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| 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 |
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| Cover | |
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Cover of Physical Review Letters, Volume 119, Number 8, published August 25, 2017. |
| PRL Editor's Suggestion, Featured in Physics, August 23, 2017 | |
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(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 | |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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 | |
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Additional Figure 1:
Distributions of the transverse momentum ratio from Z+jet and photon+jet [50] correlations in PbPb collisions. |
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Additional Figure 2:
Distributions of the transverse momentum ratio from Z+jet and photon+jet [50] correlations in pp collisions. |
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Additional Figure 3:
The mean value of the transverse momentum ratio distributions from Z+jet and photon+jet [50] correlations in PbPb collisions. |
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Additional Figure 4:
The mean value of the transverse momentum ratio distributions from Z+jet and photon+jet [50] correlations in pp collisions. |
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Additional Figure 5:
The average number of jet partners per boson from Z+jet and photon+jet [50] correlations in PbPb collisions. |
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Additional Figure 6:
The average number of jet partners per boson from Z+jet and photon+jet [50] correlations in pp collisions. |
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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]. |
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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]. |
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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]. |
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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]. |
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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]. |
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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]. |
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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]. |
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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]. |
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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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