Modification of jet shapes in PbPb collisions at sqrt(s[NN]) = 2.76 TeV

TL;DR

This paper reports the first measurement of jet shapes in PbPb collisions at √sNN = 2.76 TeV, comparing centrality-dependent PbPb data to pp references to reveal modifications in the radial distribution of jet transverse momentum. Using anti-kT jets with R = 0.3 and charged tracks with pT > 1 GeV, the study constructs the differential jet-shape ρ(r) and subtracts the heavy-ion underlying event via an η-reflected background method, with cross-checks from event-mixing and MC embedding. The results show minimal modification in peripheral collisions but clear intra-jet energy redistribution in central events: depletion at intermediate radii and enhanced energy at larger radii within the jet cone, implying in-medium shower broadening. These findings, supported by MC closure tests, constrain models of jet quenching and the interaction of parton showers with the quark-gluon plasma, advancing understanding of medium properties and energy-loss mechanisms.

Abstract

The first measurement of jet shapes, defined as the fractional transverse momentum radial distribution, for inclusive jets produced in heavy-ion collisions is presented. Data samples of PbPb and pp collisions, corresponding to integrated luminosities of 150 inverse microbarns and 5.3 inverse picobarns respectively, were collected at a nucleon-nucleon centre-of-mass energy of sqrt(s[NN]) = 2.76 TeV with the CMS detector at the LHC. The jets are reconstructed with the anti-kt algorithm with a distance parameter R = 0.3, and the jet shapes are measured for charged particles with transverse momentum pt > 1 GeV. The jet shapes measured in PbPb collisions in different collision centralities are compared to reference distributions based on the pp data. A centrality-dependent modification of the jet shapes is observed in the more central PbPb collisions, indicating a redistribution of the energy inside the jet cone. This measurement provides information about the parton shower mechanism in the hot and dense medium produced in heavy-ion collisions.

Figures (3)

• Figure 1: (Color online) Differential jet shapes obtained from the jet cone before background subtraction (filled circles), and from the "$\eta$-reflected" background cone (open circles), as a function of the distance from the jet axis in two ${PbPb}$ centrality intervals: 70--100% (left) and 0--10% (right). The measurements use inclusive jets with $p_{\mathrm{T}}\xspace^\text{jet} > 100{\,\text{Ge\spaceV\space/\space}c}\xspace$ and $0.3 < \lvert \eta \rvert< 2$, and charged particles with $p_{\mathrm{T}}\xspace^\text{track} >1{\,\text{Ge\spaceV\space/\space}c}\xspace$.
• Figure 2: (Color online) Ratio of jet shapes for jets generated with pythia and embedded into heavy-ion background events simulated by hydjet, to those obtained from the pythia signal alone. The analysis uses the same background subtraction procedure ("$\eta$-reflected") as in data. The measurements use inclusive jets with $p_{\mathrm{T}}\xspace^\text{jet} > 100{\,\text{Ge\spaceV\space/\space}c}\xspace$ and $0.3 < \lvert \eta \rvert< 2$, and charged particles with $p_{\mathrm{T}}\xspace^\text{track} > 1{\,\text{Ge\spaceV\space/\space}c}\xspace$.
• Figure 3: (Color online) Top row: Differential jet shapes in ${PbPb}$ collisions (filled circles) as a function of distance from the jet axis for inclusive jets with $p_{\mathrm{T}}\xspace^\text{jet} >100{\,\text{Ge\spaceV\space/\space}c}\xspace$ and $0.3 < \lvert \eta \rvert < 2$ in five ${PbPb}$ centrality intervals. The measurements use charged particles with $p_{\mathrm{T}}\xspace^\text{track} > 1{\,\text{Ge\spaceV\space/\space}c}\xspace$. The $\mathrm{p}$$\mathrm{p}$-based reference shapes (with centrality-based adjustments as described in the text) are shown with open symbols. Each spectrum is normalised to an integral of unity. The shaded regions represent the systematic uncertainties for the measurement performed in ${PbPb}$ collisions, with the statistical uncertainties too small to be visible. Bottom row: Jet shape nuclear modification factors, $\rho(r)^{PbPb}\xspace/\rho(r)^{\mathrm{p}\mathrm{p}\xspace}$. The error bars show the statistical uncertainties, and the shaded boxes indicate the systematic uncertainties.