CMS-PAS-HIN-16-014

CMS-PAS-HIN-16-014
Comparison of jet fragmentation for isolated-photon+jet pairs in PbPb and pp collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV
CMS Collaboration
August 2017

Abstract: Measurements of fragmentation functions for jets paired with an isolated photon in pp and, for the first time, PbPb collisions are presented. The analysis uses data from the CMS detector at the CERN LHC, with both systems at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Fragmentation functions are constructed using charged particles with transverse momentum $p_{\mathrm{T}}^{\mathrm{trk}} >$ 1 GeV/ $c$ inside jets with transverse momentum $p_{\mathrm{T}}^{\mathrm{jet}} >$ 30 GeV/ $c$ for events containing an isolated photon with $p_{\mathrm{T}}^{\gamma} >$ 60 GeV/ $c$ . For central PbPb collisions, modifications of the jet fragmentation function with respect to that found for pp collisions are observed, while no significant differences are found in the 50% most peripheral collisions. The modifications seen in central events indicate an enhancement for particles at low $p_{\mathrm{T}}$ and a depletion at high $p_{\mathrm{T}}$ , with a transition around 3 GeV/ $c$ . These measurements provide information about the longitudinal modifications of a parton shower whose initial kinematics are tightly constrained by the properties of the associated photon.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 121 (2018) 242301. The superseded preliminary plots can be found here.

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Top: The centrality dependence of the $\xi ^{\mathrm {jet}}$ distribution for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. Bottom: The ratios of the PbPb to pp distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate the systematic uncertainties.
png pdf	Figure 2: Top: The centrality dependence of the $\xi ^\gamma _{{\mathrm {T}}}$ distribution for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. Bottom: The ratios of the PbPb to pp distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate the systematic uncertainties.

Summary

In summary, the fragmentation functions of jets associated with isolated photons are presented in pp and, for the first time, in PbPb collisions at

$\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV. The fragmentation functions are constructed for jets with

$p_{\mathrm{T}}^{\mathrm{jet}} >$ 30 GeV/

$c$ and charged particles with

$p_{\mathrm{T}}^{\mathrm{trk}} >$ 1 GeV/

$c$ that are associated with an isolated photon with

$p_{\mathrm{T}}^{\gamma} >$ 60 GeV/

$c$ , and are studied as functions of

$\xi ^{\mathrm {jet}}$ and

$\xi ^\gamma _{{\mathrm {T}}}$ , in four different PbPb event centrality intervals. The modifications of the

$\xi ^{\mathrm {jet}}$ and

$\xi ^\gamma _{{\mathrm {T}}}$ distributions in central PbPb collisions indicate that there is an excess of low energy particles and a depletion of high energy particles inside the jet. This picture is more apparent in the

$\xi ^\gamma _{{\mathrm {T}}}$ distributions, where the photon-based selection allows for the tagging of the initial parton kinematics (before the quenching happened). The association with isolated photons provides, for the first time, information on the medium modification of parton showers in events with well-defined initial parton kinematics.

Additional Figures
png pdf	Additional Figure 1: Top: The ${\xi ^\mathrm {jet}}$ distribution from two event centralities for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. Bottom: The ratios of the PbPb to pp distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate the systematic uncertainties.
png pdf	Additional Figure 2: Top: The ${\xi ^\gamma _{\mathrm {T}}}$ distribution from two event centralities for jets associated with an isolated photon for PbPb (full markers) and pp (open markers) collisions. Bottom: The ratios of the PbPb to pp distributions. The vertical lines through the points represent statistical uncertainties, while the colored boxes indicate the systematic uncertainties.
png pdf	Additional Figure 3: Subtraction of the background tracks (red crosses) from the ${\xi ^\mathrm {jet}}$ distribution for jets associated with an isolated photon (black squares) for 0-10% centrality PbPb collisions. The distribution after background track subtraction (blue circles) are further subtracted for background jets (corresponding to orange squares in Fig. 4-aux).
png pdf	Additional Figure 4: Subtraction of the background jets (violet crosses) from the ${\xi ^\mathrm {jet}}$ distribution for background track subtracted jets associated with an isolated photon (orange squares) for 0-10% centrality PbPb collisions. The subtraction gives the background track and background jet subtracted distribution (green circles).
png pdf	Additional Figure 5: Distribution of the sum of photon isolation variables in Pythia+Hydjet simulation for signal (red histogram) and background (green histogram) events.
png pdf	Additional Figure 6: ${ {p_{\mathrm {T}}} ^\mathrm {jet}}$ calculated as a function of ${\xi ^\mathrm {jet}}$ for three different ${ {p_{\mathrm {T}}} ^\mathrm {trk}}$ selections where $\Delta R$ between the track and the jet is 0.
png pdf	Additional Figure 7: ${ {p_{\mathrm {T}}} ^\mathrm {trk}}$ calculated as a function of ${\xi ^\mathrm {jet}}$ for three different ${ {p_{\mathrm {T}}} ^\mathrm {jet}}$ selections where $\Delta R$ between the track and the jet is 0.
png pdf	Additional Figure 8: ${ {p_{\mathrm {T}}} ^\mathrm {jet}}$ calculated as a function of ${\xi ^\mathrm {jet}}$ for four different ${\eta ^\mathrm {jet}}$ and for each of them two different $\| {\eta ^\mathrm {trk}} \| - \| {\eta ^\mathrm {jet}} \|$ selections where ${ {p_{\mathrm {T}}} ^\mathrm {trk}} =$ 1 GeV/ $c$ and $\Delta \phi$ between the track and the jet is 0.
png pdf	Additional Figure 9: ${ {p_{\mathrm {T}}} ^\mathrm {trk}}$ calculated as a function of ${\xi ^\mathrm {jet}}$ for four different ${\eta ^\mathrm {jet}}$ and for each of them two different $\| {\eta ^\mathrm {trk}} \| - \| {\eta ^\mathrm {jet}} \|$ selections where ${ {p_{\mathrm {T}}} ^\mathrm {jet}} =$ 30 GeV/ $c$ and $\Delta \phi$ between the track and the jet is 0.
png pdf	Additional Figure 10: ${ {p_{\mathrm {T}}} ^\gamma }$ calculated as a function of ${\xi ^\gamma _{\mathrm {T}}}$ for three different ${ {p_{\mathrm {T}}} ^\mathrm {trk}}$ selections where $\Delta \phi$ between the track and the photon is $\pi$ .
png pdf	Additional Figure 11: ${ {p_{\mathrm {T}}} ^\mathrm {trk}}$ calculated as a function of ${\xi ^\gamma _{\mathrm {T}}}$ for three different ${ {p_{\mathrm {T}}} ^\gamma }$ selections where $\Delta \phi$ between the track and the photon is $\pi$ .
png pdf	Additional Figure 12: ${ {p_{\mathrm {T}}} ^\gamma }$ calculated as a function of ${\xi ^\gamma _{\mathrm {T}}}$ for two different selections of the $\Delta \phi$ between the track and the photon where the ${ {p_{\mathrm {T}}} ^\mathrm {trk}}$ is 1 GeV/ $c$ .
png pdf	Additional Figure 13: ${ {p_{\mathrm {T}}} ^\mathrm {trk}}$ calculated as a function of ${\xi ^\gamma _{\mathrm {T}}}$ for two different selections of the $\Delta \phi$ between the track and the photon where the ${ {p_{\mathrm {T}}} ^\gamma }$ is 60 GeV/ $c$ .

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