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| CMS-HIN-19-003 ; CERN-EP-2021-039 | ||
| Constraints on the initial state of PbPb collisions via measurements of Z boson yields and azimuthal anisotropy at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 25 March 2021 | ||
| Phys. Rev. Lett. 127 (2021) 102002 | ||
| Abstract: The CMS experiment at the LHC has measured the differential cross sections of Z bosons decaying to pairs of leptons, as functions of transverse momentum and rapidity, in lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The measured Z boson elliptic azimuthal anisotropy coefficient is compatible with zero, showing that Z bosons do not experience significant final-state interactions in the medium produced in the collision. Yields of Z bosons are compared to Glauber model predictions and are found to deviate from these expectations in peripheral collisions, indicating the presence of initial collision geometry and centrality selection effects. The precision of the measurement allows, for the first time, for a data-driven determination of the nucleon-nucleon integrated luminosity as a function of lead-lead centrality, thereby eliminating the need for its estimation based on a Glauber model. | ||
| Links: e-print arXiv:2103.14089 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The ${v_{2}}$ of Z bosons in PbPb collisions for various centrality bins. The error bars represent statistical uncertainties, while the boxes represent systematic uncertainties. A measurement from the ATLAS Collaboration at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV is also shown [5]. |
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Figure 2:
The Z boson differential cross section as a function of $ {| {y_{\mathrm{Z}}} |}$. The error bars represent statistical uncertainties, while the boxes represent systematic uncertainties. Predictions using one PDF and two different nPDF sets are also shown. |
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Figure 3:
The Z boson differential cross section as a function of ${{p_{\mathrm {T}}} ^{\mathrm{Z}}}$. The error bars represent statistical uncertainties, while the boxes represent systematic uncertainties. Predictions using one PDF and two different nPDF sets are also shown. The lower panel shows the ratio of the predictions to data. |
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Figure 4:
The ${T_{\mathrm {AA}}} $-normalized yields of Z bosons as a function of centrality. The error bars, hollow boxes, and solid gray boxes represent the statistical, systematic, and ${T_{\mathrm {AA}}}$ uncertainties, respectively. The value of the 0-90% data point, and the scaled HG-PYTHIA model are shown for comparison, with the width of the bands representing the contribution from the total 0-90% data point uncertainty. |
| Summary |
| In summary, Z boson yields and the elliptic flow coefficient (${v_{2}}$) have been measured with high precision as functions of centrality in lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The Z boson ${v_{2}}$ is compatible with zero, consistent with the expectation of no significant final-state interactions in the quark-gluon plasma (QGP). The differential cross section of Z boson production as a function of rapidity is found to lie on the upper edge of theoretical next-to-leading order (NLO) predictions derived with two different nuclear parton distribution functions. Discrepancies between data and NLO calculations are also observed in various regions of Z boson transverse momentum, indicative of missing higher-order theoretical corrections. Appropriately scaled Z boson yields are constant versus impact parameter for central and semi-central collisions, but a decreasing trend is seen for the first time for more peripheral events. This is compatible with the HG-PYTHIA model prediction, which accounts for initial collision geometry and centrality selection effects. These results provide a new experimental proxy for estimating the average nucleon-nucleon integrated luminosity as a function of centrality in heavy ion collisions. The ratio of Z boson yields in PbPb over pp collisions can be used as an alternative to Glauber-model-based scaling for hard scattering processes, which also automatically accounts for potential effects related to event selection and centrality calibration. Such a method provides a useful tool for future searches for the onset of QGP effects on colored hard probes in the medium produced in peripheral heavy ion collisions and small colliding systems. |
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Compact Muon Solenoid LHC, CERN |
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