CMS-PAS-HIN-19-006

CMS-PAS-HIN-19-006
Studies of parton-medium interactions using Z-tagged charged particles in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV
CMS Collaboration
May 2020

Abstract: The first measurements of Z boson-tagged charged hadron spectra are reported in PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5 TeV, using the PbPb collision data recorded by the CMS experiment at the LHC in 2018, corresponding to an integrated luminosity of 1.7 nb $^{-1}$ . Hadronic collision data with at least one Z boson with transverse momentum $p_{\,\mathrm{T}} >$ 30 GeV/ $c$ are analyzed. The azimuthal angle distributions with respect to the Z bosons, which are sensitive to medium recoils and modification of in-medium parton shower, are measured in PbPb collisions. The results indicate a modification of the angular correlation functions with respect to the reference measured in pp interactions at the same collision energy. The first measurements of Z-tagged fragmentation functions and charged particle $p_{\,\mathrm{T}}$ spectra are also reported. These measurements are sensitive to the modification of the longitudinal structure of the parton shower inside the medium. Significant modifications of the fragmentation functions and charged particle $p_{\,\mathrm{T}}$ spectra are observed.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Top : Distributions of ${\Delta \phi _{{\,\mathrm {trk,Z}}}}$ in pp collisions compared to PbPb collisions in 70-90% (left) 50-70%, 30-50%, and 0-30% (right) centrality intervals. Bottom : Difference between the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Figure 2: Top : Distributions of ${\xi ^{\,\mathrm {Z,trk}_{{\,\mathrm {T}}}}}$ in pp collisions compared to PbPb collisions in 70-90% (left), 50-70%, 30-50%, and 0-30% (right) centrality intervals. Bottom : ratio of the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Figure 3: Top : Distributions of ${{p_{{\,\mathrm {T}}} ^\text {trk}}}$ in pp collisions compared to PbPb collisions in 70-90% (left) 50-70%, 30-50%, and 0-30% (right) centrality intervals. Bottom : ratio of the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.

Summary

In summary, the first measurements of Z-tagged charged hadron spectra are reported in PbPb collisions at

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV, using the data collected in 2018. The corresponding integrated luminosity of the PbPb data is 1.7 nb

$^{-1}$ , which made the analysis possible for the first time. Collision data with at least one Z boson with

$p_{{\,\mathrm{T}}}^{\mathrm{Z}} >$ 30 GeV/

$c$ are analyzed. The azimuthal angle distributions with respect to the Z bosons, which are sensitive to the modification of in-medium parton shower and medium recoils, are measured in PbPb collisions. Comparison of the PbPb results to the reference pp results indicates a modification of the angular correlation functions. This modification extends to azimuthal angles close to the Z boson in most central PbPb events. The first measurement of Z-tagged fragmentation functions and

$p_{{\,\mathrm{T}}}^{\text{trk}}$ spectra, which are sensitive to the modification of the longitudinal structure of the parton shower inside the medium, are also reported. Significant modifications of the fragmentation functions and

$p_{{\,\mathrm{T}}}^{\text{trk}}$ spectra coming from the recoiled partons are observed. These results are complementary to the measurements using inclusive jets, dijet and photon-tagged jet events.

Additional Figures
png pdf	Additional Figure 1: Top: Distributions of ${\Delta \phi _{\mathrm {trk,Z}}}$ in pp collisions compared to PbPb collisions in 70-90% (left), 50-70%, 30-50%, and 0-30% (right) centrality intervals. Bottom: Ratio of the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 2: Top: Distributions of ${\Delta \phi _{\mathrm {trk,Z}}}$ in pp collisions compared to PbPb collisions in 70-90% (left) and 0-30% (right) centrality intervals. Bottom: Difference between the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 3: Top: Distributions of ${\xi ^\mathrm {Z,trk}_{\mathrm {T}}}$ in pp collisions compared to PbPb collisions in 70-90% (left) and 0-30% (right) centrality intervals. Bottom: Ratio of the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 4: Top: Distributions of ${{p_{\mathrm {T}}} ^\text {trk}}$ in pp collisions compared to PbPb collisions in 70-90% (left) and 0-30% (right) centrality intervals. Bottom: Ratio of the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 5: Top: Distributions of ${\Delta \phi _{\mathrm {trk,Z}}}$ in pp collisions compared to PbPb collisions in 70-90% (left) and 0-30% (right) centrality intervals. Bottom: Ratio of the PbPb and pp distributions. The vertical bars through the points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 6: Top: Distributions of ${\xi ^\mathrm {Z,trk}_{\mathrm {T}}}$ in pp collisions compared to PbPb collisions in 30-50% (left) and 0-30% (right) centrality intervals. Bottom: Ratio of the PbPb and pp distributions compared to calculations from the ${{\mathrm {{{SCET}}_G}}}$ [1,2] and Hybrid [3] theoretical models. The ${{\mathrm {{{SCET}}_G}}}$ curves represent different values of $g$ , the coupling between the jet and QCD medium. The vertical bars through the data points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 7: Top: Distributions of ${{p_{\mathrm {T}}} ^\text {trk}}$ in pp collisions compared to PbPb collisions in 30-50% (left) and 0-30% (right) centrality intervals. Bottom: Ratio of the PbPb and pp distributions compared to calculations from the ${{\mathrm {{{SCET}}_G}}}$ [1,2] and Hybrid [3] theoretical models. The ${{\mathrm {{{SCET}}_G}}}$ curves represent different values of $g$ , the coupling between the jet and QCD medium. The vertical bars through the data points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 8: Top: Distributions of ${\Delta \phi _{\mathrm {trk,Z}}}$ in pp collisions compared to PbPb collisions in 30-50% (left) and 0-30% (right) centrality intervals. Bottom: Difference between the PbPb and pp distributions compared to calculations from the Hybrid [3] theoretical model. The vertical bars through the data points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 9: Top: Distributions of ${\Delta \phi _{\mathrm {trk,Z}}}$ in pp collisions compared to PbPb collisions in 30-50% (left) and 0-30% (right) centrality intervals. Bottom: Ratio of the PbPb and pp distributions compared to calculations from the Hybrid [3] theoretical model. The vertical bars through the data points represent statistical uncertainties, while the shaded boxes indicate systematic uncertainties.
png pdf	Additional Figure 10: Invariant mass distributions of the selected muon pairs in PbPb data (markers) compared to the Z boson events simulated using {pythia} + {hydjet} [4,5] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.
png pdf	Additional Figure 11: Invariant mass distributions of the selected electron pairs in PbPb data (markers) compared to the Z boson events simulated using {pythia} + {hydjet} [4,5] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.
png pdf	Additional Figure 12: Invariant mass distributions of the selected muon pairs in pp data (markers) compared to the Z boson events simulated using {pythia} [4] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.
png pdf	Additional Figure 13: Invariant mass distributions of the selected electron pairs in pp data (markers) compared to the Z boson events simulated using {pythia} [4] (histogram). The simulation result is normalized to the number of opposite sign pairs in data.
png pdf	Additional Figure 14: 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.
png pdf	Additional Figure 15: Estimated contribution from background tracks (blue crosses) are subtracted from the ${\xi ^\mathrm {Z,trk}_{\mathrm {T}}}$ distribution (black squares) to obtain the distribution subtracted for background tracks (red circles) in 0-30% centrality PbPb collisions.
png pdf	Additional Figure 16: 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.
png pdf	Additional Figure 17: 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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