The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |eta|<2.5 and pT>500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at eta = 0 is measured to be 1.333 +/- 0.003 (stat.) +/- 0.040 (syst.), which is 5-15% higher than the Monte Carlo models predict.
Average value of charged particle multiplicity per event and unit of pseudorapidity in the pseudorapidity range from -0.2 to 0.2.
Charged particle multiplicity as a function of pseudorapidity.
Charged particle multiplicity as a function of transverse momentum.
Dijet angular distributions from the first LHC pp collisions at center-of-mass energy sqrt(s) = 7 TeV have been measured with the ATLAS detector. The dataset used for this analysis represents an integrated luminosity of 3.1 pb-1. Dijet $\chi$ distributions and centrality ratios have been measured up to dijet masses of 2.8 TeV, and found to be in good agreement with Standard Model predictions. Analysis of the $\chi$ distributions excludes quark contact interactions with a compositeness scale $\Lambda$ below 3.4 TeV, at 95% confidence level, significantly exceeding previous limits.
CHI distribution for mass bin 340 to 520 GeV.
CHI distribution for mass bin 520 to 800 GeV.
CHI distribution for mass bin 800 to 1200 GeV.
A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the Large Hadron Collider (LHC) and correspond to an integrated luminosity of 315 nb^-1 collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% CL the q* mass interval 0.30 < mq* < 1.26 TeV, extending the reach of previous experiments.
The dijet mass distribution (NUMBER OF EVENTS).
95 PCT CL upper limit of the cross section x acceptance.
Jet cross sections have been measured for the first time in proton-proton collisions at a centre-of-mass energy of 7 TeV using the ATLAS detector. The measurement uses an integrated luminosity of 17 nb-1 recorded at the Large Hadron Collider. The anti-kt algorithm is used to identify jets, with two jet resolution parameters, R = 0.4 and 0.6. The dominant uncertainty comes from the jet energy scale, which is determined to within 7% for central jets above 60 GeV transverse momentum. Inclusive single-jet differential cross sections are presented as functions of jet transverse momentum and rapidity. Dijet cross sections are presented as functions of dijet mass and the angular variable $\chi$. The results are compared to expectations based on next-to-leading-order QCD, which agree with the data, providing a validation of the theory in a new kinematic regime.
Inclusive jet double-differential cross sections in the |rapidity| range 0 to 0.3, using a jet resolution R value of 0.4. The three (sys) errors are respectively, the Absolute JES, the Unfolding and the Luminosity uncertainties.
Inclusive jet double-differential cross sections in the |rapidity| range 0.3 to 0.8, using a jet resolution R value of 0.4. The three (sys) errors are respectively, the Absolute JES, the Unfolding and the Luminosity uncertainties.
Inclusive jet double-differential cross sections in the |rapidity| range 0.8 to 1.2, using a jet resolution R value of 0.4. The three (sys) errors are respectively, the Absolute JES, the Unfolding and the Luminosity uncertainties.
First measurements of the W -> lnu and Z/gamma* -> ll (l = e, mu) production cross sections in proton-proton collisions at sqrt(s) = 7 TeV are presented using data recorded by the ATLAS experiment at the LHC. The results are based on 2250 W -> lnu and 179 Z/gamma* -> ll candidate events selected from a data set corresponding to an integrated luminosity of approximately 320 nb-1. The measured total W and Z/gamma*-boson production cross sections times the respective leptonic branching ratios for the combined electron and muon channels are $\stotW$ * BR(W -> lnu) = 9.96 +- 0.23(stat) +- 0.50(syst) +- 1.10(lumi) nb and $\stotZg$ * BR(Z/gamma* -> ll) = 0.82 +- 0.06(stat) +- 0.05(syst) +- 0.09(lumi) nb (within the invariant mass window 66 < m_ll < 116 GeV). The W/Z cross-section ratio is measured to be 11.7 +- 0.9(stat) +- 0.4(syst). In addition, measurements of the W+ and W- production cross sections and of the lepton charge asymmetry are reported. Theoretical predictions based on NNLO QCD calculations are found to agree with the measurements.
Measured fiducial cross section times leptonic branching ratio for W+ production in the W+ -> e+ nu final state.
Measured fiducial cross section times leptonic branching ratio for W- production in the W- -> e- nubar final state.
Measured fiducial cross section times leptonic branching ratio for W+/- production in the combined W+ -> e+ nu and W- -> e- nubar final state.
Measurements are presented from proton-proton collisions at centre-of-mass energies of sqrt(s) = 0.9, 2.36 and 7 TeV recorded with the ATLAS detector at the LHC. Events were collected using a single-arm minimum-bias trigger. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the relationship between the mean transverse momentum and charged-particle multiplicity are measured. Measurements in different regions of phase-space are shown, providing diffraction-reduced measurements as well as more inclusive ones. The observed distributions are corrected to well-defined phase-space regions, using model-independent corrections. The results are compared to each other and to various Monte Carlo models, including a new AMBT1 PYTHIA 6 tune. In all the kinematic regions considered, the particle multiplicities are higher than predicted by the Monte Carlo models. The central charged-particle multiplicity per event and unit of pseudorapidity, for tracks with pT >100 MeV, is measured to be 3.483 +- 0.009 (stat) +- 0.106 (syst) at sqrt(s) = 0.9 TeV and 5.630 +- 0.003 (stat) +- 0.169 (syst) at sqrt(s) = 7 TeV.
Charged-particle multiplicities in proton-proton collisions at a centre-of mass energy of 900 GeV as a function of pseudorapidity for events with the number of charged particles >=1 having transverse momentum >500 MeV and absolute(pseudorapidity) <2.5.
Charged-particle multiplicities in proton-proton collisions at a centre-of mass energy of 2360 GeV as a function of pseudorapidity for events with the number of charged particles >=1 having transverse momentum >500 MeV and absolute(pseudorapidity) <2.5.
Charged-particle multiplicities in proton-proton collisions at a centre-of mass energy of 7000 GeV as a function of pseudorapidity for events with the number of charged particles >=1 having transverse momentum >500 MeV and absolute(pseudorapidity) <2.5.
A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a centre-of-mass energy sqrt(s) = 7TeV is presented. The measurement covers the pseudorapidity ranges |eta|<1.37 and 1.52<|eta|<1.81 in the transverse energy range 15 < E_T <100 GeV. The results are based on an integrated luminosity of 880 nb-1, collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading order perturbative QCD calculations.
The measured prompt photon cross section as a function of transverse energy for the |pseudorapidity| range < 0.6.
The measured prompt photon cross section as a function of transverse energy for the |pseudorapidity| range 0.6 to 1.37.
The measured prompt photon cross section as a function of transverse energy for the |pseudorapidity| range 1.52 to 1.81.
Jet shapes have been measured in inclusive jet production in proton-proton collisions at sqrt(s) = 7 TeV using 3 pb^{-1} of data recorded by the ATLAS experiment at the LHC. Jets are reconstructed using the anti-kt algorithm with transverse momentum 30 GeV < pT < 600 GeV and rapidity in the region |y| < 2.8. The data are corrected for detector effects and compared to several leading-order QCD matrix elements plus parton shower Monte Carlo predictions, including different sets of parameters tuned to model fragmentation processes and underlying event contributions in the final state. The measured jets become narrower with increasing jet transverse momentum and the jet shapes present a moderate jet rapidity dependence. Within QCD, the data test a variety of perturbative and non-perturbative effects. In particular, the data show sensitivity to the details of the parton shower, fragmentation, and underlying event models in the Monte Carlo generators. For an appropriate choice of the parameters used in these models, the data are well described.
Measured Differential Jet Shape RHO as a function of r for jet transverse momentum from 30 to 40 GeV and absolute values of the jet rapidity from 0 to 2.8.
Measured Differential Jet Shape RHO as a function of r for jet transverse momentum from 40 to 60 GeV and absolute values of the jet rapidity from 0 to 2.8.
Measured Differential Jet Shape RHO as a function of r for jet transverse momentum from 60 to 80 GeV and absolute values of the jet rapidity from 0 to 2.8.
Searches for new phenomena in high-mass diphoton final states with the ATLAS experiment at the LHC are presented. The analysis is based on $pp$ collision data corresponding to an integrated luminosity of 36.7 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s}=13$ TeV recorded in 2015 and 2016. Searches are performed for resonances with spin 0, as predicted by theories with an extended Higgs sector, and for resonances with spin 2, using a warped extra-dimension model as a benchmark model, as well as for non-resonant signals, assuming a large extra-dimension scenario. No significant deviation from the Standard Model is observed. Upper limits are placed on the production cross section times branching ratio to two photons as a function of the resonance mass. In addition, lower limits are set on the ultraviolet cutoff scale in the large extra-dimensions model.
Upper limits on the fiducial cross section times branching ratio to two photons at centre-of-mass energy of 13 TeV of a narrow-width (Γ_X = 4 MeV) spin-0 resonance as a function of its mass m_X.
Upper limits on the fiducial cross section times branching ratio to two photons at centre-of-mass energy of 13 TeV of a spin-0 resonance as a function of its mass m_X. The decay width of the resonance equals to 2% of m_X.
Upper limits on the fiducial cross section times branching ratio to two photons at centre-of-mass energy of 13 TeV of a spin-0 resonance as a function of its mass m_X. The decay width of the resonance equals to 6% of m_X.
A detailed study of multi-particle azimuthal correlations is presented using $pp$ data at $\sqrt{s}=5.02$ and 13 TeV, and $p$+Pb data at $\sqrt{s_{\rm{NN}}}=5.02$ TeV, recorded with the ATLAS detector at the LHC. The azimuthal correlations are probed using four-particle cumulants $c_{n}\{4\}$ and flow coefficients $v_n\{4\}=(-c_{n}\{4\})^{1/4}$ for $n=2$ and 3, with the goal of extracting long-range multi-particle azimuthal correlation signals and suppressing the short-range correlations. The values of $c_{n}\{4\}$ are obtained as a function of the average number of charged particles per event, $\left\langle N_{\rm{ch}} \right\rangle$, using the recently proposed two-subevent and three-subevent cumulant methods, and compared with results obtained with the standard cumulant method. The three-subevent method is found to be least sensitive to short-range correlations, which originate mostly from jets with a positive contribution to $c_{n}\{4\}$. The three-subevent method gives a negative $c_{2}\{4\}$, and therefore a well-defined $v_2\{4\}$, nearly independent of $\left\langle N_{\rm{ch}} \right\rangle$, which provides direct evidence that the long-range multi-particle azimuthal correlations persist to events with low multiplicity. Furthermore, $v_2\{4\}$ is found to be smaller than the $v_2\{2\}$ measured using the two-particle correlation method, as expected for long-range collective behavior. Finally, the measured values of $v_2\{4\}$ and $v_2\{2\}$ are used to estimate the number of sources relevant for the initial eccentricity in the collision geometry.
The c_2{4} values calculated for charged particles with 0.3 < pT < 3 GeV with the standard cumulant method from the 13 TeV pp data. The event averaging is performed for N_{ch}^{Sel} calculated for 0.3 < pT < 3 GeV.
The c_2{4} values calculated for charged particles with 0.3 < pT < 3 GeV with the standard cumulant method from the 13 TeV pp data. The event averaging is performed for N_{ch}^{Sel} calculated for pT > 0.2 GeV.
The c_2{4} values calculated for charged particles with 0.3 < pT < 3 GeV with the standard cumulant method from the 13 TeV pp data. The event averaging is performed for N_{ch}^{Sel} calculated for pT > 0.4 GeV.
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