Production of light flavour hadrons at intermediate and high pT in pp, p-Pb and Pb-Pb collisions measured with ALICE

Light flavour transverse momentum spectra at intermediate and high pT provide an important baseline for the measurement of perturbative QCD processes in pp, for the evaluation of initial state effects in p-Pb, and for investigating the suppression from parton energy loss in Pb-Pb collisions. The measurement of the nuclear modification factor RpPb for inclusive charged particles is extended up to 50 GeV/c in pT compared to our previous measurement and remains consistent with unity up the largest momenta. Results on RpPb of charged pions, kaons and protons that cover up to 14 GeV/c in pT are presented and compared to RpPb of inclusive charged particles. On the production of charged pions, kaons, and protons up to pT = 20 GeV/c in Pb-Pb collisions final results for RAA are presented.Comment: Proceedings of the XXIV International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2014), Darmstadt, May 19-24 201

Transverse momentum distributions of primary charged particles in pp, p–Pb and Pb–Pb collisions measured with ALICE at the LHC

According to the standard model of Big Bang cosmology the earliest universe contained an extremely hot and dense medium that subsequently expanded and cooled. The evolution of the early universe happened through a phase with of deconfined quarks and gluons: the quark-gluon plasma (QGP). This phase ended about ten microseconds after the Big Bang when the temperature dropped below the critical temperature Tc and quarks and gluons became confined into hadrons. The existence of a QGP phase at high temperature is also predicted by Quantum Chromodynamics (QCD), the fundamental field theory describing the strong interaction of quarks and gluons. In high-energy collisions of heavy nuclei a QGP can be created and studied experimentally. The energy loss of high energy partons in the hot QCD medium results in a suppression of particle production at large transverse momenta. Measurements of the parton energy loss can be used to characterize the QGP properties. The Large Hadron Collider (LHC) at CERN provides hadron-hadron (pp), nucleus-nucleus (Pb–Pb) and proton-nucleus (p–Pb) collisions at the highest energies reached in an accelerator so far. The ALICE experiment at the LHC is dedicated to the investigation of the QGP in heavy-ion collisions and complemented by the study of pp and p–Pb collisions. In p–Pb collisions the QGP is not expected to be formed and the nuclear initial state and cold nuclear matter effects can be studied. Measurements in pp collisions serve as a reference for p–Pb and Pb–Pb collisions. They also allow to tune phenomenological models and are a test of theoretical predictions from perturbative QCD. The data analyzed for this thesis were collected in pp, p–Pb and Pb–Pb collisions by ALICE in 2010-2013 with different minimum bias triggers using the two VZERO scintillators and the Silicon Pixel Detector (SPD). Charged tracks are reconstructed using combined information from the Inner Tracking System (ITS) and the Time Projection Chamber (TPC), the main tracking detectors of ALICE. Events used for the analysis are required to have a reconstructed primary vertex. The selection of tracks is optimized to provide high purity of primary particles and optimal pT resolution. Measured pT distributions are corrected for tracking efficiency and acceptance effects as well as contamination from secondary particles originating from particle decays or particle production in the detector material. Both corrections are evaluated from full detector simulations using GEANT3 for particle transport through the detector with events generated from the Monte Carlo event generators PYTHIA (pp), DPMJET (p–Pb) and HIJING (Pb–Pb). Corrections for the finite momentum resolution of the detector are derived from the measured pT resolution and the pT spectra in an unfolding procedure. Spectra are normalized to inelastic yields and cross sections (pp), respectively non-single-diffractive yields (p–Pb), taking into account the efficiencies of the trigger and the vertex reconstruction. Possible sources of systematic uncertainties are identified and their contribution is estimated. Transverse momentum (pT) distributions of primary charged particles have been measured at mid rapidity |eta| < 0.8 in inelastic pp collisions at center-of-mass energies of sqrt(s) = 0.9 TeV (for 0.15 < pT < 20 GeV/c), sqrt(s) = 2.76 TeV (for 0.15 < pT < 32 GeV/c) and sqrt(s) = 7 TeV (for 0.15 < pT < 50 GeV/c). In non-single-diffractive p–Pb collisions at the nucleon-nucleon center-of-mass energy sqrt(sNN) = 5.02 TeV pT distributions have been obtained for three different pseudorapidity intervals (|etaCMS| < 0.3, 0.3 <etaCMS < 0.8 and 0.8 < etaCMS < 1.3). The analysis of the first p–Pb collisions at the LHC in 2012 covered 0.5 < pT < 20 GeV/c, the kinematic range was extended to 0.15 < pT < 50 GeV/c with the data collected during the long p–Pb run in 2013. Pb–Pb collisions were analyzed in nine intervals of centrality covering 80% of the total hadronic cross section. Transverse momentum spectra were measured at mid rapidity |eta| < 0.8 and cover 0.15 < pT < 50 GeV/c for all centrality classes. The nuclear modification factor RAA (RpPb) is calculated as the pT differential yield measured in Pb–Pb (p–Pb) collisions divided by the cross section in pp collisions, scaled by the nuclear overlap function calculated in a Monte Carlo Glauber approach. Cold and hot nuclear matter effects can be quantified with the nuclear modification factors as the deviation from binary collision scaling. To obtain the nuclear modification factor RpPb a pp reference is required at sqrt(s) = 5.02 TeV, where no measurement is available. At large pT, the pp reference is constructed from measured spectra at sqrt(s) = 7 TeV multiplied by scaling factors from NLO pQCD calculations. At low pT, where perturbative calculations are not reliable, the pp reference is interpolated between the measurements at sqrt(s) = 2.76 TeV and sqrt(s) = 7 TeV, assuming a power law behavior of the cross section as function of sqrt(s). As pp reference for RAA the measurement at sqrt(s) = 2.76 TeV is used, with a parameterization of the data for 5 < pT < 32 GeV/c and an extrapolation to 32 < pT < 50 GeV/c. Fully corrected transverse momentum spectra are also obtained as a function of the measured particle multiplicity. The average transverse momentum as a function of the true multiplicity is constructed using a reweighting procedure with effective corrections obtained from Monte Carlo simulations which account for limited acceptance, tracking efficiency and contamination. The multiplicity dependence of the average transverse momentum is compared to predictions from Monte Carlo event generators and (in p–Pb and Pb–Pb collisions) to expectations from binary collision scaling. In pp collisions the expected power-law behavior at large transverse momentum is observed, but the measured cross section does not agree with next-to-leading order (NLO) perturbative QCD calculations. At small transverse momenta the shape of the spectrum is approximately exponential. The data are compared to the MC event generators PHOJET and PYTHIA; none of them agrees with the data over the full pT range. In p–Pb collisions pT spectra are softer at forward pseudorapidity (in the Pb fragmentation region). The nuclear modification factor RpPb reveals that at low pT approximate participant scaling is in place. In the intermediate pT region, a hint of Cronin enhancement is visible, but at the edge of the experimental uncertainties. At high pT no deviation from binary collision scaling is observed and RpPb is consistent with unity. Theoretical predictions from a Color Glass Condensate initial state model are in agreement with the measured RpPb, as well as as calculations based on NLO pQCD with EPS09s nuclear parton distribution functions. In Pb–Pb collisions particle production is suppressed compared to the expectation from binary collision scaling. The suppression is largest for central collisions but remains substantial also for peripheral collisions. For central (0-5%) collisions the nuclear modification factor is about 0.4 at the largest measured momenta. The observations are in quantitative agreement only with part of theoretical models that incorporate medium-induced parton energy loss. In the future, improvements in the simulations and analysis procedure are expected to reduce the systematic uncertainties to about half the current values. With the inclusion of Pb–Pb data recorded in 2011 and the use of other triggers than minimum bias the pT reach could be extended up to 100 GeV/c

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions at root(NN)-N-S=2.76 TeV

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Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Elliptic flow of muons from heavy-flavour hadron decays at forward rapidity in Pb-Pb collisions at root s(NN)=2.76TeV

The elliptic flow, v(2), of muons from heavy-flavour hadron decays at forward rapidity (2.5 <y <4) is measured in Pb-Pb collisions at root s(NN)= 2.76TeVwith the ALICE detector at the LHC. The scalar product, two- and four-particle Q cumulants and Lee-Yang zeros methods are used. The dependence of the v(2) of muons from heavy-flavour hadron decays on the collision centrality, in the range 0-40%, and on transverse momentum, p(T), is studied in the interval 3 <p(T)<10 GeV/c. A positive v(2) is observed with the scalar product and two-particle Q cumulants in semi-central collisions (10-20% and 20-40% centrality classes) for the p(T) interval from 3 to about 5GeV/c with a significance larger than 3 sigma, based on the combination of statistical and systematic uncertainties. The v(2) magnitude tends to decrease towards more central collisions and with increasing pT. It becomes compatible with zero in the interval 6 <p(T)<10 GeV/c. The results are compared to models describing the interaction of heavy quarks and open heavy-flavour hadrons with the high-density medium formed in high-energy heavy-ion collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe

Pseudorapidity and transverse-momentum distributions of charged particles in proton-proton collisions at root s=13 TeV

The pseudorapidity (eta) and transverse-momentum (p(T)) distributions of charged particles produced in proton-proton collisions are measured at the centre-of-mass energy root s = 13 TeV. The pseudorapidity distribution in vertical bar eta vertical bar <1.8 is reported for inelastic events and for events with at least one charged particle in vertical bar eta vertical bar <1. The pseudorapidity density of charged particles produced in the pseudorapidity region vertical bar eta vertical bar <0.5 is 5.31 +/- 0.18 and 6.46 +/- 0.19 for the two event classes, respectively. The transverse-momentum distribution of charged particles is measured in the range 0.15 <p(T) <20 GeV/c and vertical bar eta vertical bar <0.8 for events with at least one charged particle in vertical bar eta vertical bar <1. The evolution of the transverse momentum spectra of charged particles is also investigated as a function of event multiplicity. The results are compared with calculations from PYTHIA and EPOS Monte Carlo generators. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe