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| 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. | ||
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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These preliminary results are superseded in this paper, PRL 119 (2017) 082301. The superseded preliminary plots can be found here. |
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| Figures | |
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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). |
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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). |
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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). |
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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). |
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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. |
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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. |
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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. |
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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. |
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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. |
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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). |
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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 | |
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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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