresponds to an integrated luminosity of $4.7{\mathrm{fb}}^{-1}$ of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The presence of additional HF ($b$ or $c$) quarks in the $t\overline{t}$ sample is inferred by looking for events with at least three $b$-tagged jets, where two are attributed to the $b$ quarks from the $t\overline{t}$ decays and the third to additional HF production. The dominant background to $t\overline{t}+\mathrm{HF}$ in this sample is $t\overline{t}+\text{jet}$ events in which a light-flavor jet is misidentified as a heavy-flavor jet. To determine the heavy- and light-flavor content of the additional $b$-tagged jets, a fit to the vertex mass distribution of $b$-tagged jets in the sample is performed. The result of the fit shows that $79\pm 14(\text{stat})\pm 22(\text{syst})$ of the 105 selected extra $b$-tagged jets originate from HF quarks, 3 standard deviations away from the hypothesis of zero $t\overline{t}+\mathrm{HF}$ production. The result for extra HF production is quoted as a ratio ($R_{\mathrm{HF}}$) of the cross section for $t\overline{t}+\mathrm{HF}$ production to the cross section for $t\overline{t}$ production with at least one additional jet. Both cross sections are measured in a fiducial kinematic region within the ATLAS acceptance. $R_{\mathrm{HF}}$ is measured to be $[6.2\pm 1.1(\text{stat})\pm 1.8(\text{syst})]\%$ for jets with $p_T>25\mathrm{GeV}$ and $|\eta |<2.5$, in agreement with the expectations from Monte Carlo generators.

A combined search for the Standard Model Higgs boson with the ATLAS detector at the LHC is presented. The data sets used correspond to integrated luminosities from $4.6{\mathrm{fb}}^{-1}$ to $4.9{\mathrm{fb}}^{-1}$ of proton-proton collisions collected at $\sqrt{s}=7\mathrm{TeV}$ in 2011. The Higgs boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV are excluded at the 95% confidence level, while the range 120 GeV to 560 GeV is expected to be excluded in the absence of a signal. An excess of events is observed at Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 standard deviations ($\sigma$). The global probability for the background to produce an excess at least as significant anywhere in the entire explored Higgs boson mass range of 110–600 GeV is estimated to be $\sim 15\%$, corresponding to a significance of approximately $1\sigma$.

This paper presents a study of the possible ordering of charged hadrons in the azimuthal angle relative to the beam axis in high-energy proton-proton collisions at the Large Hadron Collider (LHC). A spectral analysis of correlations between longitudinal and transverse components of the momentum of the charged hadrons, driven by the search for phenomena related to the structure of the QCD field, is performed. Data were recorded with the ATLAS detector at center-of-mass energies of $\sqrt{s}=900\mathrm{GeV}$ and $\sqrt{s}=7\mathrm{TeV}$. The correlations measured in a kinematic region dominated by low-$p_T$ particles are not well described by conventional models of hadron production. The measured spectra show features consistent with the fragmentation of a QCD string represented by a helixlike ordered gluon chain.

A measurement of the $B_s^0\to J/\psi \varphi$ decay parameters, updated to include flavor tagging is reported using $4.9{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the ATLAS detector from $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions recorded in 2011 at the LHC. The values measured for the physical parameters are $$\begin{gathered} {\varphi }_s=0.12\pm 0.25(\text{stat})\pm 0.05(\text{syst})\text{rad} \\ \mathrm{\Delta }{\mathrm{\Gamma }}_s=0.053\pm 0.021(\text{stat})\pm 0.010(\text{syst}){\mathrm{ps}}^{-1} \\ {\mathrm{\Gamma }}_s=0.677\pm 0.007(\text{stat})\pm 0.004(\text{syst}){\mathrm{ps}}^{-1} \\ |A_{\parallel }(0){|}^2=0.220\pm 0.008(\text{stat})\pm 0.009(\text{syst}) \\ |A_0(0){|}^2=0.529\pm 0.006(\text{stat})\pm 0.012(\text{syst}) \\ {\delta }_{\perp }=3.89\pm 0.47(\text{stat})\pm 0.11(\text{syst})\text{rad} \end{gathered}$$where the parameter $\mathrm{\Delta }{\mathrm{\Gamma }}_s$ is constrained to be positive. The $S$-wave contribution was measured and found to be compatible with zero. Results for ${\varphi }_s$ and $\mathrm{\Delta }{\mathrm{\Gamma }}_s$ are also presented as 68% and 95% likelihood contours, which show agreement with the Standard Model expectations.

A search for squarks and gluinos in final states containing jets, missing transverse momentum and no high-$p_{\mathrm{T}}$ electrons or muons is presented. The data represent the complete sample recorded in 2011 by the ATLAS experiment in 7 TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of $4.7{\mathrm{fb}}^{-1}$. No excess above the Standard Model background expectation is observed. Gluino masses below 860 GeV and squark masses below 1320 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino, for squark or gluino masses below 2 TeV, respectively. Squarks and gluinos with equal masses below 1410 GeV are excluded. In minimal supergravity/constrained minimal supersymmetric Standard Model models with $\tan \beta =10$, $A_0=0$ and $\mu >0$, squarks and gluinos of equal mass are excluded for masses below 1360 GeV. Constraints are also placed on the parameter space of supersymmetric models with compressed spectra. These limits considerably extend the region of supersymmetric parameter space excluded by previous measurements with the ATLAS detector.

A measurement of the $ZZ$ production cross section in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ using data corresponding to an integrated luminosity of $1.02{\mathrm{fb}}^{-1}$ recorded by the ATLAS experiment at the LHC is presented. Twelve events containing two $Z$ boson candidates decaying to electrons and/or muons are observed, with an expected background of $0.3\pm 0.3(\mathrm{stat}{)}_{-0.3}^{+0.4}(\mathrm{syst})$ events. The cross section measured in a phase-space region with good detector acceptance and for dilepton masses within the range 66 to 116 GeV is ${\sigma }_{ZZ\to {\ell }^{+}{\ell }^{-}{\ell }^{+}{\ell }^{-}}^{\mathrm{fid}}={19.4}_{-5.2}^{+6.3}(\mathrm{stat}{)}_{-0.7}^{+0.9}(\mathrm{syst})\pm 0.7(\mathrm{lumi})\mathrm{fb}$. The resulting total cross section for on-shell $ZZ$ production, ${\sigma }_{ZZ}^{\mathrm{tot}}={8.5}_{-2.3}^{+2.7}(\mathrm{stat}{)}_{-0.3}^{+0.4}(\mathrm{syst})\pm 0.3(\mathrm{lumi})\mathrm{pb}$, is consistent with the standard model expectation of ${6.5}_{-0.2}^{+0.3}\mathrm{pb}$ calculated at the next-to-leading order in QCD. Limits on anomalous neutral triple gauge boson couplings are derived.

The ${\chi }_b(nP)$ quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at $\sqrt{s}=7\mathrm{TeV}$ and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of $4.4{\mathrm{fb}}^{-1}$, these states are reconstructed through their radiative decays to ${\rm Y}(1S,2S)$ with ${\rm Y}\to {\mu }^{+}{\mu }^{-}$. In addition to the mass peaks corresponding to the decay modes ${\chi }_b(1P,2P)\to {\rm Y}(1S)\gamma$, a new structure centered at a mass of $10.530\pm 0.005(\mathrm{stat})\pm 0.009(\mathrm{syst})\mathrm{GeV}$ is also observed, in both the ${\rm Y}(1S)\gamma$ and ${\rm Y}(2S)\gamma$ decay modes. This structure is interpreted as the ${\chi }_b(3P)$ system.

An updated search is performed for gluino, top squark, or bottom squark $R$-hadrons that have come to rest within the ATLAS calorimeter, and decay at some later time to hadronic jets and a neutralino, using 5.0 and $22.9{\mathrm{fb}}^{-1}$ of $pp$ collisions at 7 and 8 TeV, respectively. Candidate decay events are triggered in selected empty bunch crossings of the LHC in order to remove $pp$ collision backgrounds. Selections based on jet shape and muon system activity are applied to discriminate signal events from cosmic ray and beam-halo muon backgrounds. In the absence of an excess of events, improved limits are set on gluino, stop, and sbottom masses for different decays, lifetimes, and neutralino masses. With a neutralino of mass 100 GeV, the analysis excludes gluinos with mass below 832 GeV (with an expected lower limit of 731 GeV), for a gluino lifetime between $10\mu \mathrm{s}$ and 1000 s in the generic $R$-hadron model with equal branching ratios for decays to $q\overline{q}{\tilde{\chi }}^0$ and $g{\tilde{\chi }}^0$. Under the same assumptions for the neutralino mass and squark lifetime, top squarks and bottom squarks in the Regge $R$-hadron model are excluded with masses below 379 and 344 GeV, respectively.

A search is presented for dark matter pair production in association with a $W$ or $Z$ boson in $pp$ collisions representing $20.3{\mathrm{fb}}^{-1}$ of integrated luminosity at $\sqrt{s}=8\mathrm{TeV}$ using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet mass consistent with a $W$ or $Z$ boson, and with large missing transverse momentum are analyzed. The data are consistent with the standard model expectations. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of $W$ or $Z$ bosons with large missing transverse momentum are set in two fiducial regions.

Two-particle correlations in relative azimuthal angle ($\Delta \varphi$) and pseudorapidity ($\Delta \eta$) are measured in $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ $p+\mathrm{Pb}$ collisions using the ATLAS detector at the LHC. The measurements are performed using approximately $1\mu {\mathrm{b}}^{-1}$ of data as a function of transverse momentum ($p_{\mathrm{T}}$) and the transverse energy ($\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$) summed over $3.1<\eta <4.9$ in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range ($2<|\Delta \eta |<5$) “near-side” ($\Delta \varphi \sim 0$) correlation that grows rapidly with increasing $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$. A long-range “away-side” ($\Delta \varphi \sim \pi$) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$, is found to match the near-side correlation in magnitude, shape (in $\Delta \eta$ and $\Delta \varphi$) and $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ dependence. The resultant $\Delta \varphi$ correlation is approximately symmetric about $\pi /2$, and is consistent with a dominant $\cos 2\Delta \varphi$ modulation for all $\Sigma E_{\mathrm{T}}^{\mathrm{Pb}}$ ranges and particle $p_{\mathrm{T}}$.

An improved measurement of the mass of the Higgs boson is derived from a combined fit to the reconstructed invariant mass spectra of the decay channels $H\to \gamma \gamma$ and $H\to Z{Z}^{*}\to 4\ell$. The analysis uses the $pp$ collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at center-of-mass energies of 7 TeV and 8 TeV, corresponding to an integrated luminosity of $25{\mathrm{fb}}^{-1}$. The measured value of the Higgs boson mass is $m_H=125.36\pm 0.37(\text{stat})\pm 0.18(\text{syst})\mathrm{GeV}$. This result is based on improved energy-scale calibrations for photons, electrons, and muons as well as other analysis improvements, and supersedes the previous result from ATLAS. Upper limits on the total width of the Higgs boson are derived from fits to the invariant mass spectra of the $H\to \gamma \gamma$ and $H\to Z{Z}^{*}\to 4\ell$ decay channels.