Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the LHC are reported. Data were collected in proton--proton collisions at a center-of-mass energy of 8 TeV and correspond to an integrated luminosity of 20.3 fb$^{-1}$. The observed data are well described by the expected Standard Model backgrounds. The expected (observed) upper limit on the fiducial cross section for the production of such events is 6.1 (5.3) fb at 95% confidence level. Exclusion limits are presented on models of new phenomena with large extra spatial dimensions, supersymmetric quarks, and direct pair production of dark-matter candidates.
Distribution of ETmiss in the data and for the expected background in the single-muon control region. The total background expectation has been normalized to the observed number of events in this control region. Overflow is included in the final bin.
Distribution of ETmiss in the data and for the expected background in the two-muon control region. The total background expectation is normalized to the observed number of events in this control region. Overflows are included in the final bin.
Distribution of ETmiss in the data and for the expected background in the two-electron control region. The total background expectation is normalized to the observed number of events in this control region. Overflows are included in the final bin.
This article reports on a search for dark matter pair production in association with bottom or top quarks in 20.3 fb$^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.
Model dependent cross section upper limits for the b-FDM model from combined results of SR1 and SR2.
Results are presented from a search for new decaying massive particles whose presence is inferred from an imbalance in transverse momentum and which are produced in association with a single top quark that decays into a bottom quark and two light quarks. The measurement is performed using 19.7 inverse femtobarns of data from proton-proton collisions at a center-of-mass energy of 8 TeV, collected with the CMS detector at the CERN LHC. No deviations from the standard model predictions are observed and lower limits are set on the masses of new invisible bosons. In particular, scalar and vector particles, with masses below 330 and 650 GeV, respectively, are excluded at 95% confidence level, thus substantially extending a previous limit published by the CDF Collaboration.
The invariant mass of the three jets prior to the selection on their mass to be less than 250 GeV, for events with one b-tagged jet. Data are compared to the simulated backgrounds. The expectation from a model for an invisible vector particle with a mass of 700 GeV is represented by the dashed line.
The 95% CL expected and observed CLS limits as functions of the mass of a scalar invisible particle. The expected magnitude of a signal as a function of mass, calculated at leading order, is shown by the dashed curve. The confidence intervals for the expected limit are given at 68% and 95% coverage probability. Information about functional form used for the signal (labeled "scalar signal" in the paper): F = 7.52936*exp(-(0.0070828*(x^1.02681))), where x = M.
The 95% CL expected and observed CLS limits as functions of the mass of a vector invisible particle. The expected magnitude of a signal as a function of mass, calculated at leading order, is shown by the dashed curve. The confidence intervals for the expected limit are given at 68% and 95% coverage probability. Information about functional form used for the signal (labeled "vector signal" in the paper): F = 1390.91*exp(-(0.397971*(x^0.477721))), where x = M.
A measurement of the W boson helicity is presented, where the W boson originates from the decay of a top quark produced in pp collisions. The event selection, optimized for reconstructing a single top quark in the final state, requires exactly one isolated lepton (muon or electron) and exactly two jets, one of which is likely to originate from the hadronization of a bottom quark. The analysis is performed using data recorded at a center-of-mass energy of 8 TeV with the CMS detector at the CERN LHC in 2012. The data sample corresponds to an integrated luminosity of 19.7 inverse femtobarns. The measured helicity fractions are F[L] = 0.298 +/- 0.028 (stat) +\- 0.032 (syst), F[0] = 0.720 +/- 0.039 (stat) +/- 0.037 (syst), and F[R] = -0.018 +/- 0.019 (stat) +/- 0.011 (syst). These results are used to set limits on the real part of the tWb anomalous couplings, gL and gR.
Measurement of the W helicity fractions using the cos(theta*l) distribution in the muon channel. The helicity fractions corresponding to the longitudinal (F0) and left-handed (FL) polarizations are extracted from the fit while FR is obtained from the constraint of FL+FR+F0 = 1. The statistical correlation between the fit parameters is about -0.90.
Measurement of the W helicity fractions using the cos(theta*l) distribution in the electron channel. The helicity fractions corresponding to the longitudinal (F0) and left-handed (FL) polarizations are extracted from the fit while FR is obtained from the constraint of FL+FR+F0 = 1. The statistical correlation between the fit parameters is about -0.90.
Measurement of the W helicity fractions using the cos(theta*l) distribution in both electron and muon channels. The helicity fractions corresponding to the longitudinal (F0) and left-handed (FL) polarizations are extracted from the combined fit while FR is obtained from the constraint of FL+FR+F0 = 1. The total correlation between the fit parameters is about -0.80.
A search is presented for standard model (SM) production of four top quarks (t t-bar t t-bar) in pp collisions in the lepton + jets channel. The data correspond to an integrated luminosity of 19.6 inverse femtobarns recorded at a centre-of-mass energy of 8 TeV with the CMS detector at the CERN LHC. The expected cross section for SM four top quark production is approximately 1 fb. A combination of kinematic reconstruction and multivariate techniques is used to distinguish between the small signal and large background. The data are consistent with expectations of the SM, and an upper limit of 32 fb is set at a 95% confidence level on the cross section for producing four top quarks in the SM, where a limit of 32 +/- 17 fb is expected.
Measurement of an upper limit on tttt is set by performing a simultaneous maximum likelihood fit to the distributions in the BDT event discriminant for signal and background in six event categories in the single muon plus jets and single electron plus jets channels.
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.
A search for new long-lived particles decaying to leptons is presented using proton-proton collisions produced by the LHC at sqrt(s) = 8 TeV. Data used for the analysis were collected by the CMS detector and correspond to an integrated luminosity of 19.7 inverse femtobarns. Events are selected with an electron and a muon that have transverse impact parameter values between 0.02 cm and 2 cm. The search has been designed to be sensitive to a wide range of models with nonprompt e-mu final states. Limits are set on the "displaced supersymmetry" model, with pair production of top squarks decaying into an e-mu final state via R-parity-violating interactions. The results are the most restrictive to date on this model, with the most stringent limit being obtained for a top squark lifetime corresponding to c tau = 2 cm, excluding masses below 790 GeV at 95% confidence level.
Electron transverse impact parameter distribution for data and expected background processes after the preselection requirements have been applied. The event yields per bin have been rescaled to account for the varying bin sizes. The rightmost bin contains the overflow entries.
Muon transverse impact parameter distribution for data and expected background processes after the preselection requirements have been applied. The event yields per bin have been rescaled to account for the varying bin sizes. The rightmost contains the overflow entries.
Numbers of expected and observed events in the three search regions (see the text for the definitions of these regions). Background and signal expectations are quoted as $N_{\text{exp}} \pm 1\sigma$ stat $\pm 1\sigma$ syst. If the estimated background is zero in a particular search region, the estimate is instead taken from the preceding region. Since this should always overestimate the background, we denote this by a preceding "<".
The production of the $\eta_c (1S)$ state in proton-proton collisions is probed via its decay to the $p \bar{p}$ final state with the LHCb detector, in the rapidity range $2.0 < y < 4.5$ and in the meson transverse-momentum range $p_T > 6.5$ GeV/c. The cross-section for prompt production of $\eta_c (1S)$ mesons relative to the prompt $J/\psi$ cross-section is measured, for the first time, to be $\sigma_{\eta_c (1S)}/\sigma_{J/\psi} = 1.74 \pm 0.29 \pm 0.28 \pm 0.18 _{B}$ at a centre-of-mass energy $\sqrt{s} = 7$ TeV using data corresponding to an integrated luminosity of 0.7 fb$^{-1}$, and $\sigma_{\eta_c (1S)}/\sigma_{J/\psi} = 1.60 \pm 0.29 \pm 0.25 \pm 0.17 _{B}$ at $\sqrt{s} = 8$ TeV using 2.0 fb$^{-1}$. The uncertainties quoted are, in order, statistical, systematic, and that on the ratio of branching fractions of the $\eta_c (1S)$ and $J/\psi$ decays to the $p \bar{p}$ final state. In addition, the inclusive branching fraction of $b$-hadron decays into $\eta_c (1S)$ mesons is measured, for the first time, to be $B ( b \rightarrow \eta_c X ) = (4.88 \pm 0.64 \pm 0.29 \pm 0.67 _{B}) \times 10^{-3}$, where the third uncertainty includes also the uncertainty on the $J/\psi$ inclusive branching fraction from $b$-hadron decays. The difference between the $J/\psi$ and $\eta_c (1S)$ meson masses is determined to be $114.7 \pm 1.5 \pm 0.1$ MeV/c$^2$.
Differential cross-section for $\eta_c(1S)$ prompt production for $p_T > 6.5$ [GeV/$c$] and $2.0 < y < 4.5$. The reported uncertainties are total errors.
Differential cross-section for $\eta_c(1S)$ prompt production for $p_T > 6.5$ [GeV/$c$] and $2.0 < y < 4.5$. The reported uncertainties are total errors.
Differential cross-section for $\eta_c(1S)$ from inclusive charmonium production in b-hadrons decays for $p_T > 6.5$ [GeV/$c$] and $2.0 < y < 4.5$. The reported uncertainties are total errors.
The production of $\chi_b$ mesons in proton-proton collisions is studied using a data sample collected by the LHCb detector, at centre-of-mass energies of $\sqrt{s}=7$ and $8$ TeV and corresponding to an integrated luminosity of 3.0 fb$^{-1}$. The $\chi_b$ mesons are identified through their decays to $\Upsilon(1S)\gamma$ and $\Upsilon(2S)\gamma$ using photons that converted to $e^+e^-$ pairs in the detector. The relative prompt production rate of $\chi_{b1}(1P)$ and $\chi_{b2}(1P)$ mesons is measured as a function of the $\Upsilon(1S)$ transverse momentum in the $\chi_b$ rapidity range $2.0
Relative production cross section of $\chi_{b2}(1P)$ to $\chi_{b1}(1P)$ mesons in $p_T$ bins of $\Upsilon(1S)$ meson. The first uncertainty is statistical, the second is systematic and the third is due to the uncertainty on the branching fractions.
A measurement of the production processes of the recently discovered Higgs boson is performed in the two-photon final state using 5.4 fb$^{-1}$ of proton-proton collisions data at $\sqrt{s}=7$ TeV and 20.3 fb$^{-1}$ at $\sqrt{s}=8$ TeV collected by the ATLAS detector at the Large Hadron Collider. The number of observed Higgs boson decays to diphotons divided by the corresponding Standard Model prediction, called the signal strength, is found to be $\mu = 1.17 \pm 0.27$ at the value of the Higgs boson mass measured by ATLAS, $m_{H}$ = 125.4 GeV. The analysis is optimized to measure the signal strengths for individual Higgs boson production processes at this value of $m_{H}$. They are found to be $\mu_{\mathrm{ggF}} = 1.32 \pm 0.38$, $\mu_{\mathrm{VBF}} = 0.8 \pm 0.7$, $\mu_{{WH}} = 1.0 \pm 1.6 $, $\mu_{{ZH}} = 0.1 ^{+3.7}_{-0.1} $, $\mu_{{t\bar{t}H}} = 1.6 ^{+2.7}_{-1.8} $, for Higgs boson production through gluon fusion, vector-boson fusion, and in association with a $W$ or $Z$ boson or a top-quark pair, respectively. Compared with the previously published ATLAS analysis, the results reported here also benefit from a new energy calibration procedure for photons and the subsequent reduction of the systematic uncertainty on the diphoton mass resolution. No significant deviations from the predictions of the Standard Model are found.
The signal strength for a Higgs boson of mass mH = 125.4 GeV decaying via H->gammagamma as measured in the individual analysis categories, and the combined signal strength, for the combination of the 7 TeV and 8 TeV data. The VH dilepton category is not shown because with only two events in the combined sample, the fit results are not meaningful.
The signal strength for a Higgs boson of mass mH = 125.4 GeV decaying via H->gammagamma as measured in groups of categories sensitive to individual production modes, and the combined signal strength, for the combination of the 7 TeV and 8 TeV data.
Measured signal strengths, for a Higgs boson of mass mH = 125.4 GeV decaying via H->gammagamma, of the different Higgs boson production modes and the combined signal strength mu obtained with the combination of the 7 TeV and 8 TeV data.
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