CMS-PAS-HIN-16-020

CMS-PAS-HIN-16-020
Jet properties in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}} =$ 5.02 TeV
CMS Collaboration
February 2017

Abstract: Modifications to high transverse momentum ( $p_{\rm T}$ ) jets in the quark gluon plasma (QGP) are studied in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}} =$ 5.02 TeV via correlations between jets with $p_{\rm T} >$ 120 GeV and charged particles with $p_{\rm T} >$ 0.7 GeV. This measurement aims to extend results of the same correlations at $\sqrt{s_{\mathrm{NN}}} =$ 2.76 TeV, which observed significant modification of jet track correlations out to large distances from the jet axis in central PbPb events. This analysis measures correlations in relative pseudorapidity ( $\Delta \eta$ ), relative azimuth ( $\Delta \phi$ ), and relative distance from the jet axis $\Delta r = \sqrt{\Delta\eta^{2}+\Delta\phi^{2}}$ . Results are presented as a function of PbPb collision centrality and track $p_{\rm T}$ , providing a differential picture of jet modifications in the medium. Measurements include charged particle densities as a function of distance from jet axis, total charged-particle yields as a function of track- $p_{\rm T}$ (fragmentation functions), and the jet momentum density profile (''jet shape''). This analysis uses data collected with the CMS detector at collision energy $\sqrt{\rm s_{NN}} =$ 5.02 TeV in 2015, corresponding to integrated luminosities of 404 $\mu\mathrm{b}^{-1}$ for PbPb and 27.4 $\mathrm{pb}^{-1}$ for pp.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 05 (2018) 006. The superseded preliminary plots can be found here.

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Top row: distributions of charged particle yields correlated to jets with $p_{\rm T} >$ 120 GeV as a function of $\Delta \eta$ (projected over $\|\Delta \phi \| <$ 1), shown differentially for all $p_{\rm T}^{\rm trk}$ bins for pp, peripheral PbPb, and central PbPb data. Bottom row: PbPb minus pp difference in these distributions. Hatched lines on $p_{\rm T}^{\rm trk}$ -inclusive points show total systematic uncertainties.
png pdf	Figure 2: Top row: distributions of charged particle yields correlated to jets with $p_{\rm T} >$ 120 GeV as a function of $\Delta \phi$ (projected over $\|\Delta \eta \| <$ 1), shown differentially for all $p_{\rm T}^{\rm trk}$ bins for pp, peripheral PbPb, and central PbPb data. Bottom row: PbPb minus pp difference in these distributions. Hatched lines on $p_{\rm T}^{\rm trk}$ -inclusive points show total systematic uncertainties.
png pdf	Figure 3: Top row: distributions of charged particle yields correlated to jets with $p_{\rm T} >$ 120 GeV as a function of $\Delta r$ , shown differentially for all $p_{\rm T}^{\rm trk}$ bins. Bottom row: PbPb minus pp difference in these distributions. Hatched lines on $p_{\rm T}^{\rm trk}$ -inclusive points show total systematic uncertainties.
png pdf	Figure 4: Top row: integrated yields of charged particle yields correlated to jets with $p_{\rm T} >$ 120 GeV as a function of $p_{\rm T}^{\rm trk}$ bins for PbPb data, compared to pp reference. Bottom row: integrated excess yield, PbPb minus pp. New measurements of excess yields at 5.02 TeV are compared to those measured at 2.76 TeV in CMS study [12].
png pdf	Figure 5: Top row: Transverse momentum profile of jets ${\rm P}(\Delta r)$ in pp and PbPb, shown differentially in $p_{\rm T}^{\rm trk}$ . Middle row: jet shapes $\rho (\Delta r)$ (normalized to unity over $\Delta r <$ 1) in PbPb and pp. Bottom row: jet shape ratio $\rho (\Delta r)_{\rm PbPb} / \rho (\Delta r)_{\rm pp}$ . Hatched lines on $p_{\rm T}^{\rm trk}$ -inclusive points show total systematic uncertainties.

Tables
png pdf	Table 1: Systematic uncertainties in the measurement of the jet track correlations in PbPb and pp collisions. The numbers presented in this table summarize typical range of systematic uncertainty as a function of collision centrality. The upper limits of the cited values correspond to uncertainties at lowest $p_{\rm T}^{\rm trk}$ , and uncertainties decrease with rising $p_{\rm T}^{\rm trk}$ .

Summary

This analysis studies properties of high-

$p_{\rm T}$ jets in heavy ion collisions, using LHC Run-2 data collected by the CMS detector at collision energy

$\sqrt{\rm s_{NN}} =$ 5.02 TeV in 2015. These studies are based on correlations between jets with

$p_{\rm T} >$ 120 GeV, and charged-particle tracks with 0.7

$< p_{\rm T}^{\rm trk} <$ 300 GeV, analyzed differentially in

$p_{\rm T}^{\rm trk}$ and collision centrality. Charged-particle correlations are presented as a function of angular distance from the jet, individually

$\Delta\eta$ and

$\Delta\phi$ . In these studies, a low-

$p_{\rm T}$ excess (tracks with

$p_{\rm T}^{\rm trk} <$ 3 GeV) extending to large angles

$\Delta\eta =$ 1 and

$\Delta\phi =$ 1 is found in central PbPb collisions relative to pp reference. In addition to separate studies in

$\Delta\eta$ and

$\Delta\phi$ , charged-particle densities are also measured as a function of distance from the jet axis

$\Delta r$ . To summarize these measurements, correlations are integrated over

$\Delta r <$ 1 to obtain a measurement of the total number of particles in each

$p_{\rm T}^{\rm trk}$ class. A comparison of integrals shows as many as three additional low-

$p_{\rm T}$ particles per unit of

$p_{\rm T}^{\rm trk}$ in central PbPb collisions relative to pp reference. In addition to these charged-particle density studies, correlations are constructed with each track weighted by its

$p_{\rm T}^{\rm trk}$ in order to study the overall distribution of

$p_{\rm T}^{\rm trk}$ with respect to the jet axis. This is used to measure the jet shape

$\rho(\Delta r)$ to a distance of

$\Delta r =$ 1 from the jet axis, presented differentially as a function of

$p_{\rm T}^{\rm trk}$ . A comparison of central PbPb versus pp jet shape shows redistribution of

$p_{\rm T}^{\rm trk}$ from small angles (

$\Delta r <$ 0.3) near the jet axis to large angles (

$\Delta r =$ 1) away from the jet axis. In all cases, the modifications observed both in terms of yield modification and jet shapes are similar between the measurements at

$\sqrt{\rm s_{NN}} =$ 2.76 and 5.02 TeV when directly comparing PbPb to pp differences in corresponding centrality bins.

Additional Figures
png pdf	Additional Figure 1: Jet-track correlation signal shape $S(\Delta \eta ,\Delta \phi )$ for tracks with 1 $< p_{\rm T}^{\rm trk} <$ 2 GeV in 0-10% central events (left), and corresponding mixed event shape $ME(\Delta \eta ,\Delta \phi )$ for the same centrality and $p_{\rm T}^{\rm trk}$ bin (center). Their ratio gives the acceptance-corrected yield (right).
png pdf	Additional Figure 2: Acceptance-corrected 2D jet-track correlation yield (left) is projected over the range 1.5 $< \|\Delta \eta \| <$ 2.5 and re-propagated in $\Delta \eta$ to produce a 2D background distribution (center). This background is subtracted from the total yield to obtain the 2D background-subtracted yield shown on the right (for tracks with 1 $< p_{\rm T}^{\rm trk} <$ 2 GeV).
png pdf	Additional Figure 3: Jet peak correlated yield or tracks with 1 $< p_{\rm T}^{\rm trk} <$ 2 GeV, after background subtraction.
png pdf	Additional Figure 4: Illustration of charged particle correlation observables, shown for tracks with 1 $< p_{\rm T}^{\rm trk} <$ 2 GeV. Left: $\Delta \eta$ distribution of correlated yield (projected over $\|\Delta \phi \| <$ 1). Center: $\Delta \phi$ distribution of correlated yield (projected over $\|\Delta \eta \| <$ 1). Right: $\Delta r$ distribution of correlated yield (integrated in rings of $\Delta r$ )
png pdf	Additional Figure 5: Top row: Transverse momentum profile of jets ${\rm P}(\Delta r)$ in pp and PbPb, shown differentially in $p_{\rm T}^{\rm trk}$ . Middle row: jet shapes $\rho (\Delta r)$ (normalized to unity over $\Delta r < 1$ ) in PbPb and pp. Bottom row: jet shape ratio $\rho (\Delta r)_{\rm PbPb} / \rho (\Delta r)_{\rm pp}$ . Hatched lines on $p_{\rm T}^{\rm trk}$ -inclusive points show total systematic uncertainties. Generator-level PYTHIA 6 is shown with green crosses.

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