by Rongrong Ma, Andreas Morsch, Xiaoming Zhang, Daicui Zhou. Published: 19 November 2012

The 8th International Workshop on High pT Physics at LHC was held in Wuhan, China on October 21-24, 2012. In the field of ultra-relativistic heavy-ion physics, high pT partons, produced in the early stage of the collisions, are probes to study the properties of the hot, dense medium formed in the collisions. The workshop is dedicated to the experimental and theoretical results of these probes. In particular, this year’s workshop featured the first p–Pb results, as well as the Pb–Pb results from the LHC experiments, among which the ALICE collaboration has made significant impact.


When traversing the hot and dense strongly-interacting medium, high pT partons are expected to lose energy via induced gluon radiation or elastic scattering. The resulting suppression of high-pT hadrons has been observed via the measurements of the nuclear modification factor RAA, the ratio of the yield measured in nucleus–nucleus collisions to that expected from proton–proton collisions, for both single hadrons and reconstructed jets. With the unique particle identification capability, ALICE reported the suppression of different species, namely protons, kaons and pions, where we saw significant differences at low and intermediate pT (1.5 < pT < 8 GeV/c), but similarity at high pT (pT > 18 GeV/c).

These results impose strong constraints on models in terms of parton in-medium energy loss mechanisms. To quantify the medium properties using RAA measurements, we need to separate out the initial state effects that could influence the high-pT production processes in nuclear collisions. p–Pb collisions provide a tool to perform such study since the hot medium is not expected to be created.

LHC delivered a p–Pb test run in September 2012. In this workshop, the ALICE collaboration reported the first measurement of RpPb, shown as the blue points in the left panel of Fig. 1. Contrary to the large suppression in the central Pb–Pb collisions (red points in the left panel of Fig. 1), the RpPb is consistent with unity above 3 GeV/c, which indicates that the initial state effects are very small.

Figure 1: Left: nuclear modification factor of charged hadrons in Pb–Pb and p–Pb collisions. Right: nuclear modification factor of fully reconstructed jets with radius R = 0.2 in Pb–Pb collisions.

To first order, the high pT charged hadrons are the leading fragments of the high pT parent partons. Jets can be used to gain more information about the medium properties by looking at the sub-leading fragments. ALICE reported the first measurement of the jet RAA with cone radius R = 0.2 in the relatively low pT region (30 < pT < 120 GeV/c) with very low pT cutoff on the jet constituents. A rising trend is observed as a function of jet pT, as shown in the right panel of Fig. 1. And the magnitude of the jet RAA is compatible with the charged hadron RAA within the measured kinematic region. The small value of jet RAA indicates significant energy loss of the parent parton in the medium. If the main energy loss mechanism is gluon radiation, the radiated gluons are out of the jet cone.

Figure 2: RAA as a function of pT in central collisions (left) and as a function of centrality (right) for different particle species in Pb–Pb collisions at ?sNN=2.76 TeV.

Heavy quarks offer a unique access to gain insights into the transport properties of the medium. Due to the colour-charge and mass dependence of parton energy loss, one expects the following RAA ordering: RAA(light hadrons) < RAA(D) < RAA(B). This hierarchy can be tested by comparing the RAA of heavy-flavour hadrons and that of light hadrons. The measurement of azimuthal anisotropy of heavy-flavour production in non-central collisions offers the additional medium information on the medium transport properties. In the low pT region, the azimuthal anisotropy is sensitive to the degree of thermalization of heavy quarks within the partonic medium; while in the high pT region, this anisotropy brings information on the path length dependence of heavy quark in-medium energy loss. The RAA of D mesons, heavy-flavour decay electrons (both measured at mid-rapidity) and heavy-flavour decay muons (measured at forward rapidity) with ALICE, as presented in Fig. 2, shows a similar suppression for all particle species. This implies a large in-medium energy loss of heavy quarks and a different behaviour of light hadrons and heavy-flavour particles can not be concluded at present. The non-zero v2, the second order coefficient of Fourier decomposition of momentum azimuthal distribution with respect to the reaction plane, of D mesons and heavy-flavour decay electrons presented in Fig. 3 confirms that strongly interacting of heavy quarks, alike lighter partons, are sensitive to the initial geometrical anisotropy of the medium. A reasonable description of RAA of D mesons and heavy flavour decay leptons is achieved in theoretical calculations when taking into account the in-medium parton energy loss. However a simultaneous description of RAA and v2 of heavy flavour production is still a challenge for models.

Figure 3: v2 of D mesons (left) and heavy-flavour decay electrons (right) as a function of pT in Pb–Pb collisions at ?sNN = 2.76 TeV. Results are compared to various theoretical calculations.

The RAA of J/?, (left plot of Fig. 4), shows an enhancement of J/? production in low pT region and it is distinctly different from that at RHIC. The RAA and non-zero v2 (right plot of Fig. 4) of J/? can be reproduced by both the statistical hadronization model and transport model. Discriminating these two models description will provide an answer to fundamental questions related to the fate of hadrons in the hot medium.

Figure 4: RAA (left) and v2 (right) of J/? as a function of pT in Pb–Pb collisions at ?s= 2.76 TeV. Results are compared to model predictions.

The precision measurements of open heavy-flavour and quarkonia production will be improved with the upcoming p–Pb data early next year and the planned ALICE upgrades.

Read also: 8th International Workshop on High pT Physics .