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CMS-HIN-19-006 ; CERN-EP-2021-014
Using Z boson events to study parton-medium interactions in PbPb collisions
Phys. Rev. Lett. 128 (2022) 122301
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 nuclei (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}}} > $ 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.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Upper: Distributions of ${\Delta \phi _{\mathrm {trk,\mathrm{Z}}}}$ in pp collisions compared toPbPb collisions (left to right) in the 70-90 (left), 50-70, 30-50, and 0-30% (right) centrality intervals. Lower: Difference between thePbPb and pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Several model calculations are added for comparison: Hybrid [36] and CoLBT [32,58].

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Figure 2:
Upper: Distributions of ${\xi ^\mathrm {trk,\mathrm{Z}}_{\mathrm {T}}}$ in pp collisions compared toPbPb collisions (left to right) in the 70-90 (left), 50-70, 30-50, and 0-30% (right) centrality intervals. Lower: ratios of thePbPb to pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Several model calculations are added for comparison: Hybrid [36], CoLBT [32,58], and SCET$_G$ [56].

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Figure A1:
Upper: Distributions of $1/\mathrm {N_{\mathrm{Z}}} \, \mathrm {d}\mathrm {N}_{\text {trk},\mathrm{Z}}/ \mathrm {d} p_{\mathrm {T}} ^{\text {trk}}$ in pp collisions compared to PbPb collisions (left to right) in the 70-90 (left), 50-70, 30-50, and 0-30% (right) centrality intervals. Lower: ratios of the PbPb to pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Several model calculations are added for comparison: Hybrid [36], CoLBT [32,58], and SCET$_G$ [56].
Tables

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Table A1:
Relative systematic uncertainties for ${\Delta \phi _{\mathrm {trk,\mathrm{Z}}}}$, ${\xi ^\mathrm {trk,\mathrm{Z}}_{\mathrm {T}}}$, and ${{p_{\mathrm {T}}} ^\text {trk}}$, averaged over the whole distribution, for pp and PbPb collisions. The relative uncertainties were calculated separately for each ${\Delta \phi _{\mathrm {trk,\mathrm{Z}}}}$, ${\xi ^\mathrm {trk,\mathrm{Z}}_{\mathrm {T}}}$, and ${{p_{\mathrm {T}}} ^\text {trk}}$ bin, and then the quoted average was calculated assuming each bin has the same weight within individual distributions.
Summary
In summary, the measurements of charged hadrons produced in the shower of a parton originating in the same hard scattering with a Z boson, are reported in lead-lead (PbPb) and proton-proton (pp) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. Collision data with a Z boson candidate with transverse momentum $ {p_{\mathrm{T}}} > $ 30 GeV/$c$ are analyzed. The Z-tagged fragmentation functions and ${p_{\mathrm{T}}^{\text{trk}}}$ spectra, which probe the longitudinal structure of the parton shower inside the medium, are measured and significant modifications are observed. Particle yields, which are sensitive to modification of the in-medium parton shower and medium recoils, are measured for all charged particles as a function of the azimuthal angle ($\phi$) with respect to the Z boson momentum vector. Comparison of the PbPb and pp results indicates a modification of the angular correlation functions extending to $\phi$ angles close to the Z boson in central PbPb events. The data favor theoretical models that include the response of the medium to the traversing parton. These results represent the first studies of parton-medium interactions over all $\phi$ angles, in which the initial state of the scattered parton is known before it enters the medium.
Additional Figures

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Additional Figure 1:
Upper: Distributions of ${\xi ^\mathrm{trk,Z}_{\mathrm{T}}}$ in pp collisions compared to PbPb collisions in the 70-90 (left), 50-70, 30-50, and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb to pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The results from the isolated-photon-tagged jets measurements [1] are also shown. Several model calculations are added for comparison: Hybrid [36], CoLBT [32,58], and SCET$_G$ [56].

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Additional Figure 2:
Upper: Distributions of ${\Delta \phi _{\mathrm{trk,Z}}}$ in pp collisions compared to PbPb collisions (left to right) in the 70-90 (left), 50-70, 30-50, and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb and pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively.

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Additional Figure 3:
Upper: Distributions of ${\Delta \phi _{\mathrm{trk,Z}}}$ in pp collisions compared to PbPb collisions in the 70-90 (left) and 0-30% (right) centrality intervals. Lower: Difference between the PbPb and pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively.

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Additional Figure 4:
Upper: Distributions of ${\xi ^\mathrm{trk,Z}_{\mathrm{T}}}$ in pp collisions compared to PbPb collisions in the 70-90 (left) and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb to pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively.

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Additional Figure 5:
Upper: Distributions of ${{p_{\mathrm{T}}} ^\text {trk}}$ in pp collisions compared to PbPb collisions in the 70-90 (left) and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb to pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively.

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Additional Figure 6:
Upper: Distributions of ${\Delta \phi _{\mathrm{trk,Z}}}$ in pp collisions compared to PbPb collisions in the 70-90 (left) and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb and pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively.

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Additional Figure 7:
Upper: Distributions of ${\xi ^\mathrm{trk,Z}_{\mathrm{T}}}$ in pp collisions compared to PbPb collisions in the 30-50 (left) and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb to pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Several model calculations are added for comparison: Hybrid [36], CoLBT [32,58], and SCET$_G$ [56].

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Additional Figure 8:
Upper: Distributions of ${{p_{\mathrm{T}}} ^\text {trk}}$ in pp collisions compared to PbPb collisions in the 30-50 (left) and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb to pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Several model calculations are added for comparison: Hybrid [36], CoLBT [32,58], and SCET$_G$ [56].

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Additional Figure 9:
Upper: Distributions of ${\Delta \phi _{\mathrm{trk,Z}}}$ in pp collisions compared to PbPb collisions in the 30-50 (left) and 0-30% (right) centrality intervals. Lower: Difference between the PbPb and pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Several model calculations are added for comparison: Hybrid [36] and CoLBT [32,58].

png pdf
Additional Figure 10:
Upper: Distributions of ${\Delta \phi _{\mathrm{trk,Z}}}$ in pp collisions compared to PbPb collisions in the 30-50 (left) and 0-30% (right) centrality intervals. Lower: Ratios of the PbPb and pp distributions. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Hybrid [29] model calculations are added for comparison.

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Additional Figure 11:
Invariant mass distributions of the selected muon pairs in PbPb data (markers) compared to the Z boson events simulated using PYTHIA + HYDJET [43,44] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.

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Additional Figure 12:
Invariant mass distributions of the selected electron pairs in PbPb data (markers) compared to the Z boson events simulated using PYTHIA + HYDJET [43,44] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.

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Additional Figure 13:
Invariant mass distributions of the selected muon pairs in pp data (markers) compared to the Z boson events simulated using PYTHIA [43] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.

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Additional Figure 14:
Invariant mass distributions of the selected electron pairs in pp data (markers) compared to the Z boson events simulated using PYTHIA [43] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.

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Additional Figure 15:
Estimated contribution from background tracks (blue crosses) are subtracted from the ${\Delta \phi _{\mathrm{trk,Z}}}$ distribution (black squares) to obtain the distribution subtracted for background tracks (red circles) in 0-30% centrality PbPb collisions.

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Additional Figure 16:
Estimated contribution from background tracks (blue crosses) are subtracted from the ${\xi ^\mathrm{trk,Z}_{\mathrm{T}}}$ distribution (black squares) to obtain the distribution subtracted for background tracks (red circles) in 0-30% centrality PbPb collisions.

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Additional Figure 17:
Estimated contribution from background tracks (blue crosses) are subtracted from the ${{p_{\mathrm{T}}} ^\text {trk}}$ distribution (black squares) to obtain the distribution subtracted for background tracks (red circles) in 0-30% centrality PbPb collisions.

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Additional Figure 18:
The total energy in the forward rapidity region of 3 $ < {| \eta |} < $ 5 is calculated in each pp event where a Z boson is identified and there is no extraneous collision. The per event average of the energy is calculated as function of the Z boson ${p_{\mathrm{T}}}$.
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Compact Muon Solenoid
LHC, CERN