Heavy-ion jets go with the flow

The studies of central heavy-ion collisions at the LHC by the ALICE, ATLAS and CMS experiments show that partons traversing the produced hot and dense medium lose a significant fraction of their energy. At the same time, the structure of the jet from the quenched remnant parton is essentially unmodified. The radiated energy reappears mainly at low and intermediate transverse momentum, p_T, and at large angles with respect to the centre of the jet cone. The ALICE collaboration has studied this p_T region in PbPb collisions at a centre-of-mass energy √s_NN = 2.76 TeV by using two-particle angular correlations, with some interesting results.

In the analysis, the associated particles are counted as a function of their difference in azimuth (Δφ) and pseudorapidity (Δη) with respect to a trigger particle in bins of trigger transverse momentum, p_T,trig, and associated transverse momentum, p_T,assoc. With the aim of studying potential modifications of the near-side peak, correlations independent of Δη are subtracted by an η-gap method: the correlation found in 1 < |Δη| < 1.6 (as a function of Δφ) is subtracted from the region in |Δη| < 1. Figure 1 shows an example in one p_T bin: only the near-side peak remains, while by construction the away-side (not shown) is flat.

ALICE studies the shape of the near-side peak by extracting both its rms value (which is a standard deviation, σ, for a distribution centred at zero) in the Δη and Δφ directions and the excess kurtosis (a statistical measure of the “peakedness” of a distribution). The near-side peak shows an interesting evolution towards central collisions: it becomes eccentric.

Figure 2 presents the rms as a function of centrality in PbPb collisions as well as the one for pp collisions (shown at a centrality of 100). Towards central collisions the σ in Δη (lines) increases significantly, while the σ in Δφ (data points) remains constant within uncertainties. This is found for all of the p_T bins studied, from 1 < p_T,assoc < 2 GeV/c, 2 < p_T,trig < 3 GeV/c to 2 < p_T,assoc < 3 GeV/c, 4 < p_T,trig < 8 GeV/c (Grosse-Oetringhaus 2012).

The observed behaviour is qualitatively consistent with a picture where longitudinal flow distorts the jet shape in the η-direction (Armesto et al. 2004). The extracted rms and also the kurtosis (not shown here) are quantitatively consistent (within 20%) with Monte Carlo simulations with A MultiPhase Transport Code (AMPT) (Lin et al. 2005). This Monte Carlo correctly reproduces collective effects such as “flow” at the LHC, which stem from parton–parton and hadron–hadron rescattering in the model.

This observation suggests an interplay of the jet with the flowing bulk in central heavy-ion collisions at the LHC. The further study of the low and intermediate p_T region is a promising field for the understanding of jet quenching at the LHC, which in turn is a valuable probe of the fundamental properties of quark–gluon plasma.