The ATLAS detector at the LHC recorded 0.49nb$^{-1}$ of Pb+Pb collisions and 25pb$^{-1}$ of $pp$ collisions, both at the center-of-mass energy 5.02TeV per nucleon pair. Recently, ATLAS also recorded 3$μ$b$^{-1}$ of Xe+Xe collisions at the center-of-mass energy 5.44TeV, which provides a new opportunity to study the system-size dependence of the charged-hadron production in heavy-ion collisions. The large acceptance of the ATLAS detector allows to measure the spectra of charged hadrons in a wide range of pseudorapidity and transverse momentum. The nuclear modification factors $R_\mathrm{AA}$ are constructed as a ratio of the spectra measured in Pb+Pb or Xe+Xe collisions to that measured in $pp$ collisions. The $R_\mathrm{AA}$ obtained in the two systems are presented for different centrality intervals and the results are discussed.
The first results of single jet observables in Pb+Pb collisions at sqrt(s_NN)=2.76 TeV measured with the ATLAS detector at the LHC are presented. Full jets are reconstructed with the anti-kt algorithm with R= 0.2 and 0.4, using an event-by-event subtraction procedure to correct for the effects of the underlying event including elliptic flow. The geometrically-scaled ratio of jet yields in central and peripheral events,Rcp, indicates a clear suppression of jets with ET >100 GeV. The transverse and longitudinal distributions of jet fragments is also presented. We find little no substantial chang…
The energy loss of high-p_T partons provides insight into the transport properties of the medium created in relativistic heavy ion collisions. Evidence for this energy loss was first experimentally established through observation of high-p_T hadron suppression at RHIC. More recently, measurements of fully reconstructed jets have been performed at the LHC. In this summary the latest experimental results from the ATLAS collaboration on jet suppression are presented. In particular the jet suppression in inclusive jet yields, path length dependence of the jet suppression, photon-jet and Z^0-jet co…
Measurements of inclusive jet suppression in heavy ion collisions at the LHC provide direct sensitivity to the physics of jet quenching. In a sample of lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the anti-kt algorithm with values for the distance parameter that determines the nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dep…
A measurement of $J/ψ$ and $ψ(2\mathrm{S})$ production is presented. It is based on a data sample from Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV and $pp$ collisions at $\sqrt{s}$ = 5.02 TeV recorded by the ATLAS detector at the LHC in 2015, corresponding to an integrated luminosity of $0.42\mathrm{nb}^{-1}$ and $25\mathrm{pb}^{-1}$ in Pb+Pb and $pp$, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for $9 < p_{T}^{μμ} < 40$ GeV in dimuon transverse momentum, and $-2.0 < y_{μμ} < 2.…
In this talk the detection possibilities of R-hadrons in the ATLAS detector are studied. R-hadrons are stable hadronized gluinos, predicted by certain supersymmetric models. Making use of fully simulated R-hadrons, signatures of single R-hadrons in the ATLAS subdetector are studied, and triggering issues are addressed. Fast simulation is used to study the discovery potential. The abundant production of R-hadrons at the LHC, will allow a quick discovery for a broad range of masses.
Recent results of the ATLAS experiment at LHC on hadron spectroscopy, including exotic states, are presented. Comparison of the results with various theoretical predictions is discussed.
R-hadrons are massive, meta-stable particles predicted in several Super- symmetry scenarios. Studies exploring the discovery potential of R-hadrons at the ATLAS detector have mainly focused on gluino R-hadrons. These studies have shown that gluino R-hadrons should be discovered in early running of the LHC, that they are easily isolated by simple cuts and that their mass can be measured to an accuracy of a few percent.
The ATLAS experiment at the Large Hadron Collider (LHC) measures charged hadron spectra obtained in 2010 and 2011 lead-lead LHC data taking periods with total integrated statistics of 0.15nb${}^{-1}$. The results are compared to the $pp$ spectra of charged hadrons at the same centre-of-mass energy based on the data sample with integrated luminosity of 4.2pb${}^{-1}$ obtained by the ATLAS experiment in 2011 and 2013. This allows a detailed comparison of the two collision systems in a wide transverse momentum ($0.5<p_{\mathrm{T}}<150$GeV) and pseudorapidity ($|η|<2$) ranges in different centrali…
Recent results from the proton-proton collision data taken by the ATLAS experiment on exotic resonances are presented. A search for $J/ψp$ resonances in $Λ_{b}\rightarrow J/ψp K$ decays with large $p K$ invariant masses is reported. A search for exotic resonances in the four-muon final state is shown.
The measurement of charged particle spectra in heavy ion collisions is a way to study properties of hot and dense matter created in these interactions. The centrality dependence of the spectral shape is an important tool to understand the energy loss mechanism. The ATLAS detector at the LHC accumulated integrated luminosity equal to 0.15nb^{-1} of lead-lead data at 2.76TeV per nucleon-nucleon pair. Due to the excellent capabilities of the ATLAS detector, and its stable operation in 2010 and 2011 heavy ion physics runs, these data allow measurements of the charged particle spectra and their rat…
We present studies of the production and spectroscopy of some members of the B-hadron family. We reconstruct B ground states in their hadronic decay modes with a J/ψ or Υ in the final state. These studies are based on the 2011 7 TeV dataset collected by the ATLAS detector.
The charged hadron spectra in $p$+Pb and $pp$ collisions at $\sqrt{s_{NN}}=\sqrt{s}=5.02$TeV are measured with the ATLAS experiment at the LHC. The measurements are performed with $p$+Pb data recorded in 2013 with an integrated luminosity of 25nb${}^{-1}$ and $pp$ data recorded in 2015 with an integrated luminosity of 28pb${}^{-1}$. The $p$+Pb results are compared to $pp$ spectra, presented as a ratio of transverse momentum distributions in the two systems scaled by the number of binary nucleon-nucleon collisions, the nuclear modification factor $R_{p \mathrm{Pb}}$. The study of $R_\mathrm{pPb…
The elliptic flow of muons from the decay of charm and bottom hadrons is measured in $pp$ collisions at $\sqrt{s}=13$ TeV using a data sample with an integrated luminosity of 150 pb$^{-1}$ recorded by the ATLAS detector at the LHC. The muons from heavy-flavor decay are separated from light-hadron decay muons using momentum imbalance between the tracking and muon spectrometers. The heavy-flavor decay muons are further separated into those from charm decay and those from bottom decay using the distance-of-closest-approach to the collision vertex. The measurement is performed for muons in the tra…
The latest search for massive long-lived hadronising particles with the ATLAS detector is presented. The search is conducted with the inner detector and an integrated luminosity corresponding to 2.06 fb-1 at sqrt(s) = 7 TeV. For Split-SUSY scenarios gluino masses below 810 GeV have been excluded.
R-hadrons are predicted in a range of supersymmetric scenarios including split-supersymmetry and gauge-mediated supersymmetry breaking. In this paper, the discovery potential of the ATLAS experiment for gluino and stop-based R-hadrons is outlined. A range of final state observables such as high transverse momentum muon-like objects and information on ionization energy loss is used. It is shown that ATLAS would be able to discover such particles at comparatively modest amounts of luminosity (1fb-1) for masses up to 1 TeV.
The production of exotic heavy hadronic particles arise in several models for physics beyond the Standard Model. A study of the detection possibilities of new heavy hadron states with the ATLAS detector is presented. The focus is on R-hadrons, which are stable hadronized gluinos, predicted by certain supersymmetric models. Interactions and signatures of single R-hadrons are studied with the ATLAS simulation and reconstruction framework and issues concerning triggering on R-hadron events are addressed. The ATLAS fast simulation framework has been extended to include parameterizations for R-hadr…
Tau leptons play an important role in the physics program in ATLAS. They can be used not only in searches for new phenomena like the Higgs boson or Supersymmetry, or for electroweak measurements but also in detector related studies like the determination of the missing transverse energy scale. Identifying hadronically decaying tau leptons requires good understanding of the detector performance, combining information from calorimeter and tracking detectors. The current status of the tau reconstruction and identification with the ATLAS detector is presented. The identification efficiencies are m…
Charged-hadron and identified-hadron ($K^\mathrm{0}_\mathrm{S}$, $Λ$, $Ξ^\mathrm{-}$) yield measurements in photo-nuclear Pb+Pb and $p$+Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with ATLAS
Heavy-flavour hadron production provides information about the transport properties and microscopic structure of the quark-gluon plasma created in ultra-relativistic heavy-ion collisions. A measurement of the muons from semileptonic decays of charm and bottom hadrons produced in Pb+Pb and $pp$ collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV with the ATLAS detector at the Large Hadron Collider is presented. The Pb+Pb data were collected in 2015 and 2018 with sampled integrated luminosities of $208~\mathrm{μb}^{-1}$ and $38~\mathrm{μb^{-1}}$, respectively, and $pp$ data with a …
Correlations between charged particles provide important insight into the hadronization process. The analysis of the momentum difference between charged hadrons in $pp$, $p$-lead, and lead-lead collisions at LHC is performed by the ATLAS Collaboration in order to study the dynamics of hadron formation. The spectra of correlated hadron chains are explored and compared to the predictions based on the quantized fragmentation of a three dimensional QCD helix string. This provides an alternative view of the two-particle correlation phenomenon typically attributed to the Bose-Einstein interference.
The ATLAS inner detector is used to reconstruct secondary vertices due to hadronic interactions of primary collision products, so probing the location and amount of material in the inner region of ATLAS. Data collected in 7 TeV pp collisions at the LHC, with a minimum bias trigger, are used for comparisons with simulated events. The reconstructed secondary vertices have spatial resolutions ranging from ~200 microns to 1 mm. The overall material description in the simulation is validated to within an experimental uncertainty of about 7%. This will lead to a better understanding of the reconstru…
Azimuthal anisotropies of muons from charm and bottom hadron decays are measured in Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}= 5.02$ TeV. The data were collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2018 with integrated luminosities of $0.5~\mathrm{nb}^{-1}$ and $1.4~\mathrm{nb^{-1}}$, respectively. The kinematic selection for heavy-flavor muons requires transverse momentum $4 < p_\mathrm{T} < 30$ GeV and pseudorapidity $|η|<2.0$. The dominant sources of muons in this $p_\mathrm{T}$ range are semi-leptonic decays of charm and bottom hadrons. These heavy-flavor muons…
A search is presented for dark matter produced in association with a hadronically decaying W or Z boson using 3.2 /fb of pp collisions at $\sqrt{s}$=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet compatible with a W or Z boson and with large missing transverse momentum are analysed. The data are consistent with the Standard Model predictions and are interpreted in terms of both an effective field theory and a simplified model containing dark matter.
The b-hadron production cross section is measured with the ATLAS detector in pp collisions at sqrt(s) = 7 TeV, using 3.3 pb^-1 of integrated luminosity, collected during the 2010 LHC run. The b-hadrons are selected by partially reconstructing D*muX final states. Differential cross sections are measured as functions of the transverse momentum and pseudorapidity. The measured production cross section for a b-hadron with pT>9 GeV and |eta|<2.5 is 32.7 pm 0.8 (stat) ^{+4.5}_{-6.8} (syst) ub, higher than the next-to-leading-order QCD predictions but consistent within the experimental and theoretica…
This paper presents a measurement of the 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 centre-of-mass energies of sqrt(s) = 900 GeV and sqrt(s) = 7 TeV. The correlations measured in a phase space region dominated by low-pT particles…
A measurement of the tt production cross section in the all-hadronic decay mode is presented using 4.7 fb-1 of proton-proton collisions at a centre of mass energy of 7 TeV collected by the ATLAS experiment in 2011. Events are selected using a multi-jet trigger. Kinematic and b-tagging requirements are then applied to identify tt event candidates. A kinematic fit reconstructs the event topology of the final state extracting the top-quark mass which is then used to measure the production cross section with an unbinned likelihood fit. The result is found in good agreement with the Standard Model …
The investigation of the mechanism responsible for electroweak symmetry breaking is one of the most important tasks of the scientific program of the Large Hadron Collider. The experimental results on the search of the Standard Model Higgs boson with 1 to 2 fb^-1 of proton proton collision data at sqrt s=7 TeV recorded by the ATLAS detector are presented and discussed. No significant excess of events is found with respect to the expectations from Standard Model processes, and the production of a Higgs boson is excluded at 95% Confidence Level for the mass regions 144-232, 256-282 and 296-466 Ge…
The ATLAS experiment at the Large Hadron Collider (LHC) has been able to collect 5.25 fb-1 of data in 2011. For many physics analyses both in context of the Standard Model (SM) and Beyond the Standard Model (BSM) theories such as Higgs boson searches, tau leptons play an important role. Thus, triggering on hadronic tau decays is an essential ingredient for the success of those measurements. This contribution will summarize the developed efforts to meet this challenge. Efficiency measurements using data taken in 2011 at a center-of-mass energy of 7 TeV are described and results are presented. A…
We present the measurement of the inclusive electron spectrum in proton-proton collisions at a centre-of-mass energy of 7 TeV, using 1.3 pb-1 of data collected by the ATLAS experiment at the Large Hadron Collider. Signal electrons in the transverse momentum range 7 < pt < 26 GeV and within |eta| <2.0, excluding 1.37<|eta|<1.52, are extracted from the dominant hadron and conversion backgrounds. After subtraction of the small W/Z/gamma* contribution, the cross-section as a function of pt is found to be in good agreement with theoretical predictions for heavy flavour production from Fixed Order N…
The analysis of the momentum difference between charged hadrons in high-energy proton-proton collisions is performed in order to study coherent particle production. The observed correlation pattern agrees with a model of a helical QCD string fragmenting into a chain of ground-state hadrons. A threshold momentum difference in the production of adjacent pairs of charged hadrons is observed, in agreement with model predictions. The presence of low-mass hadron chains also explains the emergence of charge-combination-dependent two-particle correlations commonly attributed to Bose-Einstein interfere…
A search for a new heavy boson $W^{\prime}$ in proton-proton collisions at $\sqrt{s}$ = 13 TeV is presented. The search focuses on the decay of the $W^{\prime}$ to a hadronic top quark and a bottom quark, using the full Run 2 dataset of the ATLAS detector. The hadronic decay of the top quark is identified using DNN-based boosted-object techniques. The dominant background is obtained by a data-driven method with small systematic uncertainties. The results are presented as upper limits on the production cross-section times decay branching ratio for the $W^{\prime}$ boson with right-handed coupli…
This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of $Z\toττ$ and $Z(\toμμ)$+jets events selected from proton-proton collisions at a centre-of-mass ener…
Recent results on top-quark mass measurements with the ATLAS detector using proton-proton collisions at the Large Hadron Collider are presented. These results correspond to the measurements in the $t\bar{t}$ all-hadronic and dilepton channels at $\sqrt{s} = 8\ \mathrm{TeV}$ collisions and an integrated luminosity of $20\ \mathrm{fb}^{-1}$.
The uncertainty on the calorimeter energy response to jets of particles is derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the calorimeter response to single isolated charged hadrons is measured and compared to the Monte Carlo simulation using proton-proton collisions at centre-of-mass energies of sqrt(s) = 900 GeV and 7 TeV collected during 2009 and 2010. Then, using the decay of K_s and Lambda particles, the calorimeter response to specific types of particles (positively and negatively charged pions, protons, and anti-protons) is measured and compared to the Monte…
A search for production of supersymmetric particles in final states containing jets, missing transverse momentum, and at least one hadronically decaying tau lepton is presented. The data were recorded by the ATLAS experiment in sqrt(s) = 7 TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 2.05 fb-1 of data. The results are interpreted in the context of gauge mediated supersymmetry breaking models with Mmess = 250 TeV, N5 = 3, mu > 0, and Cgrav = 1. The production of supersymmetric particles is excluded at 95% C.…
The ATLAS hadronic Tile Calorimeter will undergo major upgrades to the on- and off-detector electronics in preparation for the High Luminosity program of the Large Hadron Collider (HL-LHC) in 2026, so that the system can cope with the HL-LHC increased radiation levels and out-of-time pileup. The on-detector electronics of the upgraded system will continuously digitize and transmit all photo-multiplier signals to the off-detector systems at a 40 MHz rate. The off-detector electronics will store the data in pipeline buffers, produce digital hadronic tower sums for the ATLAS Level-0 trigger syste…
A search is presented for dark matter pair production in association with a W or Z boson in pp collisions representing 20.3 fb-1 of integrated luminosity at sqrt(s)=8 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, and limits are set on the mass scale in effective field theories that describe the interaction of dark matter and Standard Model particles.
A search for heavy long-lived charged $R$-hadrons is reported using a data sample corresponding to 3.2$^{-1}$ of proton--proton collisions at $\sqrt{s} = 13$ TeV collected by the ATLAS experiment at the Large Hadron Collider at CERN. The search is based on observables related to large ionisation losses and slow propagation velocities, which are signatures of heavy charged particles travelling significantly slower than the speed of light. No significant deviations from the expected background are observed. Upper limits at 95% confidence level are provided on the production cross section of long…
Knowledge of the material in the ATLAS inner tracking detector is crucial in understanding the reconstruction of charged-particle tracks, the performance of algorithms that identify jets containing b-hadrons and is also essential to reduce background in searches for exotic particles that can decay within the inner detector volume. Interactions of primary hadrons produced in pp collisions with the material in the inner detector are used to map the location and amount of this material. The hadronic interactions of primary particles may result in secondary vertices, which in this analysis are rec…
Experimental results on high transverse momentum (leading) hadron spectra in nucleus-nucleus collisions in the range sqrt(s_NN) = 20 -- 200 GeV are reviewed with an emphasis on the observed suppression compared to free space production in proton-proton collisions at the corresponding center-of-mass energies. The transverse-momentum and collision-energy (but seemingly not the in-medium path length) dependence of the experimental suppression factors measured in central collisions is consistent with the expectations of final-state non-Abelian parton energy loss in a dense QCD medium.
Hitherto unobserved long-lived massive particles with electric and/or colour charge are predicted by a range of theories which extend the Standard Model. In this paper a search is performed at the ATLAS experiment for slow-moving charged particles produced in proton-proton collisions at 7 TeV centre-of-mass energy at the LHC, using a data-set corresponding to an integrated luminosity of 34 pb-1. No deviations from Standard Model expectations are found. This result is interpreted in a framework of supersymmetry models in which coloured sparticles can hadronise into long-lived bound hadronic sta…
Same- and opposite-sign charge asymmetries are measured in lepton+jets $t\bar{t}$ events in which a $b$-hadron decays semileptonically to a soft muon, using data corresponding to an integrated luminosity of 20.3 fb$^{-1}$ from proton--proton collisions at a centre-of-mass energy of $\sqrt{s}=8$ TeV collected with the ATLAS detector at the Large Hadron Collider at CERN. The charge asymmetries are based on the charge of the lepton from the top-quark decay and the charge of the soft muon from the semileptonic decay of a $b$-hadron and are measured in a fiducial region corresponding to the experim…
The ATLAS detector at the Large Hadron Collider at CERN is used to search for the decay of a scalar boson to a pair of long-lived particles, neutral under the Standard Model gauge group, in 20.3 fb$^{-1}$ of data collected in proton--proton collisions at $\sqrt{s}$ = 8 TeV. This search is sensitive to long-lived particles that decay to Standard Model particles producing jets at the outer edge of the ATLAS electromagnetic calorimeter or inside the hadronic calorimeter. No significant excess of events is observed. Limits are reported on the product of the scalar boson production cross section ti…
A measurement of the calorimeter response to isolated charged hadrons in the ATLAS detector at the LHC is presented. This measurement is performed with 3.2 nb$^{-1}$ of proton--proton collision data at $\sqrt{s}=7$ TeV from 2010 and 0.1 nb$^{-1}$ of data at $\sqrt{s}=8$ TeV from 2012. A number of aspects of the calorimeter response to isolated hadrons are explored. After accounting for energy deposited by neutral particles, there is a 5\% discrepancy in the modelling, using Geant4 physics lists, of the calorimeter response to isolated charged hadrons in the central calorimeter region. The desc…
This paper discusses hadron energy reconstruction for the ATLAS barrel prototype combined calorimeter (consisting of a lead-liquid argon electromagnetic part and an iron-scintillator hadronic part) in the framework of the non-parametrical method. The non-parametrical method utilizes only the known $e/h$ ratios and the electron calibration constants and does not require the determination of any parameters by a minimization technique. Thus, this technique lends itself to an easy use in a first level trigger. The reconstructed mean values of the hadron energies are within $\pm 1%$ of the true val…
Hadron energy reconstruction for the Atlas barrel prototype combined calorimeter, consisting of the lead-liquid argon electromagnetic part and the iron-scintillator hadronic part, in the framework of the non-parametrical method has been fulfilled. This method uses only the known e/h ratios and the electron calibration constants and does not require the determination of any parameters by a minimization technique and can be used for the fast energy reconstruction in the first level trigger. The obtained reconstruction of the mean values of energies is within $\pm 1%$ and the fractional energy re…
A search for the direct production of the supersymmetric partners of $τ$-leptons (staus) in final states with two hadronically decaying $τ$-leptons is presented. The analysis uses a dataset of $pp$ collisions corresponding to an integrated luminosity of $139$ fb$^{-1}$, recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of 13 TeV. No significant deviation from the expected Standard Model background is observed. Limits are derived in scenarios of direct production of stau pairs with each stau decaying into the stable lightest neutralino and one $τ$-lepton i…
A measurement of $b$-hadron pair production is presented, based on a data set corresponding to an integrated luminosity of 11.4 fb$^{-1}$ of proton--proton collisions recorded at $\sqrt{s}=8$ TeV with the ATLAS detector at the LHC. Events are selected in which a $b$-hadron is reconstructed in a decay channel containing $J/ψ\rightarrow μμ$, and a second $b$-hadron is reconstructed in a decay channel containing a muon. Results are presented in a fiducial volume defined by kinematic requirements on three muons based on those used in the analysis. The fiducial cross section is measured to be $17.7…
This Letter presents a search for pair production of heavy down-type quarks decaying via b'->Wt in the lepton + jets channel, as b'b'->WtWt->bbWWWW->lvbbqqqqqq. 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 fb^-1 from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV is excluded at the 95% confidence level.
A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the LHC at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb^-1. Events with an isolated electron or muon and a tau lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, sigma_ttbar = 186+/- 13 (stat) +/- 20 (syst) +/- 7 (…
The inner detector is the central tracking device of the ATLAS detector. In these proceedings the tracking performance of the inner detector is presented on collision data recorded at $\sqrt{s}$ = 900 GeV and 7 TeV. The identification of resonances like $Ξ$ and $Ω$ baryons in cascade decays via $K_{\mathrm{s}}$ and $Λ$ mesons is presented as well as the reconstruction of the $J / ψ$ and $ψ$(2S) mesons decaying into two muons. Furthermore, the performance of the track reconstruction and a data-driven method of estimating the track reconstruction efficiency as used in measurements of charged par…
The top quark pair production cross section in the fully hadronic final state is characterized by a six jet topology, two of which could be identified as originating from a b-quark using ATLAS b-tagging algorithms. Compared to other decay channels, this final state presents an advantageous larger branching ratio; on the other hand it suffers from a very large QCD multi-jet background, generally difficult to estimate from Monte Carlo simulation and therefore evaluated using data-driven techniques. The analysis is performed using 36pb-1 of pp collisions produced at the LHC with a center-of-mass …
Thanks to the excellent performances of ATLAS and CMS in triggering on muon signals and reconstructing these particles down to low transverse momentum, large samples of heavy-flavored hadrons have been collected in the 2011 LHC run at sqrt(s) = 7 TeV. The analysis of these samples has enabled both experiments to perform competitive measurements of heavy-flavor properties, such as quarkonium polarization, lifetime and CP-violation measurements, hadron spectroscopy and branching ratios of rare B decays.
We present heavy flavor measurements performed by the ATLAS and CMS Collaborations with data collected at the Large Hadron Collider. The production mechanism of heavy flavor hadrons is discussed as well as lifetime measurements and searches for the rare decay Bs->mumu. The large available statistics of about 5 fb-1 per experiment collected during the year 2011 together with the excellent detector performance allows to perform competitive heavy flavor measurements.
A search for decays of pair-produced neutral long-lived particles (LLPs) is presented using 139 fb$^{-1}$ of proton-proton collision data collected by the ATLAS detector at the LHC in 2015-2018 at a centre-of-mass energy of 13 TeV. Dedicated techniques were developed for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background, and limits for several benchmark signals are determined. For a SM Higgs b…
This paper presents a study of the production of $WW$ or $WZ$ boson pairs, with one $W$ boson decaying to $eν$ or $μν$ and one $W$ or $Z$ boson decaying hadronically. The analysis uses 20.2 fb$^{-1}$ of $\sqrt{s}=8$ TeV $pp$ collision data, collected by the ATLAS detector at the Large Hadron Collider. Cross-sections for $WW/WZ$ production are measured in high-$p_{T}$ fiducial regions defined close to the experimental event selection. The cross-section is measured for the case where the hadronically decaying boson is reconstructed as two resolved jets, and the case where it is reconstructed as …
Jets originating from the fragmentation of quarks and gluons are the most common, and complicated, final state objects produced at hadron colliders. A precise knowledge of their energy calibration is therefore of great importance at experiments at the Large Hadron Collider at CERN, while is very difficult to ascertain. We present in-situ techniques and results for the jet energy scale at ATLAS using recent collision data. ATLAS has demonstrated an understanding of the necessary jet energy corrections to within \approx 4% in the central region of the calorimeter.
The top-quark mass is measured in the all-hadronic top-antitop quark decay channel using proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The data set used in the analysis corresponds to an integrated luminosity of 20.2 fb$^{-1}$. The large multi-jet background is modelled using a data-driven method. The top-quark mass is obtained from template fits to the ratio of the three-jet to the dijet mass. The three-jet mass is obtained from the three jets assigned to the top quark decay. From these three jets the dijet…
