First measurement of jet mass in Pb–Pb and p–Pb collisions at the LHC

This letter presents the first measurement of jet mass in Pb–Pb and p–Pb collisions at √sNN = 2.76 TeV and √sNN = 5.02 TeV, respectively. Both the jet energy and the jet mass are expected to be sensitive to jet quenching in the hot Quantum Chromodynamics (QCD) matter created in nuclear collisions at collider energies. Jets are reconstructed from charged particles using the anti-kT jet algorithm and resolution parameter R = 0.4. The jets are measured in the pseudorapidity range |ηjet| < 0.5 and in three intervals of transverse momentum between 60 GeV/c and 120 GeV/c. The measurement of the jet mass in central Pb–Pb collisions is compared to the jet mass as measured in p–Pb reference collisions, to vacuum event generators, and to models including jet quenching. It is observed that the jet mass in central Pb–Pb collisions is consistent within uncertainties with p–Pb reference measurements. Furthermore, the measured jet mass in Pb–Pb collisions is not reproduced by the quenching models considered in this letter and is found to be consistent with PYTHIA expectations within systematic uncertaintie

J/ψ suppression at forward rapidity in Pb–Pb collisions at √sNN = 5.02 TeV

The inclusive J/ ψ production has been studied in Pb–Pb and pp collisions at the centre-of-mass energy per nucleon pair sNN=5.02 TeV , using the ALICE detector at the CERN LHC. The J/ ψ meson is reconstructed, in the centre-of-mass rapidity interval 2.5 < y < 4 and in the transverse-momentum range pT < 12 GeV/c , via its decay to a muon pair. In this Letter, we present results on the inclusive J/ ψ cross section in pp collisions at s=5.02 TeV and on the nuclear modification factor RAA . The latter is presented as a function of the centrality of the collision and, for central collisions, as a function of the transverse momentum pT of the J/ ψ . The measured RAA values indicate a suppression of the J/ ψ in nuclear collisions and are then compared to our previous results obtained in Pb–Pb collisions at sNN=2.76 TeV . The ratio of the RAA values at the two energies is also computed and compared to calculations of statistical and dynamical models. The numerical value of the ratio for central events (0–10% centrality) is 1.17±0.04(stat)±0.20(syst) . In central events, as a function of pT , a slight increase of RAA with collision energy is visible in the region 2 < pT < 6 GeV/c . Theoretical calculations qualitatively describe the measurements, within uncertainties

Event-by-event mean pT fluctuations in pp and Pb–Pb collisions at the LHC

Event-by-event fluctuations of the mean transverse momentum of charged particles produced in pp collisions at √s = 0.9, 2.76 and 7 TeV, and Pb–Pb collisions at √sNN = 2.76 TeV are studied as a function of the chargedparticle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations indicative of correlated particle emission are observed in all systems. The results in pp collisions show little dependence on collision energy. The Monte Carlo event generators PYTHIA and PHOJET are in qualitative agreement with the data. Peripheral Pb–Pb data exhibit a similar multiplicity dependence as that observed in pp. In central Pb–Pb, the results deviate from this trend, featuring a significant reduction of the fluctuation strength. The results in Pb– Pb are in qualitative agreement with previous measurements in Au–Au at lower collision energies and with expectations from models that incorporate collective phenomena

Charged jet cross sections and properties in proton-proton collisions at √s =7 TeV

The differential charged jet cross sections, jet fragmentation distributions, and jet shapes are measured in minimum bias proton-proton collisions at center-of-mass energy √s=7 TeV using the ALICE detector at the LHC. Jets are reconstructed from charged particle momenta in the midrapidity region using the sequential recombination kT and anti-kT as well as the SISCone jet finding algorithms with several resolution parameters in the range R ¼ 0.2–0.6. Differential jet production cross sections measured with the three jet finders are in agreement in the transverse momentum (pT) interval 20 < pjet;ch T < 100 GeV=c. They are also consistent with prior measurements carried out at the LHC by the ATLAS Collaboration. The jet charged particle multiplicity rises monotonically with increasing jet pT, in qualitative agreement with prior observations at lower energies. The transverse profiles of leading jets are investigated using radial momentum density distributions as well as distributions of the average radius containing 80% (hR80i) of the reconstructed jet pT. The fragmentation of leading jets with R ¼ 0.4 using scaled pT spectra of the jet constituents is studied. The measurements are compared to model calculations from event generators (PYTHIA, PHOJET, HERWIG). The measured radial density distributions and hR80i distributions are well described by the PYTHIA model (tune Perugia-2011). The fragmentation distributions are better described by HERWIG

Centrality dependence of particle production in p–Pb collisions at √sNN= 5.02 TeV

We report measurements of the primary charged particle pseudorapidity density and transverse momentum distributions in p–Pb collisions at √sNN = 5.02 TeV, and investigate their correlation with experimental observables sensitive to the centrality of the collision. Centrality classes are defined using different event activity estimators, i.e. charged particle multiplicities measured in three different pseudorapidity regions as well as the energy measured at beam rapidity (zero-degree). The procedures to determine the centrality, quantified by the number of participants (Npart), or the number of nucleon-nucleon binary collisions (Ncoll), are described. We show that, in contrast to Pb–Pb collisions, in p–Pb collisions large multiplicity fluctuations together with the small range of participants available, generate a dynamical bias in centrality classes based on particle multiplicity. We propose to use the zero-degree energy, which we expect not to introduce a dynamical bias, as an alternative event-centrality estimator. Based on zero-degree energy centrality classes, the Npart dependence of particle production is studied. Under the assumption that the multiplicity measured in the Pb-going rapidity region scales with the number of Pb-participants, an approximate independence of the multiplicity per participating nucleon measured at midrapitity of the number of participating nucleons is observed. Furthermore, at high-pT the p–Pb spectra are found to be consistent with the pp spectra scaled by Ncoll for all centrality classes. Our results represent valuable input for the study of the event activity dependence of hard probes in p–Pb collision and, hence, help to establish baselines for the interpretation of the Pb–Pb data

Suppression of Υ(1S)at forward rapidity in Pb–Pb collisions at √sNN=2.76TeV

We report on the measurement of the inclusive $Upsilon$(1S) production in Pb-Pb collisions at $sqrt{s_{rm NN}}=2.76$ TeV carried out at forward rapidity ($2.

Multiplicity dependence of charged pion, kaon, and (anti)proton production at large transverse momentum in p–Pb collisions at √sNN = 5.02 TeV

The production of charged pions, kaons and (anti)protons has been measured at mid-rapidity (−0.5 10 GeV/c), the particle ratios are consistent with those reported for pp and Pb–Pb collisions at the LHC energies. At intermediate pT the (anti)proton RpPb shows a Cronin-like enhancement, while pions and kaons show little or no nuclear modification. At high pT the charged pion, kaon and (anti)proton RpPb are consistent with unity within statistical and systematic uncertainties