CMS-PAS-HIN-18-020

CMS-PAS-HIN-18-020
Measurement of b jet shapes in pp collisions at $\sqrt{s} =$ 5.02 TeV
CMS Collaboration
November 2019

Abstract: We present the first study of the charged hadron production associated with b jets in 5.02 TeV pp collisions. The data sample used in this study was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 27.4 pb $^{-1}$ . The differential jet shapes, defined as the normalized transverse momentum flow of charged hadrons with respect to the angular distance from the jet axis, are measured for b jets via the jet-track correlation method. In addition to the jet shapes, the per-jet yields of charged particles associated with b jets are also quantified, again as a function of the angular distance with respect to the jet axis. Extracted jet shape and particle yield distributions for b jets are compared with results for inclusive jets, as well as with predictions from the PYTHIA event generator. The new results reveal a deficiency in the modeling of softer part of the jet fragmentation for both types of jets studied and set a baseline for future studies of parton flavor dependence of jet-medium interactions in heavy ion collisions.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 05 (2021) 054. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Particle yield distributions $Y(\Delta r)$ as a function of $\Delta r$ are presented differentially for all for all $p_{\mathrm {T}}^{\mathrm {trk}}$ bins studied for inclusive jets (left) and b jets (right) with ${p_{\mathrm {T}}} >$ 120 GeV. The shadowed boxes present the systematic uncertainties.
png pdf	Figure 2: Jet-associated charge particle yield $Y(\Delta r)$ for inclusive jets (left) and b jets (middle) as a function of $\Delta r$ for 1 $< p_{\mathrm {T}}^{\mathrm {trk}} <$ 20 GeV. Both jets types are selected with ${p_{\mathrm {T}}} >$ 120 GeV. Charged particle tracks with $p_{\mathrm {T}}^{\mathrm {trk}} >$ 1 GeV are used to construct the distributions as a function of $\Delta r$ for pp data (red) and PYTHIA simulation (blue), respectively. The right plot shows the particle yield difference of b jets and inclusive jets as a function of $\Delta r$ for pp data and PYTHIA simulation (blue line). The shadowed boxes present systematic uncertainties.
png pdf	Figure 3: The radial jet momentum distribution $\rho (\Delta r)$ of inclusive jets (left) and b jets (right) with ${p_{\mathrm {T}}} >$ 120 GeV as a function of $\Delta r$ shown differentially for all $p_{\mathrm {T}}^{\mathrm {trk}}$ bins for pp. The shadowed boxes present the systematic uncertainties.
png pdf	Figure 4: The radial jet momentum distribution $\rho (\Delta r)$ of inclusive jets (left) and b jets (middle), both with ${p_{\mathrm {T}}} >$ 120 GeV and $p_{\mathrm {T}}^{\mathrm {trk}} >$ 1 GeV shown as a function of $\Delta r$ for both data(red markers) and the PYTHIA 6 simulation(blue markers) respectively. The right plot shows the b jets to inclusive jets radial jet momentum distribution ratio as a function of $\Delta r$ for pp data and PYTHIA 6 simulation(blue band). The shadowed boxes show the systematic uncertainty.

Tables
png pdf	Table 1: Systematic uncertainties in percentage for the measurements of the jet-track correlations. Where an uncertainty range is given, the upper edge of the range corresponds to the bin with the smallest $p_{\mathrm {T}}^{\mathrm {trk}}$ values. The sources from the decontamination and tagging bias are exclusive for b jets.

Summary

In this note, the first charged-particle track yields and jet shapes for b and inclusive jets in pp collisions using data collected with the CMS detector at the LHC at nucleon-nucleon center-of-mass energy of

$\sqrt{s} =$ 5.02 TeV are presented. The correlations of charged particles having transverse momentum

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

$| \eta | <$ 2.4 with the jet axis of jets having

${p_{\mathrm{T}}} >$ 120 GeV and

$| \eta | <$ 1.6 are studied. Charged particle yields associated with jets are presented as a function of relative angular distance

$\Delta r = \sqrt{\smash[b]{(\Delta\eta)^{2}+(\Delta\phi)^{2}}}$ from the jet axis. In these studies, a higher number of associated charged particle tracks in the low

$\Delta r$ is found for b jets with respect to inclusive jets produced predominantly by light parton flavors. The trends observed in pp data for particle yield distributions associated with both types of jets are reproduced by PYTHIA calculations.

In addition to the charged-particle yields, we examine the jet transverse momentum profile variable

$\rho(\Delta r)$ , defined using the distribution of charged-particle tracks in annular rings around the jet axis with each particle weighted by its

$p_{\mathrm{T}}^{\mathrm{trk}}$ value. The measured shapes of b jets are broader than those of inclusive jets. The reported shapes for both types of jets are reproduced by PYTHIA calculations at small angular distances from the jet axis; however, measured transverse momenta distributions at larger

$\Delta r$ are underpredicted by Monte Carlo simulations for both b and inclusive jets, with larger data-to-simulation differences observed for b jets.

This result provides new constraints on pQCD calculations for flavor dependencies in parton fragmentation and gluon radiation, as well as relative contributions of different processes to b quark production. These measurements are also expected to offer an important reference for the future detailed studies of flavor effects for parton interactions with the quark-gluon plasma.

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