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

TL;DR

This CMS study measures the jet fragmentation function for inclusive jets in PbPb and pp collisions at $\sqrt{s_{NN}}=2.76$ TeV, using charged tracks with $p_T>1$ GeV/$c$ inside a cone of $R=0.3$ around jets with $p_T>100$ GeV/$c$. By comparing PbPb data to a high-statistics pp reference and applying data-driven background subtraction, jet-energy, and fragmentation-bias corrections, the analysis reveals a centrality-dependent enhancement of soft fragments ($p_T<3$ GeV/$c$) in central PbPb jets, with PbPb/pp ratios up to ~1.5–1.6 in the 0–10% most central bin. Peripheral collisions show little to no modification, while the most central events show increased low-$p_T$ activity and a mild depletion at intermediate $\xi$, consistent across jet $p_T$ ranges. The results constrain models of medium-induced energy loss and the medium response, highlighting the transfer of energy from hard jets to soft hadrons in the QGP.

Abstract

The jet fragmentation function of inclusive jets with transverse momentum pt above 100 GeV in PbPb collisions has been measured using reconstructed charged particles with pt above 1 GeV in a cone of radius 0.3 around the jet axis. A data sample of PbPb collisions collected in 2011 at a nucleon-nucleon center-of-mass energy of sqrt(s[NN]) = 2.76 TeV corresponding to an integrated luminosity of 150 inverse microbarns is used. The results for PbPb collisions as a function of collision centrality and jet transverse momentum are compared to reference distributions based on pp data collected at the same center-of-mass energy in 2013, with an integrated luminosity of 5.3 inverse picobarns. A centrality-dependent modification of the fragmentation function is found. For the most central collisions, a significant enhancement is observed in the PbPb / pp fragmentation function ratio for charged particles with pt less than 3 GeV. This enhancement is observed for all jet pt bins studied.

Figures (8)

• Figure 1: (Color online) (Top) The PbPb fragmentation function in bins of centrality (increasing from left to right) overlaid with $\mathrm{p}$$\mathrm{p}$ reference data. Jets have $100 < p_{\mathrm{T}}\xspace < 300{\,\text{Ge\spaceV\space/\space}c}\xspace$, and tracks have $p_{\mathrm{T}}\xspace > 1{\,\text{Ge\spaceV\space/\space}c}\xspace$. (Bottom) The ratio of each PbPb fragmentation function to its $\mathrm{p}$$\mathrm{p}$ reference. Error bars are statistical, and boxes show the systematic uncertainty.
• Figure 2: (Color online) (Top) The PbPb fragmentation function in bins of centrality (increasing from left to right) overlaid with $\mathrm{p}$$\mathrm{p}$ reference data. Jets have $100 < p_{\mathrm{T}}\xspace < 120{\,\text{Ge\spaceV\space/\space}c}\xspace$, and tracks have $p_{\mathrm{T}}\xspace > 1{\,\text{Ge\spaceV\space/\space}c}\xspace$. (Bottom) The ratio of each PbPb fragmentation function to its $\mathrm{p}$$\mathrm{p}$ reference. Error bars are statistical, and boxes show the systematic uncertainty.
• Figure 3: (Color online) (Top) The PbPb fragmentation function in bins of centrality (increasing from left to right) overlaid with $\mathrm{p}$$\mathrm{p}$ reference data. Jets have $120 < p_{\mathrm{T}}\xspace < 150{\,\text{Ge\spaceV\space/\space}c}\xspace$, and tracks have $p_{\mathrm{T}}\xspace > 1{\,\text{Ge\spaceV\space/\space}c}\xspace$. (Bottom) The ratio of each PbPb fragmentation function to its $\mathrm{p}$$\mathrm{p}$ reference. Error bars are statistical, and boxes show the systematic uncertainty.
• Figure 4: (Color online) (Top) The PbPb fragmentation function in bins of centrality (increasing from left to right) overlaid with $\mathrm{p}$$\mathrm{p}$ reference data. Jets have $150 < p_{\mathrm{T}}\xspace < 300{\,\text{Ge\spaceV\space/\space}c}\xspace$, and tracks have $p_{\mathrm{T}}\xspace > 1{\,\text{Ge\spaceV\space/\space}c}\xspace$. (Bottom) The ratio of each PbPb fragmentation function to its $\mathrm{p}$$\mathrm{p}$ reference. Error bars are statistical, and boxes show the systematic uncertainty.
• Figure 5: (Color online) The spectrum of tracks inside the cone of jets with $100 < p_{\mathrm{T}}\xspace^\text{jet} < 300{\,\text{Ge\spaceV\space/\space}c}\xspace$, as a function of track $p_{\mathrm{T}}$, for PbPb (with increasing centrality from left to right) and pp. Both the PbPb and $\mathrm{p}$$\mathrm{p}$ results are background subtracted, in the same manner as for the fragmentation function. The bottom panels show the difference of PbPb and $\mathrm{p}$$\mathrm{p}$ spectra, demonstrating an excess of low-$p_{\mathrm{T}}$ tracks in the PbPb events.