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

Abstract

We report the measured transverse momentum (p T) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy √ s NN = 5.02 TeV in the kinematic range of 0.15 < p T < 50 GeV/c and |η| < 0.8. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at √ s NN = 2.76 TeV, as well as in p-Pb collisions at √ s NN = 5.02 TeV, 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 p T spectra are suppressed by more than a factor of 7 around 6-7 GeV/c with a significant reduction in suppression towards higher momenta up to 30 GeV/c. The nuclear modification factor R pPb , 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 √ s NN = 5.02 TeV compared to 2.76 TeV, the nuclear modification factors show no significant 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.

Intriguing similarities of high- pT particle production between pp and A−A collisions

“…give us the probability that partons normally produced in the multiple binary pp collision get degraded in the hot and dense system created in the heavy-ion collision, resulting in a suppression of the ratio (R AA < 1). For instance, in the 0-5% Pb-Pb collisions at the LHC energies the suppression is about 7-8 for p T of around 6-7 GeV/c [24,14]. For higher p T , R AA exhibits a continuous rise and approaches unity [24].…”

“…in Pb-Pb collisions at √ s NN = 2.76 TeV and 5.02 TeV [14] and Xe-Xe collisions at √ s NN = 5.44 TeV [15] scale with dN ch /dη . This suggests that multiplicity (or energy density) may play an important role to describe the high-p T particle production in heavy-nuclei collisions.…”

New results on hard probes in heavy-ion collisions with ALICE

“…In addition, it is already visible in the comparison of high-p T identified particles in pp collisions at large √ s that leading Figure 1: Left: Nuclear modification factor for charged particles in three p T ranges for different colliding systems and energies [2] Right: Nuclear modification in p-Pb as well as in peripheral and central Pb-Pb [3,4] order and next-to-leading order (LO and NLO) Monte Carlo Models with parton showering provide a better description of the data than NLO perturbative QCD calculation with one dimensional fragmentation functions [6] extracted at a lower collisions energy. Figure 2: Left: Nuclear modification factor for charged particles over the full centrality range [7] Right: Comparison of the apparent suppression to the expectation of the HG-PYTHIA model [8] The comparison of the charged particle production in Pb-Pb at √ s NN = 5.02 TeV [3] to recent ALICE results obtained with lighter Xe-ions at similar energies ( √ s NN = 5.44 TeV, [2]) is important to disentangle effects of the collision geometry. In particular this can be done by studying the nuclear modification factor in three distinct p T ranges as shown in Fig.…”

“…In the measurement of single charged particles at high p T the ALICE collaboration recently published the results for Pb-Pb collisions at the highest collision energy so far, √ s NN = 5.02 TeV, with significantly improved systematic uncertainties and a p T reach up to 50 GeV [3]. The nuclear modification factor R AA shows only little variation from the measurements at √ s NN = 2.76 TeV, which can be understood when considering that the expected larger parton energy loss is compensated by a harder parton spectrum by which the energy loss is filtered.…”

Charged-particle production in Pb+Pb and Xe+Xe collisions measured with the ATLAS detector

“…Yields of charged hadrons are suppressed in Pb+Pb collisions relative to pp collisions [1,2,3,4,5,6]. The Xe+Xe collisions at similar center-of-mass energy offer an opportunity to study properties of the quark-gluon plasma in the system with different geometry [7,8].…”