CMS-HIN-15-013

CMS-HIN-15-013 ; CERN-EP-2017-002
Study of jet quenching with Z+jet correlations in PbPb and pp collisions at $\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV
CMS Collaboration
3 February 2017
Phys. Rev. Lett. 119 (2017) 082301
Abstract: The production of jets in association with Z bosons, reconstructed via the $\mu^{+}\mu^{-}$ and $\mathrm{ e }^{+}\mathrm{ e }^{-}$ decay channels, is studied in pp and, for the first time, in PbPb collisions. Both data samples were collected by the CMS experiment at the LHC, at a center-of-mass energy of 5.02 TeV. The PbPb collisions were analyzed in the 0-30% centrality range. The back-to-back azimuthal alignment was studied in both pp and PbPb collisions for Z bosons with transverse momentum $p_{\mathrm{T}}^{\mathrm{Z}} >$ 60 GeV/ $c$ and a recoiling jet with $p_{\mathrm{T}}^{\text{jet}} >$ 30 GeV/ $c$ . The $p_{\mathrm{T}}$ imbalance, $x_{\mathrm{jZ}}= p_{\mathrm{T}}^{\text{jet}}/p_{\mathrm{T}}^{\mathrm{Z}}$ , as well as the average number of jet partners per Z, $R_{\mathrm{jZ}}$ , were studied in intervals of $p_{\mathrm{T}}^{\mathrm{Z}}$ , in both pp and PbPb collisions. The $R_{\mathrm{jZ}}$ is found to be smaller in PbPb than in pp collisions, which suggests that in PbPb collisions a larger fraction of partons, associated with the Z bosons, lose energy and fall below the 30 GeV/ $c$ $p_{\mathrm{T}}^{\text{jet}}$ threshold.
Links: e-print arXiv:1702.01060 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures	Summary	Additional Figures	References	CMS Publications

Cover
png ; pdf	Cover of Physical Review Letters, Volume 119, Number 8, published August 25, 2017.

PRL Editor's Suggestion, Featured in Physics, August 23, 2017
png	(not a CMS figure) Figure from ``Synopsis: A Precise Probe of the Quark-Gluon Plasma'', by Katherine Wright. Properties of the quark-gluon plasma can be inferred from measurements of jets and Z bosons simultaneously produced in the ion collisions that create the plasma.

Figures
png pdf	Figure 1: Invariant mass distributions of the selected dimuons (top) and dielectrons (bottom), compared to PYTHIA+HYJET ${\mathrm{ Z } } (\ell \ell )$ +jet events. The MC histogram is normalized to the number of events in the data.
png pdf	Figure 1-a: Invariant mass distributions of the selected dimuons (top) and dielectrons (bottom), compared to PYTHIA+HYJET ${\mathrm{ Z } } (\ell \ell )$ +jet events. The MC histogram is normalized to the number of events in the data.
png pdf	Figure 1-b: Invariant mass distributions of the selected dimuons (top) and dielectrons (bottom), compared to PYTHIA+HYJET ${\mathrm{ Z } } (\ell \ell )$ +jet events. The MC histogram is normalized to the number of events in the data.
png pdf	Figure 2: Distributions of the azimuthal angle difference ${\Delta \phi _{\mathrm {jZ}}}$ between the Z boson and the jet (top), and of the transverse momentum ratio ${x_{\mathrm {jZ}}}$ between the jet and the Z boson with ${\Delta \phi _{\mathrm {jZ}}} > 7 \pi /8$ (bottom). The distributions are normalized by the number of Z events, $N_{{\mathrm{ Z } } }$ . Vertical lines (bands) indicate statistical (systematic) uncertainties.
png pdf	Figure 2-a: Distributions of the azimuthal angle difference ${\Delta \phi _{\mathrm {jZ}}}$ between the Z boson and the jet (top), and of the transverse momentum ratio ${x_{\mathrm {jZ}}}$ between the jet and the Z boson with ${\Delta \phi _{\mathrm {jZ}}} > 7 \pi /8$ (bottom). The distributions are normalized by the number of Z events, $N_{{\mathrm{ Z } } }$ . Vertical lines (bands) indicate statistical (systematic) uncertainties.
png pdf	Figure 2-b: Distributions of the azimuthal angle difference ${\Delta \phi _{\mathrm {jZ}}}$ between the Z boson and the jet (top), and of the transverse momentum ratio ${x_{\mathrm {jZ}}}$ between the jet and the Z boson with ${\Delta \phi _{\mathrm {jZ}}} > 7 \pi /8$ (bottom). The distributions are normalized by the number of Z events, $N_{{\mathrm{ Z } } }$ . Vertical lines (bands) indicate statistical (systematic) uncertainties.
png pdf	Figure 3: The mean value of the ${x_{\mathrm {jZ}}}$ distribution (top) and the average number of jet partners per Z boson ${R_{\mathrm {jZ}}}$ (bottom), as a function of ${p_{\mathrm {T}}^{{\mathrm{ Z } } }}$ . Vertical lines (bands) indicate statistical (systematic) uncertainties.
png pdf	Figure 3-a: The mean value of the ${x_{\mathrm {jZ}}}$ distribution (top) and the average number of jet partners per Z boson ${R_{\mathrm {jZ}}}$ (bottom), as a function of ${p_{\mathrm {T}}^{{\mathrm{ Z } } }}$ . Vertical lines (bands) indicate statistical (systematic) uncertainties.
png pdf	Figure 3-b: The mean value of the ${x_{\mathrm {jZ}}}$ distribution (top) and the average number of jet partners per Z boson ${R_{\mathrm {jZ}}}$ (bottom), as a function of ${p_{\mathrm {T}}^{{\mathrm{ Z } } }}$ . Vertical lines (bands) indicate statistical (systematic) uncertainties.
png pdf	Figure 4: Comparison of the measured pp (top) and PbPb (bottom) ${x_{\mathrm {jZ}}}$ distributions with several theoretical models, smeared by the respective jet energy resolution: JEWEL [26], Hybrid [25], and GLV [27]. The JEWEL error bars represent statistical uncertainties while the widths of the Hybrid bands represent systematic variations. A MadGraph5_amc@nlo calculation [36] is also shown.

Summary

In summary, correlations of

$p_{\mathrm{T}}^{\mathrm{Z}} >$ 40 GeV/

$c$ Z bosons with

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

$c$ jets have been studied in pp and, for the first time, in PbPb collisions. The data were collected with the CMS experiment during the 2015 data taking period, at

$\sqrt{ s_{\mathrm{NN}} } =$ 5.02 TeV. Distributions of the azimuthal angle difference between the Z and the jet suggest that the peak at

$\Delta \phi_{\mathrm{jZ}} = \pi$ is slightly narrower in PbPb than in pp data, however significant differences were not established with the current precision. The

$x_{\mathrm{jZ}}$ distributions indicate that the PbPb values tend to be lower than those measured in pp collisions. Correspondingly, the average value of the transverse momentum ratio

$< x_{\mathrm{jZ}} >$ is smaller in PbPb than in pp collisions, for all

$p_{\mathrm{T}}^{\mathrm{Z}}$ intervals. The average number of jet partners per Z,

$R_{\mathrm{jZ}}$ , is lower in PbPb than in pp collisions, for all

$p_{\mathrm{T}}^{\mathrm{Z}}$ intervals, which suggests that in PbPb collisions a larger fraction of partons associated with the Z boson lose energy and fall below the 30 GeV/

$c$

$p_{\mathrm{T}}^{\text{jet}}$ threshold. These measurements provide new input for the determination of jet quenching parameters using a selection of partons with well-defined flavor and initial kinematics.

Additional Figures
png pdf	Additional Figure 1: Distributions of the transverse momentum ratio from Z+jet and photon+jet [50] correlations in PbPb collisions.
png pdf	Additional Figure 2: Distributions of the transverse momentum ratio from Z+jet and photon+jet [50] correlations in pp collisions.
png pdf	Additional Figure 3: The mean value of the transverse momentum ratio distributions from Z+jet and photon+jet [50] correlations in PbPb collisions.
png pdf	Additional Figure 4: The mean value of the transverse momentum ratio distributions from Z+jet and photon+jet [50] correlations in pp collisions.
png pdf	Additional Figure 5: The average number of jet partners per boson from Z+jet and photon+jet [50] correlations in PbPb collisions.
png pdf	Additional Figure 6: The average number of jet partners per boson from Z+jet and photon+jet [50] correlations in pp collisions.
png pdf	Additional Figure 7: Comparison of the measured PbPb ${\Delta \phi _{\mathrm {jZ}}}$ (azimuthal angle difference between the Z boson and the jet) distributions with several theoretical models smeared by the jet energy resolution in PbPb : JEWEL [26] and Hybrid [25].
png pdf	Additional Figure 8: Comparison of the measured pp and ${\Delta \phi _{\mathrm {jZ}}}$ (azimuthal angle difference between the Z boson and the jet) distributions with several theoretical models smeared by the jet energy resolution in pp : JEWEL [26] and Hybrid [25]. The curve labeled MG5aMC@NLO represents the MadGraph5_aMC@NLO calculation [36].
png pdf	Additional Figure 9: Comparison of the measured PbPb ${x_{\mathrm {jZ}}}$ (transverse momentum ratio) distributions with several theoretical models smeared by the jet energy resolution in PbPb : JEWEL [26], Hybrid [25], and GLV [27].
png pdf	Additional Figure 10: Comparison of the measured pp and ${x_{\mathrm {jZ}}}$ (transverse momentum ratio) distributions with several theoretical models smeared by the jet energy resolution in pp : JEWEL [26], Hybrid [25], and GLV [27]. The curve labeled MG5aMC@NLO represents the MadGraph5_aMC@NLO calculation [36].
png pdf	Additional Figure 11: The mean value of the ${x_{\mathrm {jZ}}}$ distribution as a function of ${p_{\mathrm {T}}^{{\mathrm {Z}}}}$ in PbPb collisions are compared to predictions from JEWEL [26].
png pdf	Additional Figure 12: The mean value of the ${x_{\mathrm {jZ}}}$ distribution as a function of ${p_{\mathrm {T}}^{{\mathrm {Z}}}}$ in pp collisions are compared to predictions from JEWEL [26] and MadGraph5_aMC@NLO calculation [36].
png pdf	Additional Figure 13: The average number of jet partners per Z boson ${R_{\mathrm {jZ}}}$ as a function of ${p_{\mathrm {T}}^{{\mathrm {Z}}}}$ in PbPb collisions are compared to predictions from Hybrid model [25] and JEWEL [26].
png pdf	Additional Figure 14: The average number of jet partners per Z boson ${R_{\mathrm {jZ}}}$ as a function of ${p_{\mathrm {T}}^{{\mathrm {Z}}}}$ in pp collisions are compared to predictions from Hybrid model [25], JEWEL [26], and MadGraph5_aMC@NLO calculation [36].
png pdf	Additional Figure 15: ${\Delta \phi _{\mathrm {jZ}}}$ (azimuthal angle difference between the Z boson and the jet) in PbPb collisions before (open squares) and after (red circles) background subtraction where the background (blue triangles) is estimated with the mixed event method.

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