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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 937 records · Page 52Linked to original sources

arXiv2016

Measurement of the Inelastic Proton-Proton Cross Section at $\sqrt{s} = 13$ TeV with the ATLAS Detector at the LHC

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

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arXiv2016

Measurements of $ψ(2S)$ and $X(3872) \to J/ψπ^+π^-$ production in $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

Differential cross sections are presented for the prompt and non-prompt production of the hidden-charm states $X(3872)$ and $ψ(2S)$, in the decay mode $J/ψπ^+π^-$, measured using 11.4 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. The ratio of cross-sections $X(3872)/ψ(2S)$ is also given, separately for prompt and non-prompt components, as well as the non-prompt fractions of $X(3872)$ and $ψ(2S)$. Assuming independent single effective lifetimes for non-prompt $X(3872)$ and $ψ(2S)$ production gives $R_B = \frac{\mathcal{B}(B \rightarrow X(3872)\textrm{ + any}) \mathcal{B}(X(3872 \rightarrow J/ψπ^+π^-)}{\mathcal{B}(B \rightarrow ψ(2S)\textrm{ + any}) \mathcal{B}(ψ(2S) \rightarrow J/ψπ^+π^-)} = (3.95 \pm 0.32 \mathrm{(stat)} \pm 0.08\mathrm{(sys)}) \times 10^{-2}$, while separating short- and long-lived contributions, assuming that the short-lived component is due to $B_c$ decays, gives $R_B = (3.57 \pm 0.33\mathrm{(stat)} \pm 0.11\mathrm{(sys)}) \times 10^{-2}$, with the fraction of non-prompt $X(3872)$ produced via $B_c$ decays for $p_\mathrm{T}(X(3872)) > 10$ GeV being $(25 \pm 13\mathrm{(stat)} \pm 2\mathrm{(sys)} \pm 5\mathrm{(spin)})\%$. The distributions of the dipion invariant mass in the $X(3872)$ and $ψ(2S)$ decays are also measured and compared to theoretical predictions.

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arXiv2016

Measurement of the inclusive isolated prompt photon cross section in $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

A measurement of the cross section for the inclusive production of isolated prompt photons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV is presented. The measurement covers the pseudo rapidity ranges $|η^γ| < 1.37$ and $1.56 \leq |η^γ| < 2.37$ in the transverse energy range $25 < E_{\rm T}^γ < 1500$ GeV. The results are based on an integrated luminosity of 20.2 fb$^{-1}$, recorded by the ATLAS detector at the LHC. Photon candidates are identified by combining information from the calorimeters and the inner tracker. The background is subtracted using a data-driven technique, based on the observed calorimeter shower-shape variables and the deposition of hadronic energy in a narrow cone around the photon candidate. The measured cross sections are compared with leading-order and next-to-leading order perturbative QCD calculations and are found to be in a good agreement over ten orders of magnitude.

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arXiv2016

The performance of the jet trigger for the ATLAS detector during 2011 data taking

The performance of the jet trigger for the ATLAS detector at the LHC during the 2011 data taking period is described. During 2011 the LHC provided proton-proton collisions with a centre-of-mass energy of 7 TeV and heavy ion collisions with a 2.76 TeV per nucleon-nucleon collision energy. The ATLAS trigger is a three level system designed to reduce the rate of events from the 40 MHz nominal maximum bunch crossing rate to the approximate 400 Hz which can be written to offline storage. The ATLAS jet trigger is the primary means for the online selection of events containing jets. Events are accepted by the trigger if they contain one or more jets above some transverse energy threshold. During 2011 data taking the jet trigger was fully efficient for jets with transverse energy above 25 GeV for triggers seeded randomly at Level 1. For triggers which require a jet to be identified at each of the three trigger levels, full efficiency is reached for offline jets with transverse energy above 60 GeV. Jets reconstructed in the Event Filter and corresponding to offline jets with transverse energy greater than 60 GeV, are reconstructed with a resolution in transverse energy of better than 4% in the central region and better than 2.5% in the forward direction.

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arXiv2016

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in $pp$ collisions at $\sqrt{s}=8$ TeV using the ATLAS detector

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, $t\bar{t}$ system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=8$ TeV}. The observables have been chosen to emphasize the $t\bar{t}$ production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb$^{-1}$, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a $b$-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.

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arXiv2016

Search for new phenomena in different-flavour high-mass dilepton final states in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search is performed for a heavy particle decaying into different flavour dilepton pairs ($eμ$, $eτ$ or $μτ$), using 3.2 $fb^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV collected in 2015 by the ATLAS detector at the Large Hadron Collider. No excess over the Standard Model prediction is observed. Limits at the 95% credibility level are set on the mass of a $Z^{\prime}$ boson with lepton-flavour-violating couplings at 3.0, 2.7 and 2.6 TeV, and on the mass of a supersymmetric $τ$ sneutrino with $R$-parity-violating couplings at 2.3, 2.2 and 1.9 TeV, for $eμ$, $eτ$ and $μτ$ final states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.

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arXiv2016

Search for charged Higgs bosons produced in association with a top quark and decaying via $H^{\pm} \rightarrow τν$ using $pp$ collision data recorded at $\sqrt{s} = 13$ TeV by the ATLAS detector

Charged Higgs bosons produced in association with a single top quark and decaying via $H^{\pm} \rightarrow τν$ are searched for with the ATLAS experiment at the LHC, using proton-proton collision data at $\sqrt{s} = 13$ TeV corresponding to an integrated luminosity of 3.2 $\mathrm{fb}^{-1}$. The final state is characterised by the presence of a hadronic $τ$ decay and missing transverse momentum, as well as a hadronically decaying top quark, resulting in the absence of high-transverse-momentum electrons and muons. The data are found to be consistent with the expected background from Standard Model processes. A statistical analysis leads to $95\%$ confidence-level upper limits on the production cross section times branching fraction, $σ(pp \rightarrow [b]t H^{\pm}) \times \mbox{BR}(H^{\pm} \rightarrow τν)$, between 1.9 pb and 15 fb, for charged Higgs boson masses ranging from 200 to 2000 GeV. The exclusion limits for this search surpass those obtained with the proton-proton collision data recorded at $\sqrt{s} = 8$ TeV.

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arXiv2016

Observation of long-range elliptic anisotropies in $\sqrt{s}=$13 and 2.76 TeV $pp$ collisions with the ATLAS detector

ATLAS has measured two-particle correlations as a function of relative azimuthal-angle, $Δϕ$, and pseudorapidity, $Δη$, in $\sqrt{s}$=13 and 2.76 TeV $pp$ collisions at the LHC using charged particles measured in the pseudorapidity interval $|η|$<2.5. The correlation functions evaluated in different intervals of measured charged-particle multiplicity show a multiplicity-dependent enhancement at $Δϕ\sim 0$ that extends over a wide range of $Δη$, which has been referred to as the "ridge". Per-trigger-particle yields, $Y(Δϕ)$, are measured over 2<$|Δη|$<5. For both collision energies, the $Y(Δϕ)$ distribution in all multiplicity intervals is found to be consistent with a linear combination of the per-trigger-particle yields measured in collisions with less than 20 reconstructed tracks, and a constant combinatoric contribution modulated by $\cos{(2Δϕ)}$. The fitted Fourier coefficient, $v_{2,2}$, exhibits factorization, suggesting that the ridge results from per-event $\cos{(2ϕ)}$ modulation of the single-particle distribution with Fourier coefficients $v_2$. The $v_2$ values are presented as a function of multiplicity and transverse momentum. They are found to be approximately constant as a function of multiplicity and to have a $p_{\mathrm{T}}$ dependence similar to that measured in $p$+Pb and Pb+Pb collisions. The $v_2$ values in the 13 and 2.76 TeV data are consistent within uncertainties. These results suggest that the ridge in $pp$ collisions arises from the same or similar underlying physics as observed in $p$+Pb collisions, and that the dynamics responsible for the ridge has no strong $\sqrt{s}$ dependence.

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arXiv2016

Study of the rare decays of $B^0_s$ and $B^0$ into muon pairs from data collected during the LHC Run 1 with the ATLAS detector

A study of the decays $B^0\to μ^+μ^-$ and $B^0_s\to μ^+μ^-$ has been performed using data corresponding to an integrated luminosity of $25$ fb$^{-1}$ of $7$ TeV and $8$ TeV proton--proton collisions collected with the ATLAS detector during the LHC Run 1. For $B^0$, an upper limit on the branching fraction is set at ${\cal B}(B^0 \to μ^+μ^-) < 4.2 \times 10^{-10}$ at $95\%$ confidence level. For $B^0_s$, the branching fraction ${\cal B}(B^0_s \to μ^+μ^-) = \left( 0.9^{+1.1}_{-0.8} \right) \times 10^{-9}$ is measured. The results are consistent with the Standard Model expectation with a $p$-value of $4.8\%$, corresponding to $2.0$ standard deviations.

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arXiv2016

Measurement of the $W^{\pm}Z$ boson pair-production cross section in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

The production of $W^{\pm}Z$ events in proton--proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb$^{-1}$. The $W^{\pm}Z$ candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is $σ_{W^\pm Z \rightarrow \ell^{'} ν\ell \ell}^{\textrm{fid.}} = 63.2 \pm 3.2$ (stat.) $\pm 2.6$ (sys.) $\pm 1.5$ (lumi.) fb. In comparison, the next-to-leading-order Standard Model prediction is $53.4^{+3.6}_{-2.8}$ fb. The extrapolation of the measurement from the fiducial to the total phase space yields $σ_{W^{\pm}Z}^{\textrm{tot.}} = 50.6 \pm 2.6$ (stat.) $\pm 2.0$ (sys.) $\pm 0.9$ (th.) $\pm 1.2$ (lumi.) pb, in agreement with a recent next-to-next-to-leading-order calculation of $48.2^{+1.1}_{-1.0}$ pb. The cross section as a function of jet multiplicity is also measured, together with the charge-dependent $W^+Z$ and $W^-Z$ cross sections and their ratio.

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arXiv2016

Measurement of the top quark mass in the $t\bar{t}\to$ dilepton channel from $\sqrt{s}=8$ TeV ATLAS data

The top quark mass is measured in the $t\bar{t}\to$ dilepton channel (lepton $= e, μ$) using ATLAS data recorded in the year 2012 at the LHC. The data were taken at a proton--proton centre-of-mass energy of $\sqrt{s}=$ 8 TeV and correspond to an integrated luminosity of about 20.2 fb$^{-1}$. Exploiting the template method, and using the distribution of invariant masses of lepton--b-jet pairs, the top quark mass is measured to be $m_{top} =$ 172.99 $\pm$ 0.41 (stat) $\pm$ 0.74 (syst) GeV, with a total uncertainty of 0.84 GeV. Finally, a combination with previous ATLAS $m_{top}$ measurements from $\sqrt{s}=$7 TeV data in the $t\bar{t}\to$ dilepton and $t\bar{t}\to$ lepton+jets channels results in $m_{top} =$ 172.84 $\pm$ 0.34 (stat) $\pm$ 0.61 (syst) GeV, with a total uncertainty of 0.70 GeV.

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arXiv2016

Charged-particle distributions at low transverse momentum in $\sqrt{s}$=13 TeV pp interactions measured with the ATLAS detector at the LHC

Measurements of distributions of charged particles produced in proton-proton collisions with a centre-of-mass energy of 13 TeV are presented. The data were recorded by the ATLAS detector at the LHC and correspond to an integrated luminosity of 151 $μ$b$^{-1}$. The particles are required to have a transverse momentum greater than 100 MeV and an absolute pseudorapidity less than 2.5. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on multiplicity are measured in events containing at least two charged particles satisfying the above kinematic criteria. The results are corrected for detector effects and compared to the predictions from several Monte Carlo event generators.

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arXiv2016

Measurement of jet activity in top quark events using the $eμ$ final state with two $b$-tagged jets in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

Measurements of the jet activity in $t\bar{t}$ events produced in proton--proton collisions at $\sqrt{s}=8$ TeV are presented, using $20.3\,$fb$^{-1}$ of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dilepton $eμ$ decay channel with two identified $b$-jets. The numbers of additional jets for various jet transverse momentum ($p_T$) thresholds, and the normalised differential cross-sections as a function of $p_T$ for the five highest-$p_T$ additional jets, were measured in the jet pseudorapidity range $|η|<4.5$. The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimum $p_T$ of a single jet or the scalar sum of $p_T$ of all additional jets. These fractions were also measured in different regions of the invariant mass of the $eμb\bar{b}$ system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-order $t\bar{t}$ event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.

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arXiv2016

Search for scalar leptoquarks in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment

An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in $pp$ collisions at $\sqrt{s}$ = 13 TeV at the Large Hadron Collider, have been considered. An integrated luminosity of 3.2 fb$^{-1}$, corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 GeV and 1050 GeV (1160 GeV and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.

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arXiv2016

Search for Higgs and $Z$ Boson Decays to $ϕ\,γ$ with the ATLAS Detector

A search for the decays of the Higgs and $Z$ bosons to a $ϕ$ meson and a photon is performed with a $pp$ collision data sample corresponding to an integrated luminosity of 2.7 fb$^{-1}$ collected at $\sqrt{s}=$13 TeV with the ATLAS detector at the LHC. No significant excess of events is observed above the background, and 95% confidence level upper limits on the branching fractions of the Higgs and $Z$ boson decays to $ϕ\,γ$ of 1.4$\times$10$^{-3}$ and 8.3$\times$10$^{-6}$, respectively, are obtained.

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arXiv2016

Measurement of fiducial differential cross sections of gluon-fusion production of Higgs bosons decaying to $WW^{\ast}{\rightarrow\,}eνμν$ with the ATLAS detector at $\sqrt{s}=8$ TeV

This paper describes a measurement of fiducial and differential cross sections of gluon-fusion Higgs boson production in the $H{\rightarrow\,}WW^{\ast}{\rightarrow\,}eνμν$ channel, using 20.3 fb$^{-1}$ of proton-proton collision data. The data were produced at a centre-of-mass energy of $\sqrt{s} = 8$ TeV at the CERN Large Hadron Collider and recorded by the ATLAS detector in 2012. Cross sections are measured from the observed $H{\rightarrow\,}WW^{\ast}{\rightarrow\,}eνμν$ signal yield in categories distinguished by the number of associated jets. The total cross section is measured in a fiducial region defined by the kinematic properties of the charged leptons and neutrinos. Differential cross sections are reported as a function of the number of jets, the Higgs boson transverse momentum, the dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto efficiency, or fraction of events with no jets above a given transverse momentum threshold, is also reported. All measurements are compared to QCD predictions from Monte Carlo generators and fixed-order calculations, and are in agreement with the Standard Model predictions.

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arXiv2016

Search for high-mass new phenomena in the dilepton final state using proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search is conducted for both resonant and non-resonant high-mass new phenomena in dielectron and dimuon final states. The search uses 3.2 fb$^{-1}$ of proton-proton collision data, collected at $\sqrt{s} = 13$ TeV by the ATLAS experiment at the LHC in 2015. The dilepton invariant mass is used as the discriminating variable. No significant deviation from the Standard Model prediction is observed; therefore limits are set on the signal model parameters of interest at 95% credibility level. Upper limits are set on the cross-section times branching ratio for resonances decaying to dileptons, and the limits are converted into lower limits on the resonance mass, ranging between 2.74 TeV and 3.36 TeV, depending on the model. Lower limits on the $\ell\ell qq$ contact interaction scale are set between 16.7 TeV and 25.2 TeV, also depending on the model.

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arXiv2016

Search for resonances in diphoton events at $\sqrt{s}$=13 TeV with the ATLAS detector

Searches for new resonances decaying into two photons in the ATLAS experiment at the CERN Large Hadron Collider are described. The analysis is based on proton--proton collision data corresponding to an integrated luminosity of 3.2 fb$^{-1}$ at $\sqrt{s} = 13$ TeV recorded in 2015. Two searches are performed, one targeted at a spin-2 particle of mass larger than 500 GeV, using Randall--Sundrum graviton states as a benchmark model, and one optimized for a spin-0 particle of mass larger than 200 GeV. Varying both the mass and the decay width, the most significant deviation from the background-only hypothesis is observed at a diphoton invariant mass around 750 GeV with local significances of 3.8 and 3.9 standard deviations in the searches optimized for a spin-2 and spin-0 particle, respectively. The global significances are estimated to be 2.1 standard deviations for both analyses. The consistency between the data collected at 13 TeV and 8 TeV is also evaluated. Limits on the production cross section times branching ratio to two photons for the two resonance types are reported.

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