Measurement of jet fragmentation into charged particles in pp and PbPb collisions at sqrt(s[NN]) = 2.76 TeV

TL;DR

The study probes whether the quark-gluon plasma in PbPb collisions at $√s_{NN}=2.76$ TeV alters the high-$p_T$ fragmentation of jets relative to pp collisions. Using CMS data, fragmentation functions are built from charged tracks with $p_T^{track}>4$ GeV within $ΔR<0.3$ of jets reconstructed by anti-$k_T$ with $R=0.3$, and a pp reference is carefully matched to the PbPb spectrum via smearing and reweighting. The main result is that, despite large dijet energy loss indicated by $A_J$, the fragmentation shapes inside the jet cone are consistent with vacuum fragmentation across centrality, jet momentum, and imbalance classes. This suggests energy loss occurs outside the high-$p_T$ fragment region or is carried by low-$p_T$ fragments, providing important constraints for jet-quenching and medium-response theories.

Abstract

Jet fragmentation in pp and PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair was studied using data collected with the CMS detector at the LHC. Fragmentation functions are constructed using charged-particle tracks with transverse momenta pt > 4 GeV for dijet events with a leading jet of pt > 100 GeV. The fragmentation functions in PbPb events are compared to those in pp data as a function of collision centrality, as well as dijet-pt imbalance. Special emphasis is placed on the most central PbPb events including dijets with unbalanced momentum, indicative of energy loss of the hard scattered parent partons. The fragmentation patterns for both the leading and subleading jets in PbPb collisions agree with those seen in pp data at 2.76 TeV. The results provide evidence that, despite the large parton energy loss observed in PbPb collisions, the partition of the remaining momentum within the jet cone into high-pt particles is not strongly modified in comparison to that observed for jets in vacuum.

Figures (4)

• Figure 1: Dijet asymmetry, $A_J$, distributions in (a) pp collisions, (b) peripheral (30--100%) PbPb, and (c) central (0--30%) PbPb collisions. Data are shown as black points while the histograms show pythia dijets, which when compared to PbPb data have been embedded into hydjet events. Error bars represent the statistical uncertainty.
• Figure 2: Data from pp collisions. Left: Leading and subleading jet $p_{\mathrm{T}}\xspace$ distributions (not corrected for jet-finding efficiency and not unfolded for the jet energy resolution). Right: Fragmentation functions reconstructed for the leading (open circles) and subleading (solid points) jets. The statistical uncertainties, shown as error bars, are smaller than the symbols in most cases.
• Figure 3: (a,b) Fragmentation functions reconstructed in peripheral and central PbPb data for the leading (open circles) and subleading (solid points) jets. (c,d) Ratio of each PbPb fragmentation function to its pp-based reference. Error bars are statistical, the hollow boxes represent the systematic uncertainty for the leading jet, and gray boxes show the systematic uncertainty for the subleading jet. (e,f) Jet $p_{\mathrm{T}}$ distributions in PbPb data (not corrected for efficiency and not unfolded for $p_{\mathrm{T}}$ resolution) compared to the pp-based reference (see text). Only statistical uncertainties are shown in panels a, b, e and f.
• Figure 4: (a--d) Fragmentation functions for the leading (open circles) and subleading (solid points) jets in four regions of $A_J$ in central PbPb collisions compared to the pp reference. (e--h) Ratio of each fragmentation function to its pp-based reference. Error bars shown are statistical. The systematic uncertainty is represented by hollow boxes (leading jet) or gray boxes (subleading jet). (i--l) Jet $p_{\mathrm{T}}\xspace$ distributions in PbPb collisions in four regions of $A_J$ (not corrected for efficiency and not unfolded for $p_{\mathrm{T}}$ resolution) compared to the pp-based reference (see text). Only statistical uncertainties are shown.