We present the charged-particle multiplicity distributions over a wide pseudorapidity range ($-3.4<\eta<5.0$) for pp collisions at $\sqrt{s}=$ 0.9, 7, and 8 TeV at the LHC. Results are based on information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE, extending the pseudorapidity coverage of the earlier publications and the high-multiplicity reach. The measurements are compared to results from the CMS experiment and to PYTHIA, PHOJET and EPOS LHC event generators, as well as IP-Glasma calculations.
Multiplicity distribution in the pseudorapidity region -2.0 to 2.0 for NSD collisions at a centre-of-mass energy of 900 GeV.
Multiplicity distribution in the pseudorapidity region -2.4 to 2.4 for NSD collisions at a centre-of-mass energy of 900 GeV.
Multiplicity distribution in the pseudorapidity region -3.0 to 3.0 for NSD collisions at a centre-of-mass energy of 900 GeV.
We present the charged-particle pseudorapidity density in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\,\mathrm{Te\kern-.25exV}$ in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from $-3.5$ to $5$, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0-5%) collisions we find $21\,400\pm 1\,300$ while for the most peripheral (80-90%) we find $230\pm 38$. This corresponds to an increase of $(27\pm4)\%$ over the results at $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{Te\kern-.25exV}$ previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The charged-particle pseudorapidity density of the most central collisions is compared to model calculations --- none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.
Charged-particle pseudorapidity density for ten centrality classes over a broad $\eta$ range in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}}=5.02\,\mathrm{TeV}$. Boxes around the points reflect the total uncorrelated systematic uncertainties, while the filled squares on the right reflect the correlated systematic uncertainty (evaluated at $\eta=0$). Statistical errors are generally insignificant and smaller than the markers. Also shown is the reflection of the $3.5<\eta<5$ values around $\eta=0$ (open circles). The line corresponds to fits of the difference between two Gaussians centred at $\eta=0$ ($f_{\text{GG}}$) [PLB754.373] to the data.
Charged-particle pseudorapidity density at midrapidity in most perihperhal (80-90%) Pb-Pb collisions at $\sqrt{s_{\scriptscriptstyle\mathrm{NN}}}=5.02\,\mathrm{TeV}$.
Total number of charged particles as a function of the mean number of participating nucleons [PRC88.044909]. The total charged-particle multiplicity is given as the integral over $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ over the measured region ($-3.5<\eta<5$) and extrapolations from fitted functions in the unmeasured regions. The contribution from unmeasured $\eta$ regions amounts to $\approx30\%$ of the total number of charged particles. The uncertainty on the extrapolation to the unmeasured pseudorapidity region is smaller than the size of the markers. The contribution to the systematic uncertainties from the centrality determination and electromagnetic processes are vanishing compared to the contribution from the largest differences between the fitted functions. A function inspired by factorisation [PRC83.024913] is fitted to the data, and the best fit yields $a=51.5\pm7.3$, $b=0.16\pm0.05$.
The centrality dependence of the charged-particle pseudorapidity density measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ over a broad pseudorapidity range is presented. This Letter extends the previous results reported by ALICE to more peripheral collisions. No strong change of the charged-particle pseudorapidity density distributions with centrality is observed, and when normalised to the number of participating nucleons in the collisions, the evolution over pseudorapidity with centrality is likewise small. The broad pseudorapidity range allows precise estimates of the total number of produced charged particles which we find to range from $162\pm22$ (syst.) to $17170\pm770$ (syst.) in 80-90% and 0-5 central collisions, respectively. The total charged-particle multiplicity is seen to approximately scale with the number of participating nucleons in the collision. This suggests that hard contributions to the charged-particle multiplicity are limited. The results are compared to models which describe $\mbox{d}N_{\mbox{ch}}/\mbox{d}\eta$ at mid-rapidity in the most central Pb-Pb collisions and it is found that these models do not capture all features of the distributions.
Measurement of $\mbox{d}N_{\mbox{ch}}/\mbox{d}\eta$ for all centralities and a broad $\eta$ range. Combined and symmetrised $\mbox{d}N_{\mbox{ch}}/\mbox{d}\eta$ over 30-90 PCT centrality from both SPD and FMD. Previously published results for 0-30 PCT over the full pseudorapidity range available elsewhere [PLB726.610]. Please note the systematic uncertainty from the centrality determination is encoded as a qualifier in the table header.
Full--width half--maximum of the charged--particle pseudorapidity distributions versus the average number of participants. The uncertainties on the ALICE measurements are from the fit of $f_{\text{GG}}$ only and evaluated at $95\%$ confidence level.
The charged--particle pseudorapidity density distributions scaled by the average number of participants in various pseudorapidity intervals as a function of the number of participants. Data for the 0 to 30 PCT most central events, and in ETARAP < 0.5 is available in previously published results [PLB726.610,PRC88.044910]. The uncertainties on $\left\langle N_{\text{part}}\right\rangle$ from the Glauber calculations not included (see [PRC88.044910]).
A detailed study of pseudorapidity densities and multiplicity distributions of primary charged particles produced in proton-proton collisions, at $\sqrt{s} =$ 0.9, 2.36, 2.76, 7 and 8 TeV, in the pseudorapidity range $|\eta|<2$, was carried out using the ALICE detector. Measurements were obtained for three event classes: inelastic, non-single diffractive and events with at least one charged particle in the pseudorapidity interval $|\eta|<1$. The use of an improved track-counting algorithm combined with ALICE's measurements of diffractive processes allows a higher precision compared to our previous publications. A KNO scaling study was performed in the pseudorapidity intervals $|\eta|<$ 0.5, 1.0 and 1.5. The data are compared to other experimental results and to models as implemented in Monte Carlo event generators PHOJET and recent tunes of PYTHIA6, PYTHIA8 and EPOS.
Measured pseudorapidity dependence of $dN/d\eta$ for INEL collisions at a centre-of-mass energy of 900 GeV.
Measured pseudorapidity dependence of $dN/d\eta$ for NSD collisions at a centre-of-mass energy of 900 GeV.
Measured pseudorapidity dependence of $dN/d\eta$ for INEL>0 collisions at a centre-of-mass energy of 900 GeV.
Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z-boson are presented. The measurements are performed using 20.3 inverse femtobarns of proton-proton collision data collected at a centre-of-mass energy of sqrt(s)=8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5 sigma level. The electroweak process includes the vector boson fusion production of a Z-boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z-boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the SHERPA and POWHEG event generators.
Unfolded normalised differential Z+2j cross section as a function of dijet invariant mass in the baseline region.
Unfolded normalised differential Z+2j cross section as a function of dijet invariant mass in the search region.
Unfolded normalised differential Z+2j cross section as a function of the rapidity separation between the leading jets in the baseline region.
Measurements of the production of jets of particles in association with a Z boson in pp collisions at $\sqrt{s}$ = 7 TeV are presented, using data corresponding to an integrated luminosity of 4.6/fb collected by the ATLAS experiment at the Large Hadron Collider. Inclusive and differential jet cross sections in Z events, with Z decaying into electron or muon pairs, are measured for jets with transverse momentum pT > 30 GeV and rapidity |y| < 4.4. The results are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements supplemented by parton showers.
The distribution of Inclusive jet multiplicity. The first (sys) error is the uncorrelated systematic error and the second the correlated systematic error.
The distribution of Ratio of cross sections for successive inclusive jet multiplicities n/(n-1). The first (sys) error is the uncorrelated systematic error and the second the correlated systematic error.
The distribution of exclusive jet multiplicity. The first (sys) error is the uncorrelated systematic error and the second the correlated systematic error.
The production of Kshort and Lambda hadrons is studied in inelastic pp collisions at sqrt(s) = 0.9 and 7 TeV collected with the ATLAS detector at the LHC using a minimum-bias trigger. The observed distributions of transverse momentum, rapidity, and multiplicity are corrected to hadron level in a model-independent way within well defined phase-space regions. The distribution of the production ratio of Lambdabar to Lambda baryons is also measured. The results are compared with various Monte Carlo simulation models. Although most of these models agree with data to within 15% in the Kshort distributions, substantial disagreements with data are found in the Lambda distributions of transverse momentum.
The corrected transverse momentum distribution of KS mesons at 7000 GeV.
The corrected rapidity distribution of KS mesons at 7000 GeV.
The corrected multiplicity distribution of KS mesons at 7000 GeV.
Results of three searches are presented for the production of supersymmetric particles decaying into final states with missing transverse momentum and exactly two isolated leptons, e or mu. The analysis uses a data sample collected during the first half of 2011 that corresponds to a total integrated luminosity of 1 fb^-1 of sqrt{s} = 7 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider. Opposite-sign and same-sign dilepton events are separately studied, with no deviations from the Standard Model expectation observed. Additionally, in opposite- sign events, a search is made for an excess of same-flavour over different-flavour lepton pairs. Effective production cross sections in excess of 9.9 fb for opposite-sign events containing supersymmetric particles with missing transverse momentum greater than 250 GeV are excluded at 95% CL. For same-sign events containing supersymmetric particles with missing transverse momentum greater than 100 GeV, effective production cross sections in excess of 14.8 fb are excluded at 95% CL. The latter limit is interpreted in a simplified weak gaugino production model excluding chargino masses up to 200 GeV.
The dilepton invariant mass distribution for same-sign dileptons.
The missing-mass ET distribution for same-sign dilepton events before any jet requirement.
The missing-mass ET distribution for same-sign dilepton events after requiring two high-pt jets.
The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |eta| < 2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of sqrt(s_NN) = 2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point 'tracklets' and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2 < |eta| < 4.9. Measurements are presented of the per-event charged particle density distribution, dN_ch/deta, and the average charged particle multiplicity in the pseudorapidity interval |eta|<0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with the lower sqrt(s_NN) results. The shape of the dN_ch/deta distribution is found to be independent of centrality within the systematic uncertainties of the measurement.
The measured charged particle density distributions as a fuinction of pseudorapidity in the centrality regions 0-10, 10-20, 20-30 and 30-40 %.
The measured charged particle density distributions as a fuinction of pseudorapidity in the centrality regions 40-50, 50-60, 60-70 and 70-80 %.
Mean values of the charged particle multiplicities in the pseudorapidiy range -0.5-0.5 as a function of centrality. N(C=PART), the number of participating nucleons in the collision, is also shown, determined from the muliplicity and ET of the event, with which it has been shown to be strongly correlated.
Jets are identified and their properties studied in center-of-mass energy sqrt(s) = 7 TeV proton-proton collisions at the Large Hadron Collider using charged particles measured by the ATLAS inner detector. Events are selected using a minimum bias trigger, allowing jets at very low transverse momentum to be observed and their characteristics in the transition to high-momentum fully perturbative jets to be studied. Jets are reconstructed using the anti-kt algorithm applied to charged particles with two radius parameter choices, 0.4 and 0.6. An inclusive charged jet transverse momentum cross section measurement from 4 GeV to 100 GeV is shown for four ranges in rapidity extending to 1.9 and corrected to charged particle-level truth jets. The transverse momenta and longitudinal momentum fractions of charged particles within jets are measured, along with the charged particle multiplicity and the particle density as a function of radial distance from the jet axis. Comparison of the data with the theoretical models implemented in existing tunings of Monte Carlo event generators indicates reasonable overall agreement between data and Monte Carlo. These comparisons are sensitive to Monte Carlo parton showering, hadronization, and soft physics models.
Double differential cross sections for charged particle jets as a function of the jet PT in the |rapidity| range 0.0-0.5, shown separately for the two R values. The first (sys) errors is the correlated efficiency uncertainty and the second (sys) error is the correlated vetex splitting uncertainty. The third (sys) error is the quadratic sum of all the uncorrelated systematic uncertainties.
Double differential cross sections for charged particle jets as a function of the jet PT in the |rapidity| range 0.5-1.0, shown separately for the two R values. The first (sys) errors is the correlated efficiency uncertainty and the second (sys) error is the correlated vetex splitting uncertainty. The third (sys) error is the quadratic sum of all the uncorrelated systematic uncertainties.
Double differential cross sections for charged particle jets as a function of the jet PT in the |rapidity| range 1.0-1.5, shown separately for the two R values. The first (sys) errors is the correlated efficiency uncertainty and the second (sys) error is the correlated vetex splitting uncertainty. The third (sys) error is the quadratic sum of all the uncorrelated systematic uncertainties.
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