Andrea Rossi, of the University of Padova, gave a seminar in the CERN Main Auditorium on 4 October, reporting on "Recent ALICE measurements on open heavy-flavour production in p-p, p-Pb, and Pb-Pb collisions at the LHC". In this article the author provides a summary of his talk.
In the last year ALICE has done important progresses towards the characterization of charm- and beauty-quark dynamics in the Quark Gluon Plasma formed in Pb-Pb collisions at the LHC. Two physical quantities were investigated: the nuclear modification factor (RAA) and the elliptic flow (v2). RAA is defined as the ratio of the production of a given particle in nucleus-nucleus collisions and the expectation from proton-proton collisions in the absence of nuclear effects. High-energy quarks and gluons lose energy interacting with the medium constituents. This energy loss results in a “redshift” of the transverse-momentum (pT) spectra of the produced particles, causing RAA to be smaller than unity at intermediate/high pT. The elliptic flow is used to quantify the azimuthal anisotropy of the produced particles with respect to the reaction plane defined by the nuclei impact-parameter vector and the beam direction. At low pT v2 provides information on how much heavy-quarks participate to the system collective motion, while at high pT it gives information on the dependence of energy loss from the distance covered by the quarks in the medium.
ALICE has excellent tracking and particle-identification capabilities for reconstructing D mesons as well as electrons from charm- and beauty-hadron decays (heavy-flavour electrons in what follows) at mid-rapidity and muons from charm- and beauty-hadron decays (heavy-flavour muons in what follows) at forward rapidity.
Recently, ALICE released final results for the RAA of non-strange D mesons (JHEP 1603 081, 2016), Ds (JHEP1603, 082, 2016), heavy-flavour electrons (arXiv:1609.07104), beauty electrons (arXiv:1609.03898), and for the v2 of heavy-flavour electrons (arXiv:1606.00321) and muons (PLB 753, 41, 2016 41) in Pb-Pb collisions at a center-of-mass energy √sNN = 2.76 TeV. Moreover, a first preliminary measurement of the RAA of heavy-flavour muons in Pb-Pb collisions at √sNN = 5.02 TeV was performed with data from run-2. The measurements evidence a significant suppression of particles originated by charm and beauty quarks with intermediate/high pT at mid- as well as forward rapidity. These results indicate that charm and beauty quarks lose a significant amount of energy while interacting strongly with the medium constituents. The data are reproduced, within uncertainties, by models that include a dependence of in-medium partonic energy loss from the quark mass and predict a higher RAA for B mesons than D mesons. The more direct confirmation of this hierarchy is provided by the observation that the RAA of D-mesons is smaller than that of J/y from beauty-hadron decays measured by CMS.
A positive v2 coefficient was measured in semi-central collisions for low-pT heavy-flavour electrons and muons and for D mesons. Models that include mechanisms, like collisional energy loss and hadron formation via coalescence, that can transfer to charm quarks the elliptic flow induced during the system expansion of the medium, can reproduce the data within uncertainties.
Together, the RAA and v2 measurements set stringent constraints to theoretical predictions. Models taking into account initial-state effects, heavy-quark interactions via both radiative and collisional processes, possible hadron-formation via coalescence of heavy quarks with light quarks from the medium, and implementing a realistic medium evolution are in better agreement with data over a wider momentum range. This rich fauna of possible effects is one of the nicer features of heavy-flavour physics in heavy-ion collisions. However, it also introduces several degrees of freedom inside theoretical models. A major goal for ALICE in run-2 and run-3 is to perform new and more precise measurements that can set boundaries to the various effects. In particular, coalescence should enhance the ratio of Ds and non-strange D-meson yields. The first measurement done with run-1 data shows an intriguing trend in this direction. Precise measurements of Ds RAA and v2 will allow for a quantification of the effect. Initial-state effects instead can be constrained by measuring particle production in p-Pb collisions. Recently, the production of D mesons was measured down to pT=0 in pp and p-Pb collisions for the first time ever at mid-rapidity (arXiv:1605.07569). The statistical uncertainty is still large but the road is traced for making a precise measurement with the large-size dataset that will be collected with the upcoming p-Pb run.