The ALICE experiment has measured the production of the charmed mesons D0 and D+ in Pb-Pb collisions at the LHC. In central (head-on) collisions, a large suppression with respect to expectations is observed at large transverse momentum pt, indicating that charm quarks undergo a strong energy loss in the hot and dense state of QCD matter formed in these collisions. D meson suppression is measured for the first time directly in central nucleus-nucleus collisions.
Andrea Dainese/ALICENuclear modification factor as a function of transverse momentum for D0, D+, and ?+ mesons in central Pb-Pb collisions at?sNN = 2.76 TeV.
Heavy-flavour particles are regarded as effective probes of the conditions of the highly excited system (medium) formed in nucleus-nucleus collisions: they are expected to be sensitive to its energy density, through the mechanism of in-medium energy loss. The nuclear modification factor RAA - the ratio of the yield measured in nucleus-nucleus to that expected from proton-proton collisions - is well-established as a sensitive observable for the study of the interaction of hard partons with the medium. Due to the QCD nature of parton energy loss, quarks are predicted to lose less energy than gluons (that have a higher colour charge) and, in addition, the ‘dead-cone’ effect and other mechanisms are expected to reduce the energy loss of heavy with respect to light partons. Therefore, one should observe a pattern of gradually decreasing RAA suppression when going from the mostly gluon-originated light-flavour hadrons (e.g. pions) to D and to B mesons: RAA(?) < RAA(D) < RAA(B). The measurement and comparison of these different medium probes provides, therefore, a unique test of the colour-charge and mass dependence of parton energy loss.
Experiments at RHIC measured indirectly the suppression of heavy flavour hadrons in Au-Au collisions at 200 GeV through the RAA of their inclusive decay electrons. Using data from the first Pb-Pb run at the LHC (?sNN = 2.76 TeV), the ALICE Collaboration has measured the production of prompt D mesons via decay vertex reconstruction in the channels D0 ? K-?+ and D+ ? K-?+?+. As shown in the figure, a suppression of a factor 4-5, as large as for charged pions, is observed above 5 GeV/c. At lower momenta, there is an indication of smaller suppression for D than for ? mesons. Higher statistics data, expected from the 2011 Pb-Pb run, will allow to study this region with more precision and address the intriguing mass dependence of QCD energy loss.
This result implies a strong in-medium energy loss for heavy quarks, also suggested by the suppression measured by the ALICE Collaboration for electrons and muons from heavy flavour decays, and by the CMS Collaboration for J/? particles from B meson decays.
Andrea Dainese, & for the ALICE Collaboration (2011). Heavy-flavour production in Pb-Pb collisions at the LHC, measured with the ALICE detector Proceedings of Quark Matter 2011 arXiv: 1106.4042v1