CMS logoCMS event Hgg
Compact Muon Solenoid
LHC, CERN

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
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.
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 ${p_{\mathrm {T}}}$.
References
1 J. C. Collins and M. J. Perry Superdense matter: Neutrons or asymptotically free quarks? PRL 34 (1975) 1353
2 F. Karsch The phase transition to the quark gluon plasma: recent results from lattice calculations NP A 590 (1995) 367 hep-lat/9503010
3 D. A. Appel Jets as a probe of quark-gluon plasmas PRD 33 (1986) 717
4 J. P. Blaizot and L. D. McLerran Jets in expanding quark-gluon plasmas PRD 34 (1986) 2739
5 M. Gyulassy and M. Plumer Jet quenching in dense matter PLB 243 (1990) 432
6 STAR Collaboration Transverse-momentum and collision-energy dependence of high-$ p_{\mathrm{t}} $ hadron suppression in Au+Au collisions at ultrarelativistic energies PRL 91 (2003) 172302 nucl-ex/0305015
7 PHENIX Collaboration Suppression pattern of neutral pions at high transverse momentum in Au+Au collisions at $ \sqrt{s_{\rm{NN}}} = $ 200 GeV and constraints on medium transport coefficients PRL 101 (2008) 232301 0801.4020
8 ALICE Collaboration Centrality dependence of charged particle production at large transverse momentum in Pb--Pb collisions at $ \sqrt{s_{\rm{NN}}} = $ 2.76 TeV PLB 720 (2013) 52 1208.2711
9 ATLAS Collaboration Measurement of charged-particle spectra in Pb+Pb collisions at $ \sqrt{s_{_{\mathrm{NN}}}} = $ 2.76 TeV with the ATLAS detector at the LHC JHEP 09 (2015) 050 1504.04337
10 CMS Collaboration Charged-particle nuclear modification factors in PbPb and pPb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV JHEP 04 (2017) 039 CMS-HIN-15-015
1611.01664
11 CMS Collaboration Measurement of inclusive jet cross sections in pp and PbPb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV PRC 96 (2017) 015202 CMS-HIN-13-005
1609.05383
12 ATLAS Collaboration Centrality and rapidity dependence of inclusive jet production in $ \sqrt{s_\mathrm{NN}} = $ 5.02 TeV proton--lead collisions with the ATLAS detector PLB 748 (2015) 392 1412.4092
13 ALICE Collaboration Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb-Pb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV JHEP 09 (2015) 170 1506.03984
14 STAR Collaboration Dijet imbalance measurements in Au+Au and pp collisions at $ \sqrt{s_{_{\mathrm{NN}}}} = $ 200 GeV at STAR PRL 119 (2017) 062301 1609.03878
15 ATLAS Collaboration Centrality, rapidity and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV measured with the ATLAS detector PRC 93 (2016) 034914 1506.08552
16 CMS Collaboration Measurement of isolated photon production in pp and PbPb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV PLB 710 (2012) 256 CMS-HIN-11-002
1201.3093
17 CMS Collaboration Study of W boson production in PbPb and pp collisions at $ \sqrt{s_\mathrm{NN}}= $ 2.76 TeV PLB 715 (2012) 66 CMS-HIN-11-008
1205.6334
18 CMS Collaboration Study of Z production in PbPb and pp collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 2.76 TeV in the dimuon and dielectron decay channels JHEP 03 (2015) 022 CMS-HIN-13-004
1410.4825
19 V. Kartvelishvili, R. Kvatadze, and R. Shanidze On Z and Z+jet production in heavy ion collisions PLB 356 (1995) 589 hep-ph/9505418
20 X.-N. Wang, Z. Huang, and I. Sarcevic Jet quenching in the opposite direction of a tagged photon in high-energy heavy ion collisions PRL 77 (1996) 231 hep-ph/9605213
21 Z.-B. Kang, I. Vitev, and H. Xing Vector-boson-tagged jet production in heavy ion collisions at energies available at the CERN large hadron collider PRC 96 (2017) 014912 1702.07276
22 CMS Collaboration Observation and studies of jet quenching in PbPb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV PRC 84 (2011) 024906 CMS-HIN-10-004
1102.1957
23 CMS Collaboration The CMS experiment at the CERN LHC JINST 3 (2008) S08004 CMS-00-001
24 CMS Collaboration The CMS trigger system JINST 12 (2017) P01020 CMS-TRG-12-001
1609.02366
25 T. Sjostrand, S. Mrenna, and P. Z. Skands A brief introduction to PYTHIA 8.1 CPC 178 (2008) 852 0710.3820
26 CMS Collaboration Extraction and validation of a new set of CMS PYTHIA8 tunes from underlying-event measurements EPJC 80 (2020) CMS-GEN-17-001
1903.12179
27 J. Alwall et al. The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations JHEP 07 (2014) 079 1405.0301
28 I. P. Lokhtin and A. M. Snigirev A model of jet quenching in ultrarelativistic heavy ion collisions and high-$ p_{\mathrm{T}} $ hadron spectra at RHIC EPJC 45 (2006) 211 hep-ph/0506189
29 GEANT4 Collaboration GEANT4--a simulation toolkit NIMA 506 (2003) 250
30 CMS Collaboration Performance of Electron Reconstruction and Selection with the CMS Detector in Proton-Proton Collisions at $ \sqrt{s} = $ 8 TeV JINST 10 (2015), no. 06, P06005 CMS-EGM-13-001
1502.02701
31 CMS Collaboration Performance of CMS Muon Reconstruction in $ pp $ Collision Events at $ \sqrt{s} = $ 7 TeV JINST 7 (2012) P10002 CMS-MUO-10-004
1206.4071
32 CMS Collaboration Description and performance of track and primary-vertex reconstruction with the CMS tracker JINST 9 (2014) P10009 CMS-TRK-11-001
1405.6569
33 CMS Collaboration Observation of Medium-Induced Modifications of Jet Fragmentation in Pb-Pb Collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV Using Isolated Photon-Tagged Jets PRL 121 (2018), no. 24, 242301 CMS-HIN-16-014
1801.04895
34 CMS Collaboration Jet Shapes of Isolated Photon-Tagged Jets in Pb-Pb and pp Collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV PRL 122 (2019), no. 15, 152001 CMS-HIN-18-006
1809.08602
35 B. Efron Bootstrap methods: Another look at the jackknife Ann. Statist. 7 (1979), no. 1
36 W. Chen et al. Effects of jet-induced medium excitation in $ \gamma $-hadron correlation in A+A collisions PLB 777 (2018) 86 1704.03648
37 R. Kunnawalkam Elayavalli and K. C. Zapp Medium response in JEWEL and its impact on jet shape observables in heavy ion collisions JHEP 07 (2017) 141 1707.01539
Compact Muon Solenoid
LHC, CERN