Probing quark–gluon plasma with charmed mesons

The ALICE collaboration has released a new measurement of the production of D⁰, D⁺, D^*+ and D_s⁺ mesons, which contain a charm quark, in lead–lead (PbPb) collisions at a centre-of-mass energy per nucleon pair (√s_NN) of 5.02 TeV. These measurements probe the propagation of charm quarks in the quark–gluon plasma (QGP) produced in high-energy heavy-ion collisions. Charm quarks are produced early in the collision and subsequently experience the whole system evolution, losing part of their energy via inelastic (gluon radiation) or elastic (“collisional”) scattering processes. The charm quarks emerge from the collision in D mesons, which are identified by their characteristic decays.

The result is reported in terms of the nuclear modification factor (R_AA), which is the ratio between the measured p_T distribution in heavy-ion and proton–proton (pp) collisions, scaled by the average number of binary nucleon–nucleon collisions in each nuclear collision. The figure shows the average R_AA of non-strange D mesons (D⁰, D⁺, D^*+) and strange (D⁺_s) mesons in central (0–10%) PbPb collisions. For the non-strange mesons, a minimum of R_AA ≈ 0.2 for p_T = 6–10 GeV/c indicates a significant energy loss for charm quarks. The R_AA is compatible with that of charged particles for p_T > 8 GeV/c, while it is larger at lower p_T. The comparison to light-flavour hadrons helps to study the colour-charge and quark-mass dependence of the in-medium parton energy loss.

The R_AA of D_s⁺ mesons is larger than that of non-strange D mesons. Though the experimental uncertainty is still large, such a difference would suggest that charm quarks also form hadrons by recombining with the surrounding light quarks in the QGP. This mechanism differs from the fragmentation process that is thought to be the main hadronisation mechanism in the absence of a medium. The recombination mechanism enhances the yield of particles with strangeness because strange quarks are copiously produced in the QGP.

The R_AA at LHC Run 2 is compatible with that measured at a lower centre-of-mass energy per nucleon pair of 2.76 TeV, but the larger collected data sample at Run 2 made it possible to reduce the uncertainties by a factor of about two. A similar suppression at the two energies is expected by the “Djordjevic model” (figure, right) due to the combination of a stronger suppression in the denser medium and a harder p_T distribution at 5.02 TeV with respect to 2.76 TeV.

The next PbPb run at the end of 2018, and the subsequent upgrade of the ALICE detector, will allow us to improve the measurement. This will shed further light on the energy loss and hadronisation of heavy quarks in the QGP and allow researchers to determine the transport coefficients describing the scattering power of the QGP and the diffusion of charm quarks in the medium.