103 research outputs found
Production of charged pions, kaons and protons at large transverse momenta in pp and Pb-Pb collisions at = 2.76 TeV
Transverse momentum spectra of and up to = 20 GeV/c at mid-rapidity, |y| 0.8, in pp and Pb-Pb collisions at = 2.76 TeV have been measured using the ALICE detector at the LHC. At intermediate (2-8 GeV/c) an enhancement of the proton-to-proton ratio, (p + \bar{p})/(\pi^+ + \pi^-\(), with respect to pp collisions is observed and the ratio reaches 0.80 in central Pb-Pb collisions. The measurement of the nuclear modification factors for \(\pi^{\pm}, K^{\pm} and indicates that within the systematic and statistical uncertainties they are the same at high (> 10 GeV/c), suggesting that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets.publishedVersio
Multiplicity dependence of the average transverse momentum in pp, p-Pb, and Pb-Pb collisions at the LHC
The average transverse momentum versus the charged-particle multiplicity was measured in p-Pb collisions at a collision energy per nucleon-nucleon pair = 5.02 TeV and in pp collisions at collision energies of = 0.9, 2.76, and 7 Tev in the kinematic range 0.15 with is observed, which is much stronger than that measured in Pb-Pb collisions. For pp collisions, this could be attributed, within a model of hadronizing strings, to multiple-parton interactions and to a final-state color reconnection mechanism. The data in p-Pb and Pb-Pb collisions cannot be described by an incoherent superposition of nucleon-nucleon collisions and pose a challenge to most of the event generators.publishedVersio
Measurement of the Cross Section for Electromagnetic Dissociation with Neutron Emission in Pb-Pb Collisions at âsNN = 2.76 TeV
The first measurement of neutron emission in electromagnetic dissociation of 208Pb nuclei at the LHC is presented. The measurement is performed using the neutron Zero Degree Calorimeters of the ALICE experiment, which detect neutral particles close to beam rapidity. The measured cross sections of single and mutual electromagnetic dissociation of Pb nuclei at âsNN = 2.76 TeV with neutron emission are Ď_single EMD = 187.2Âą0.2 (stat.) +13.8â12.0 (syst.) b and Ď_mutual EMD = 6.2 Âą 0.1 (stat.) Âą0.4 (syst.) b respectively. The experimental results are compared to the predictions from a relativistic electromagnetic dissociation model.publishedVersio
Energy dependence of the transverse momentum distributions of charged particles in pp collisions measured by ALICE
Differential cross sections of charged particles in inelastic pp collisions as a function of have been measured at = 0.9, 2.76 and 7 TeV at the LHC. The spectra are compared to NLO-pQCD calculations. Though the differential cross section for an individual cannot be described by NLO-pQCD, the relative increase of cross section with is in agreement with NLO-pQCD. Based on these measurements and observations, procedures are discussed to construct pp reference spectra at = 2.76 and 5.02 TeV up to = 50 GeV/c as required for the calculation of the nuclear modification factor in nucleus-nucleus and proton-nucleus collisions.publishedVersio
Mid-rapidity anti-baryon to baryon ratios in pp collisions at = 0.9, 2.76 and 7 TeV measured by ALICE
The ratios of yields of anti-baryons to baryons probes the mechanisms of baryon-number transport. Results for anti-proton/proton, anti-, anti- and anti- in pp collisions at = 0.9, 2.76 and 7 TeV, measured with the ALICE detector at the LHC, are reported. Within the experimental uncertainties and ranges covered by our measurement, these ratios are independent of rapidity, transverse momentum and multiplicity for all measured energies. The results are compared to expectations from event generators, such as PYTHIA and HIJING/B, that are used to model the particle production in pp collisions. The energy dependence of anti-proton/proton, anti-, anti- and anti-, reaching values compatible with unity for = 7 TeV, complement the earlier anti-proton/proton measurement of ALICE. These dependencies can be described by exchanges with the Regge-trajectory intercept of â 0.5, which are suppressed with increasing rapidity interval , Any significant contribution of an exchange not suppressed at large (reached at LHC energies) is disfavoured.publishedVersio
Centrality dependence of the pseudorapidity density distribution for charged particles in Pb-Pb collisions at = 2.76 TeV
We present the first wide-range measurement of the charged-particle pseudorapidity density distribution, for different centralities (the 0-5%, 5-10%, 10-20%, and 20-30% most central events) in Pb-Pb collisions at = 2.76 TeV at the LHC. The measurement is performed using the full coverage of the ALICE detectors, -5.0 < < 5.5, and employing a special analysis technique based on collisions arising from LHC ' satellite' bunches. We present the pseudorapidity density as a function of the number of participating nucleons as well as an extrapolation to the total number of produced charged particles = 17165 +/- 772 for the 0-5% most central collisions). From the measured d\)._{ch}\).d\).eta\) distribution we derive the rapidity density distribution, d\)._{ch}\).dy, under simple assumptions. The rapidity density distribution is found to be significantly wider than the predictions of the Landau model, which reproduce data well at RHIC energies. We assess the validity of longitudinal scaling by comparing to lower energy results from RHIC. Finally the mechanisms of the underlying particle production are discussed based on a comparison with various theoretical models.publishedVersio
Underlying Event measurements in pp collisions at and 7 TeV with the ALICE experiment at the LHC
J/Ψ production and nuclear effects in p-Pb collisions at âsNN=5.02 TeV
Inclusive J/Ď production has been studied with the ALICE detector in p-Pb collisions at the nucleonânucleon center of mass energy âsNN = 5.02TeV at the CERN LHC. The measurement is performed in the center of mass rapidity domains 2.03 < ycms < 3.53 and â4.46 < ycms < â2.96, down to zero transverse momentum, studying the Îź+Îźâ decay mode. In this paper, the J/Ď production cross section and the nuclear modification factor RpPb for the rapidities under study are presented. While at forward rapidity, corresponding to the proton direction, a suppression of the J/Ď yield with respect to binary-scaled pp collisions is observed, in the backward region no suppression is present. The ratio of the forward and backward yields is also measured differentially in rapidity and transverse momentum. Theoretical predictions based on nuclear shadowing, as well as on models including, in addition, a contribution from partonic energy loss, are in fair agreement with the experimental results
Measurement of charged jet suppression in Pb-Pb collisions at âsNN = 2.76 TeV
A measurement of the transverse momentum spectra of jets in PbâPb collisions at âsNN = 2.76TeV is reported. Jets are reconstructed from charged particles using the anti-kT jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudorapidity |n| < 0.5. The transverse momentum pT of charged particles is measured down to 0.15GeV/c which gives access to the low pT fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R = 0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high pT leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R = 0.2 and R = 0.3 is found to be similar in PbâPb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R < 0.3
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