Inclusive J/ψ production at forward and backward rapidity in p-Pb collisions at √sNN=8.16 TeV

Inclusive J/psi production is studied in p-Pb interactions at a centre-of-mass energy per nucleon-nucleon collision root s(NN) = 8.16 TeV, using the ALICE detector at the CERN LHC. The J/psi meson is reconstructed, via its decay to a muon pair, in the centreof-mass rapidity intervals 2.03 < Y-cms < 3.53 and -4.46 < Y-cms < -2.96, where positive and negative Y(cms )refer to the p-going and Pb-going direction, respectively. The transverse momentum coverage is PT < 20 GeV/c. In this paper, Y-cms- and P-T-differential cross sections for inclusive J/psi production are presented, and the corresponding nuclear modification factors R-pPb are shown. Forward results show a suppression of the J/psi yield with respect to pp collisions, concentrated in the region P-T less than or similar to 5 GeV/c. At backward rapidity no significant suppression is observed. The results are compared to previous measurements by ALICE in p-Pb collisions at root s(NN) = 5.02 TeV and to theoretical calculations. Finally, the ratios R-FB between forward- and backward-Y-cms R-pPb values are shown and discussed

Transverse momentum spectra and nuclear modification factors of charged particles in pp, p-Pb and Pb-Pb collisions at the LHC

We report the measured transverse momentum (pT) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy p sNN = 5 : 02TeV in the kinematic range of 0 : 15 < pT < 50 GeV/c and jj < 0 : 8. A signi fi cant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at p sNN = 2 : 76TeV, as well as in p-Pb collisions at p sNN = 5 : 02TeV, which is also presented. Spectra from Pb-Pb collisions are presented in nine centrality intervals and are compared to a reference spectrum from pp collisions scaled by the number of binary nucleon-nucleon collisions. For central collisions, the pT spectra are suppressed by more than a factor of 7 around 6-7 GeV/c with a signi fi cant reduction in suppression towards higher momenta up to 30 GeV/c. The nuclear modi fi cation factor RpPb, constructed from the pp and p-Pb spectra measured at the same collision energy, is consistent with unity above 8 GeV/c. While the spectra in both pp and Pb-Pb collisions are substantially harder at p sNN = 5 : 02TeV compared to 2.76TeV, the nuclear modi fi cation factors show no signi fi cant collision energy dependence. The obtained results should provide further constraints on the parton energy loss calculations to determine the transport properties of the hot and dense QCD matter

Direct photon production at low transverse momentum in proton-proton collisions at root s=2.76 and 8 TeV

Measurements of inclusive and direct photon production at midrapidity in pp collisions at root s = 2.76 and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of 0.4 7 GeV/c is at least one sigma above unity and consistent with expectations from next-to-leading order pQCD calculations

Energy dependence of exclusive J/ photoproduction off protons in ultra-peripheral p-Pb collisions at sNN=5.02 TeV mathrmJ/psimathrm J/psi J / ψ photoproduction off protons in ultra-peripheral p–Pb collisions at sqrtsmathrmscriptscriptstyleNN=5.02sqrts_mathrm scriptscriptstyle NN = 5.02 s NN = 5.02 TeV

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of J/ vector mesons off proton targets in ultra-peripheral p-Pb collisions at a centre-of-mass energy per nucleon pair =5.02 TeV. The e+e- and +- decay channels are used to measure the cross section as a function of the rapidity of the J/ in the range -2.5<2.7, corresponding to an energy in the p centre-of-mass in the interval 40<550 GeV. The measurements, which are consistent with a power law dependence of the exclusive J/ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements

Charged-particle pseudorapidity density at mid-rapidity in p–Pb collisions at √sNN = 8.16 TeV

The pseudorapidity density of charged particles, dN(ch)/d eta, in p-Pb collisions has been measured at a centre of-mass energy per nucleon-nucleon pair of root S-NN = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, vertical bar eta vertical bar /2 is 4.73 +/- 0.20, where is the average number of participating nucleons, is 9.5% higher than the corresponding value for p-Pb collisions at root S-NN = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agree within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the dN(ch)/d eta distribution. Saturation-based models reproduce the distributions well for eta > -1.3. The dN(ch)/d eta is also measured for different centrality estimators, based both on the charged particle multiplicity and on the energy deposited in the Zero Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p-Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity

Measurement of dielectron production in central Pb-Pb collisions at sNN=2.76TeV

The first measurement of dielectron (e(+)e(-)) production in central (0-10%) Pb-Pb collisions at root S-NN = 2.76 TeV at the LHC is presented. The dielectron invariant-mass spectrum is compared to the expected contributions from hadron decays in the invariant-mass range 0 < m(ee) < 3.5 GeV/c(2). The ratio of data and the cocktail of hadronic contributions without vacuum rho(0) is measured in the invariant-mass range 0.15 < m(ee) < 0.7 GeV/c(2), where an excess of dielectrons is observed in other experiments, and its value is 1.40 +/- 0.28 (stat.) +/- 0.08 (syst.) +/- 0.27 (cocktail). The dielectron spectrum measured in the invariant mass range 0 < m(ee) < 1 GeV/c(2) is consistent with the predictions from two theoretical model calculations that include thermal dielectron production from both partonic and hadronic phases with in-medium broadened rho(0) meson. The fraction of direct virtual photons over inclusive virtual photons is extracted for dielectron pairs with invariant mass 0.1 < m(ee) < 0.3 GeV/c(2) and in the transverse-momentum intervals 1 <( )p(T,ee) < 2 GeV/c and 2 < p(T,ee) < 4 GeV/c. The measured fraction of virtual direct photons is consistent with the measurement of real direct photons by ALICE and with the expectations from previous dielectron measurements at RHIC within the experimental uncertainties

Jet fragmentation transverse momentum measurements from di-hadron correlations in √s=7 TeV pp and √sNN=5.02 TeV p–Pb collisions

The transverse structure of jets was studied via jet fragmentation transverse momentum (j(T)) distributions, obtained using two-particle correlations in proton-proton and proton-lead collisions, measured with the ALICE experiment at the LHC. The highest transverse momentum particle in each event is used as the trigger particle and the region 3 < p(Tt) < 15GeV/c is explored in this study. The measured distributions show a clear narrow Gaussian component and a wide non-Gaussian one. Based on Pythia simulations, the narrow component can be related to non-perturbative hadronization and the wide component to quantum chromodynamical splitting. The width of the narrow component shows a weak dependence on the transverse momentum of the trigger particle, in agreement with the expectation of universality of the hadronization process. On the other hand, the width of the wide component shows a rising trend suggesting increased branching for higher transverse momentum. The results obtained in pp collisions at root s TeV and in p-Pb collisions at root sNN=5.02 TeV are compatible within uncertainties and hence no significant cold nuclear matter effects are observed. The results are compared to previous measurements from CCOR and PHENIX as well as to Pythia 8 and Herwig 7 simulations

Multiplicity dependence of light-flavor hadron production in pp collisions at root s=7 TeV

Comprehensive results on the production of unidentified charged particles, pi(+/-), K-+/-, K-s(0), K*(892)(0), p, (p) over bar, phi(1020), Lambda, (Lambda) over bar, Xi(-) , (Xi) over bar (+), Omega(-), and (Omega) over bar (+) hadrons in proton-proton (pp) collisions at root s = 7 TeV at midrapidity (vertical bar y vertical bar < 0.5) as a function of charged-particle multiplicity density are presented. In order to avoid autocorrelation biases, the actual transverse momentum (p(T)) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particleto-pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at Large Hadron Collider energies. The obtained p(T), distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions

Event-Shape Engineering for the D-meson elliptic flow in mid-central Pb-Pb collisions at √sNN=5.02 TeV

The production yield of prompt D mesons and their elliptic flow coefficient v(2) were measured with the Event-Shape Engineering (ESE) technique applied to mid-central (10-30% and 30-50% centrality classes) Pb-Pb collisions at the centre-of-mass energy per nucleon pair sNN=5.02 TeV, with the ALICE detector at the LHC. The ESE technique allows the classification of events, belonging to the same centrality, according to the azimuthal anisotropy of soft particle production in the collision. The reported measurements give the opportunity to investigate the dynamics of charm quarks in the Quark-Gluon Plasma and provide information on their participation in the collective expansion of the medium. D mesons were reconstructed via their hadronic decays at mid-rapidity, || < 0.8, in the transverse momentum interval 1 < p(T) < 24 GeV/c. The v(2) coefficient is found to be sensitive to the event-shape selection confirming a correlation between the D-meson azimuthal anisotropy and the collective expansion of the bulk matter, while the per-event D-meson yields do not show any significant modification within the current uncertainties

Anisotropic flow in Xe–Xe collisions at sNN=5.44 TeV

The first measurements of anisotropic flow coefficients v(n) for mid-rapidity charged particles in Xe-Xe collisions at root s(NN)=5.44TeV are presented. Comparing these measurements to those from Pb-Pb collisions at root s(NN)=5.02 TeV, v(2) is found to be suppressed for mid-central collisions at the same centrality, and enhanced for central collisions. The values of v(3) are generally larger in Xe-Xe than in Pb-Pb at a given centrality. These observations are consistent with expectations from hydrodynamic predictions. When both v(2) and v(3) are divided by their corresponding eccentricities for a variety of initial state models, they generally scale with transverse density when comparing Xe-Xe and Pb-Pb, with some deviations observed in central Xe-Xe and Pb-Pb collisions. These results assist in placing strong constraints on both the initial state geometry and medium response for relativistic heavy-ion collisions. (c) 2018 European Organization for Nuclear Research. Published by Elsevier B.V