by Andrea Dainese and Francesco Prino. Published: 06 August 2012

When heavy nuclei collide at high energies, a high-density colour deconfined state of strongly interacting matter is expected to form. According to lattice QCD calculations, the confinement of coloured quarks and gluons into colourless hadrons vanishes under the conditions of high energy-density and temperature that are reached in these collisions and a phase transition to a quark–gluon plasma (QGP) occurs.

The LHC, operating with heavy ions, is nowadays the frontier machine for exploring the QGP experimentally, but such studies began 25 years ago with fixed-target experiments at the Alternating Gradient Synchrotron at Brookhaven and the Super Proton Synchrotron at CERN. The field entered the collider era in 2000 with Brookhaven’s Relativistic Heavy-Ion Collider (RHIC). Experiments there showed that initial hard-partonic collisions produce energetic quarks and gluons that interact with the hot and dense QGP, probing its properties and, more generally, those of the strong interaction in an extended many-body system. The abundant production of these “hard probes” constitutes one of the leading opportunities that have opened up at the LHC – where collisions of heavy ions have nearly a 14-fold increase in centre-of-mass energy with respect to RHIC – and their extensive study is a leading feature of the heavy-ion programmes of the ALICE, ATLAS and CMS experiments.

High-momentum partons are created in hard-scattering processes that occur in the early stage of the nuclear collision. They subsequently traverse the hot QGP, losing energy as they interact with its constituents. This energy loss is expected to occur via inelastic processes (gluon radiation induced in the medium, or radiative energy loss, analogous to bremsstrahlung in QED) and via elastic processes (collisional energy loss).

"The abundant production of these “hard probes” constitutes one of the leading opportunities that have opened up at the LHC – where collisions of heavy ions have nearly a 14-fold increase in centre-of-mass energy with respect to RHIC"

The massive c and b quarks (mc ~ 1.5 GeV/c2 , mb ~ 5 GeV/c2 ) are useful probes of these energy-loss mechanisms. In QCD, quarks have a lower colour-coupling strength than gluons, thus the energy loss should be smaller for quarks than for gluons. At LHC energies, hadrons containing light flavours originate mainly from gluons.

Therefore, charmed mesons provide an experimental tag for a low colour-charge, quark parent. In addition, the “dead-cone effect” should reduce small-angle gluon radiation for heavy quarks that have moderate energy-over-mass values, i.e. for c and b quarks with momenta up to about 10 GeV/c .

The nuclear modification factor, RAA , is one of the observables that are sensitive to the interaction of hard partons with the medium. This quantity is defined as the ratio of particle production measured in nucleus–nucleus (AA) interactions to that expected on the basis of the proton–proton (pp) spectrum, scaled by the average number of binary nucleon–nucleon collisions occurring in the collisions of the nuclei.

Loss of energy in the medium leads to a suppression of hadrons at moderate-to-high transverse momentum (pt > 2 GeV/c ), so RAA < 1. In the range pt < 10 GeV/c , where the masses of the heavy c and b quarks are not negligible with respect to their momenta, the properties of parton energy-loss described above mean that an increase in RAA (i.e. a smaller suppression) is expected when going from the mostly gluon-originated light-flavour hadrons (such as pions) to D and B mesons with c quarks and b quarks, respectively: RAA (?) < RAA (D) < RAA (B).

In the ALICE experiment at the LHC, the charmed mesons D0, D+ and D*+ are reconstructed in the central barrel through their decays to charged hadrons, namely D0 ? K–?+, D+ ? K–?+?+ and D*+ ? D0?+ , followed by D0?K–?+ . The signal is extracted from the invariant-mass distributions of the combinations of charged tracks reconstructed in the inner tracking system (ITS) and the time-projection chamber (TPC). The high-multiplicity environment of lead–lead (PbPb) interactions, where about 1600 primary charged particles per unit of rapidity are produced for head-on collisions, is particularly challenging for the exclusive reconstruction of D-meson decays because of the large combinatorial background.

Figure 1. Average RAA of D mesons in the 0–20% centrality class compared with the nuclear-modification factors of charged hadrons and nonprompt J/? from B decays in the same centrality class. The boxes at RAA=1 represent the relative normalization uncertainties."

Figure 1 shows the average RAA of the three D-meson species as a function of the transverse momentum for the most central collisions (ALICE collaboration 2012b). To study the expected dependences of the energy loss on colour charge and parton mass, the nuclear modification factor is compared with those of charged hadrons measured by ALICE and those of nonprompt J/? mesons (from B decays) measured by the CMS experiment for pt > 6.5 GeV/c (CMS collaboration 2012). The charged-hadron nuclear-modification factor is dominated by light flavours and coincides with that of charged pions above pt ? 5 GeV/c .

This comparison between the values of RAA for D mesons and charged hadrons shows that the average nuclear modification factor for the D mesons is close to that of charged hadrons. However, considering that the systematic uncertainties of D mesons are not fully correlated with pt, there is an indication for RAA (D) > RAA (charged). The suppression of J/? from B decays is clearly weaker than that of charged hadrons, while the comparison with D mesons is not conclusive and requires more differential and precise measurements of the transverse momentum dependence.

Apart from final-state effects, which are related to the formation of a hot and deconfined medium, initial-state effects are also expected to influence the nuclear-modification factor, because it is nuclei rather than nucleons that collide. In particular, the modification of the parton distribution functions (PDFs) of the nucleons in the nuclei affects the initial hard-scattering probability and, thus, the yields of energetic partons, including heavy quarks. In the kinematic range relevant for charm production at LHC energies, the main effect is nuclear shadowing, which induces a reduction in the yields of D mesons at low momentum.

Perturbative QCD calculations supplemented with a phenomenological parameterization of the nuclear modification of the PDFs indicates that the shadowing-induced effect on RAA is limited to ± 15% for pt > 6 GeV/c . This suggests that the strong suppression observed in the high-pt data is a final-state effect, arising predominantly from energy loss of c quarks in the medium.

"The precision of the measurements will be improved in the future, using the large sample of PbPb collisions recorded in 2011. In addition, proton–lead collisions will provide insight into possible initial-state effects, which may play an important role, mainly in the low-momentum region"

In conclusion, the first ALICE results on the nuclear-modification factor RAA for charm hadrons in PbPb collisions at a centre-of-mass energy ?sNN = 2.76 TeV indicate strong in-medium energy loss for charm quarks. There is a possible indication, which is not fully significant with the current level of experimental uncertainties, that RAA (D) > RAA (charged).

The precision of the measurements will be improved in the future. First preliminary results from the large sample of PbPb collisions recorded in 2011 are going to be presented in Quark Matter 2012 Conference. In addition, the results from the analysis of the first LHC run with proton–lead collisions, expected in January 2013, will provide insight into possible initial-state effects, which may play an important role, mainly in the low-momentum region.

See also: The Quark Matter 2012 conference webpage