CMS-PAS-HIN-15-013

CMS-PAS-HIN-15-013
Study of Z+jet correlations in PbPb and pp collisions at $\sqrt{s_{\rm NN}} =$ 5.02 TeV
CMS Collaboration
May 2016

Abstract: The production of Z+jet pairs is measured for the first time in pp and central PbPb collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair, using data samples collected by the CMS experiment at the LHC. The Z+jet azimuthal angle correlations and $p_{\mathrm{T}}$ imbalance are analysed for events containing a Z boson with transverse momentum $p_{\mathrm{T}}^{\mathrm{Z}} >$ 60 GeV/ $c$ and an associated jet with $p_{\mathrm{T}}^{\, \text{Jet}} >$ 30 GeV/ $c$ . A moderate shift in the $p_{\mathrm{T}}^{\, \text{Jet}}/p_{\mathrm{T}}^{\,\mathrm{Z}}$ ratio is seen in central PbPb collisions with respect to the ratio found using pp data.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 119 (2017) 082301. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1-a: Dimuon invariant mass distributions of Z boson candidates in pp collisions (a) and in 0-30% centrality PbPb collisions (b), as measured (circles) and simulated (filled histogram).
png pdf	Figure 1-b: Dimuon invariant mass distributions of Z boson candidates in pp collisions (a) and in 0-30% centrality PbPb collisions (b), as measured (circles) and simulated (filled histogram).
png pdf	Figure 2-a: Dielectron invariant mass distributions of Z boson candidates in pp collisions (a) and in 0-30% centrality PbPb collisions (b), as measured (circles) and simulated (filled histogram).
png pdf	Figure 2-b: Dielectron invariant mass distributions of Z boson candidates in pp collisions (a) and in 0-30% centrality PbPb collisions (b), as measured (circles) and simulated (filled histogram).
png pdf	Figure 3: Normalized number of Z+jet pairs before (black squares) and after (red circles) mixed-event background subtraction. The distribution of the Z+jet pairs from the mixed event method is also shown, as blue triangles.
png pdf	Figure 4-a: Azimuthal angle correlation ${\Delta \phi _{JZ}}$ between the Z boson and the jet (a), and transverse momentum ratio ${x_{JZ}}$ between the Z boson and the jets, with the azimuthal angle separation ${\Delta \phi _{JZ}}>$ 7/8 $\pi$ (b). The distributions are normalized by the number of Z boson events. Transverse momentum thresholds are ${p_{\mathrm{T}}^{\mathrm{Z}}} >$ 60 GeV/ $c$ for Z bosons and ${p_{\mathrm{T}}^{\text{Jet}}} >$ 30 GeV/ $c$ for jets. The mean of the ${x_{JZ}}$ distributions in PbPb and pp data are indicated with red and green arrows, respectively.
png pdf	Figure 4-b: Azimuthal angle correlation ${\Delta \phi _{JZ}}$ between the Z boson and the jet (a), and transverse momentum ratio ${x_{JZ}}$ between the Z boson and the jets, with the azimuthal angle separation ${\Delta \phi _{JZ}}>$ 7/8 $\pi$ (b). The distributions are normalized by the number of Z boson events. Transverse momentum thresholds are ${p_{\mathrm{T}}^{\mathrm{Z}}} >$ 60 GeV/ $c$ for Z bosons and ${p_{\mathrm{T}}^{\text{Jet}}} >$ 30 GeV/ $c$ for jets. The mean of the ${x_{JZ}}$ distributions in PbPb and pp data are indicated with red and green arrows, respectively.
png pdf	Figure 5-a: Azimuthal angle correlation ${\Delta \phi _{JZ}}$ (a) and transverse momentum ratio ${x_{JZ}}$ after the ${\Delta \phi _{JZ}} >$ 7/8 $\pi$ cut (b), as measured in pp data (circles) and simulated with PYTHIA and MadGraph5-aMC@NLO MC (filled histograms). The distributions are normalized by the number of Z boson events. Transverse momentum thresholds are ${p_{\mathrm{T}}^{\mathrm{Z}}} >$ 60 GeV/ $c$ for Z bosons and ${p_{\mathrm{T}}^{\text{Jet}}} >$ 30 GeV/ $c$ for jets.
png pdf	Figure 5-b: Azimuthal angle correlation ${\Delta \phi _{JZ}}$ (a) and transverse momentum ratio ${x_{JZ}}$ after the ${\Delta \phi _{JZ}} >$ 7/8 $\pi$ cut (b), as measured in pp data (circles) and simulated with PYTHIA and MadGraph5-aMC@NLO MC (filled histograms). The distributions are normalized by the number of Z boson events. Transverse momentum thresholds are ${p_{\mathrm{T}}^{\mathrm{Z}}} >$ 60 GeV/ $c$ for Z bosons and ${p_{\mathrm{T}}^{\text{Jet}}} >$ 30 GeV/ $c$ for jets.
png pdf	Figure 6-a: The mean value of the ${x_{JZ}}$ distribution as a function of the Z boson transverse momentum (a) and the ${R_{JZ}}$ probability to find a back-to-back jet partner for a Z boson candidate (b).
png pdf	Figure 6-b: The mean value of the ${x_{JZ}}$ distribution as a function of the Z boson transverse momentum (a) and the ${R_{JZ}}$ probability to find a back-to-back jet partner for a Z boson candidate (b).

Tables
png pdf	Table 1: Number of events surviving each selection step, for the pp and 0-30% centrality PbPb collisions.

Summary

This is the first study of Z+jet correlations in PbPb collisions, made with the CMS experiment. The collisions of PbPb and pp at 5.02 TeV center-of-mass energy, corresponding to an integrated luminosity of 404

$\mu$ b

$^{-1}$ and 25.8 pb

$^{-1}$ , respectively, were collected during the 2015 data-taking period. The Z+jet correlations were studied using the transverse momentum ratio

${x_{JZ}}$ , the azimuthal angle correlation

${\Delta \phi _{JZ}}$ , and the probability to find a back-to-back jet partner

${R_{JZ}}$ . The Z boson and jets were required to have transverse momentum greater than 60 GeV/

$c$ and 30 GeV/

$c$ , respectively. The transverse momentum ratio

${x_{JZ}}$ for the 0-30% centrality PbPb collisions is shifted to lower values with respect to pp collisions. The study of the average value of the transverse momentum ratio in Z boson

$p_{\mathrm{T}}$ bins shows that

$<{x_{JZ}}>$ is systematically smaller for PbPb collisions than in pp for all

$p_{\mathrm{T}}$ bins with

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

$c$ . These observations are in agreement with expected jet quenching effects. For all Z boson

$p_{\mathrm{T}}$ bins,

${R_{JZ}}$ in PbPb collisions is lower than in pp collisions, which suggests that in PbPb collisions a larger fraction of partons associated with the Z boson lost energy and fell below the 30 GeV/

$c$ jet

$p_{\mathrm{T}}$ threshold.

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