Using Z boson events to study parton-medium interactions in PbPb collisions

The collaboration
Phys.Rev.Lett. 128 (2022) 122301, 2022.

Abstract
The spectra measurements of charged hadrons produced in the shower of a parton originating in the same hard scattering with a leptonically decaying Z boson, are reported in lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. Both PbPb and pp data sets are recorded by the CMS experiment at the LHC, and correspond to an integrated luminosity of 1.7 nb$^{-1}$ and 320 pb$^{-1}$, respectively. Hadronic collision data with one reconstructed Z boson candidate with the transverse momentum $p_\mathrm{T}$$\gt$ 30 GeV/$c$ are analyzed. The Z boson constrains the initial energy and direction of the associated parton. In heavy ion events, azimuthal angular distributions of charged hadrons with respect to the direction of a Z boson are sensitive to modifications of the in-medium parton shower and medium response. Compared to reference data from pp interactions, the results for central PbPb collisions indicate a modification of the angular correlations. The measurements of the fragmentation functions and $p_\mathrm{T}$ spectra of charged particles in Z boson events, which are sensitive to medium modifications of the parton shower longitudinal structure, are also reported. Significant modifications in central PbPb events compared to pp reference data are also found for these observables.

• #### Table 1

Data from top panel of Fig. 1

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Distributions of $\Delta\phi_{\mathrm{trk,Z}}$ in pp collisions at 5.02 TeV.

• #### Table 2

Data from top panel of Fig. 1

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Distributions of $\Delta\phi_{\mathrm{trk,Z}}$ in 70-90% centrality PbPb collisions at 5.02 TeV.

• #### Table 3

Data from top panel of Fig. 1

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Distributions of $\Delta\phi_{\mathrm{trk,Z}}$ in 50-70% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 1

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Distributions of $\Delta\phi_{\mathrm{trk,Z}}$ in 30-50% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 1

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Distributions of $\Delta\phi_{\mathrm{trk,Z}}$ in 0-30% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 1

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PbPb - pp difference of $\Delta\phi_{\mathrm{trk,Z}}$ distributions for 70-90% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 1

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PbPb - pp difference of $\Delta\phi_{\mathrm{trk,Z}}$ distributions for 50-70% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 1

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PbPb - pp difference of $\Delta\phi_{\mathrm{trk,Z}}$ distributions for 30-50% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 1

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PbPb - pp difference of $\Delta\phi_{\mathrm{trk,Z}}$ distributions for 0-30% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 2

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Distributions of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ in pp collisions at 5.02 TeV.

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Data from top panel of Fig. 2

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Distributions of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ in 70-90% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 2

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Distributions of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ in 50-70% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 2

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Distributions of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ in 30-50% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 2

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Distributions of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ in 0-30% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 2

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PbPb / pp ratio of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ distributions for 70-90% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 2

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PbPb / pp ratio of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ distributions for 50-70% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 2

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PbPb / pp ratio of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ distributions for 30-50% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 2

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PbPb / pp ratio of $\xi^{\mathrm{trk,Z}}_{\mathrm{T}}$ distributions for 0-30% centrality PbPb collisions at 5.02 TeV.

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Data from top left panel of Fig. 3

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Distributions of p$^{\mathrm{trk}}_{\mathrm{T}}$ in pp collisions at 5.02 TeV.

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Data from top right panel of Fig. 3

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Distributions of p$^{\mathrm{trk}}_{\mathrm{T}}$ in pp collisions at 5.02 TeV.

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Data from top panel of Fig. 3

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Distributions of p$^{\mathrm{trk}}_{\mathrm{T}}$ in 70-90% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 3

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Distributions of p$^{\mathrm{trk}}_{\mathrm{T}}$ in 50-70% centrality PbPb collisions at 5.02 TeV.

• #### Table 23

Data from top panel of Fig. 3

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Distributions of p$^{\mathrm{trk}}_{\mathrm{T}}$ in 30-50% centrality PbPb collisions at 5.02 TeV.

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Data from top panel of Fig. 3

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Distributions of p$^{\mathrm{trk}}_{\mathrm{T}}$ in 0-30% centrality PbPb collisions at 5.02 TeV.

• #### Table 25

Data from bottom panel of Fig. 3

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PbPb / pp ratio of p$^{\mathrm{trk}}_{\mathrm{T}}$ distributions for 70-90% centrality PbPb collisions at 5.02 TeV.

• #### Table 26

Data from bottom panel of Fig. 3

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PbPb / pp ratio of p$^{\mathrm{trk}}_{\mathrm{T}}$ distributions for 50-70% centrality PbPb collisions at 5.02 TeV.

• #### Table 27

Data from bottom panel of Fig. 3

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PbPb / pp ratio of p$^{\mathrm{trk}}_{\mathrm{T}}$ distributions for 30-50% centrality PbPb collisions at 5.02 TeV.

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Data from bottom panel of Fig. 3

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PbPb / pp ratio of p$^{\mathrm{trk}}_{\mathrm{T}}$ distributions for 0-30% centrality PbPb collisions at 5.02 TeV.