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Study of Z production in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV in the dimuon and dielectron decay channels

CMS Collaboration

arXiv:1410.4825v2nucl-exhep-ex

TL;DR

The study asks whether Z-boson production is modified in PbPb collisions despite initial-state nuclear effects. Using CMS dimuon and dielectron data at 2.76 TeV, it measures yields and compares PbPb with pp after binary-collision scaling. The scaled production is consistent with unity and shows no dependence on transverse momentum, rapidity, or collision centrality within uncertainties.

  • Problem

    The study addresses whether Z-boson production can provide an unmodified control of hot and dense nuclear matter despite possible initial-state effects.

  • Method

    CMS measures Z-boson yields versus transverse momentum, rapidity, and collision centrality in PbPb and pp data, then computes the binary-collision-scaled nuclear modification factor.

  • Results

    1.06 ± 0.05 (stat) ± 0.08 (syst) is the centrality-integrated dimuon RAA, while the combined RAA shows no dependence on transverse momentum, rapidity, or centrality within uncertainties.

  • Takeaways & Limitations

    Within the measured kinematic range and uncertainties, Z-boson production is not modified in PbPb collisions relative to binary-scaled pp production.

Abstract

from arXiv · show

The production of Z bosons is studied in the dimuon and dielectron decay channels in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV, using data collected by the CMS experiment at the LHC. The PbPb data sample corresponds to an integrated luminosity of about 150 inverse microbarns, while the pp data sample collected in 2013 at the same nucleon-nucleon centre-of-mass energy has an integrated luminosity of 5.4 inverse picobarns. The Z boson yield is measured as a function of rapidity, transverse momentum, and collision centrality. The ratio of PbPb to pp yields, scaled by the number of inelastic nucleon-nucleon collisions, is found to be 1.06 +/- 0.05 (stat) +/- 0.08 (syst) in the dimuon channel and 1.02 +/- 0.08 (stat) +/- 0.15 (syst) in the dielectron channel, for centrality-integrated Z boson production. This binary collision scaling is seen to hold in the entire kinematic region studied, as expected for a colourless probe that is unaffected by the hot and dense QCD medium produced in heavy ion collisions.

1 Introduction

The introduction motivates dileptonic Z bosons as control probes of hot, dense QCD matter while noting possible initial-state effects in heavy-ion production. It also outlines the PbPb and pp datasets used to determine the nuclear modification factor.

  • Motivation: Leptons from Z-boson decays pass through the medium without strong interactions, making dileptons controls for processes expected to be heavily modified.The introduction also notes that Z-boson production can be affected by initial-state effects in heavy-ion collisions.
  • Previous measurements: Earlier CMS measurements in first-LHC PbPb collisions found electroweak bosons essentially unmodified by the hot and dense medium.The cited measurements included Z → µ+µ−, W± → µ±ν, and isolated photon production, using about 7 µb−1.
  • Datasets and observable: The analysis uses 2011 PbPb data with about 166 µb−1 and 2013 pp data at the same energy with 5.4 pb−1 to measure the nuclear modification factor.RAA is introduced as the ratio of PbPb and pp yields, with the passage continuing beyond the supplied excerpt.

2 The CMS detector

The CMS detector is built around a 6 m superconducting solenoid providing a 3.8 T magnetic field, enclosing tracking and calorimetry systems, with muons measured in detectors outside the solenoid. Muon tracking achieves 1.3–2.0% transverse-momentum resolution in the barrel for 20 < pT < 100 GeV, while electrons are measured with the ECAL.

  • Detector overview: A 6 m superconducting solenoid provides a 3.8 T magnetic field around the silicon tracker, ECAL, and HCAL.The tracker and both calorimeters have barrel and two endcap sections.
  • Muon system: Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke, covering |η| < 2.4.The muon system uses drift tubes, cathode strip chambers, and resistive-plate chambers.
  • Muon system: 1.3–2.0% relative transverse-momentum resolution is achieved for muons with 20 < pT < 100 GeV in the barrel.The corresponding resolution is better than 6% in the endcaps.
  • Electromagnetic calorimeter: Electrons are measured in a lead tungstate crystal ECAL comprising 75 848 crystals.The ECAL provides pseudorapidity coverage in the barrel region.

3 Event selection and centrality determination

The analysis selects inelastic hadronic PbPb events while suppressing ultraperipheral, beam-background, and beam-gas contamination, then uses channel-specific triggers for PbPb and pp samples. PbPb collisions are categorized by geometrical centrality, with TAA used to normalize yields for comparison with pp production.

  • Event selection: PbPb events are preselected with a reconstructed primary vertex containing at least two tracks and at least three HF towers on each side, each depositing at least 3 GeV.Ultraperipheral collisions and non-collision beam backgrounds are removed, with additional beam-gas suppression based on pixel-detector hit distributions.
  • Trigger selection: For Z →µ+µ−, single-muon triggering requires pT greater than 15 GeV/c in PbPb, while pp uses a double-muon trigger without an explicit pT threshold.The corresponding trigger efficiencies within the analysis acceptance are approximately 99% for PbPb and 98% for pp.
  • Centrality determination: PbPb centrality is defined from the geometrical overlap of the incoming nuclei and divided into classes from peripheral, with little overlap, to central, with nearly complete overlap.CMS centrality bins correspond to fractions of the total hadronic inelastic cross section observed in the relevant distribution.
  • Nuclear overlap normalization: TAA-normalized corrected Z boson yields in PbPb are compared with pp production cross sections, with TAA interpreted as the NN-equivalent integrated luminosity per AA collision.The average TAA ranges from 0.47 ± 0.07 in the peripheral 50–100% interval to 23.2 ± 1.0 in the central 0–10% interval, in mb−1.

4 Lepton reconstruction

Leptons are reconstructed with dedicated algorithms adapted to the heavy-ion environment. Muon reconstruction efficiency improves substantially, while track-quality, vertex, and detector-based selections suppress backgrounds in both channels.

  • Muon reconstruction: Muon candidates require detector hits, at least four tracker layers, and a global-fit χ2 per degree of freedom below 10.Additional requirements include pixel-detector hits and transverse (longitudinal) vertex distances below 0.2 (5.0) mm.
  • Electron reconstruction: Electron reconstruction combines pixel and strip tracker information with ECAL measurements, using superclusters to estimate photon energy after bremsstrahlung.The method is designed for the heavy-ion environment, where bremsstrahlung spreads deposits azimuthally in the ECAL.
  • Electron reconstruction: Electron selection uses supercluster-track matching, shower shape, hadronic leakage, and transverse vertex-distance variables to reduce background.The same identification variables are used in PbPb and pp collisions, with more stringent criteria in pp.

5 Signal extraction, corrections, and systematic uncertainties

Z candidates are selected from opposite-charge lepton pairs in the 60–120 GeV/c^2 mass range with channel-specific kinematic and rapidity requirements. Yields are corrected using simulation-based acceptance and efficiency factors, with pT unfolding and quantified systematic uncertainties.

  • Signal selection and background: Z candidates are selected as opposite-charge lepton pairs with invariant mass 60–120 GeV/c^2, while same-charge pairs estimate combinatorial background.The combinatorial background is negligible at the 0.1% level in the muon channel and about 8% (4%) in the electron channel for PbPb (pp) data.
  • Signal selection and background: Muon candidates require pT > 20 GeV/c and |ημ| < 2.4, while electron candidates require pT > 20 GeV/c and |ηe| < 1.44.The visible rapidity ranges are |y| < 2.0 for dimuons and |y| < 1.44 for dielectrons.
  • Acceptance and efficiency corrections: PbPb acceptance and efficiency corrections use PYTHIA electroweak signals with GEANT4 detector simulation embedded in HYDJET heavy-ion events.Corrections are calculated separately in rapidity, pT, and event-centrality bins and applied consistently to numerator and denominator.
  • Momentum-resolution correction: Z-boson pT spectra are unfolded by inverting a simulation-derived response matrix before applying acceptance and efficiency corrections.The pT resolution varies from 7% (22%) at low pT to 2.5% (2.5%) at higher pT in the dimuon (dielectron) channels.
  • Acceptance and efficiency corrections: Approximately 70 (50)% acceptance is obtained in the muon (electron) rapidity ranges, with detection efficiency of 85 (55)% in PbPb and 90 (80)% in pp collisions.The paired values correspond respectively to muon (electron) channels and PbPb (pp) collision systems.
  • Systematic uncertainties: The combined trigger, reconstruction, and selection efficiency uncertainty is 1.8% (7.4%) for dimuons (dielectrons), while acceptance uncertainties are less than 2% and unfolding uncertainties less than 1%.For pp differential cross sections, luminosity determination contributes a 3.7% systematic uncertainty; centrality-integrated TAA contributes 6.2% relative uncertainty to RAA.

6 Results

The measured PbPb Z-boson yields agree with pp-based theoretical references after nuclear-overlap scaling, showing no strong dependence on transverse momentum, rapidity, or collision centrality. Within uncertainties, no significant nuclear modification is observed in either lepton channel or their combination.

  • 6.1 pp reference: The pp differential cross sections agree with POWHEG theoretical predictions for both dimuon and dielectron Z-boson candidates.Candidates are selected in the 60–120 GeV/c^2 mass range, with |y| < 2.0 for dimuons and |y| < 1.44 for dielectrons.
  • 6.2 PbPb yields: No strong centrality dependence is observed for the PbPb Z-boson yield divided by the nuclear overlap function TAA.The centrality-integrated value is compared with the pp → Z → ℓ+ℓ− cross section from POWHEG.
  • 6.2 PbPb yields: No strong deviations are observed in PbPb yield distributions versus pT and y relative to POWHEG and nuclear-PDF theoretical references.Theoretical comparisons include predictions with and without nuclear PDF modifications through EPS09.
  • 6.2 PbPb yields: PbPb yields are consistent with theoretical predictions, indicating scaling with the number of inelastic nucleon-nucleon collisions.Isospin and shadowing effects are small compared with the statistical uncertainties and cannot be distinguished.
  • 6.3 Nuclear modification: The RAA values show no dependence on pT, y, or centrality in either the muon or electron channel within the studied kinematic range and uncertainties.This indicates no observed variation in nuclear effects across these variables.
  • 6.4 Combined channels: The combined dilepton RAA likewise shows no dependence on pT, y, or centrality, indicating no modification of PbPb Z production relative to scaled pp collisions within uncertainties.The combined measurement uses dimuon and dielectron yields and cross sections.

7 Summary

The study measures Z-boson yields and pp cross sections in PbPb and pp collisions at √sNN = 2.76 TeV across kinematic and centrality variables, using dimuon and dielectron channels.

  • Z-boson yields were measured versus pT, y, and centrality in PbPb collisions in both dimuon and dielectron channels.
  • 166 µb−1 was the approximate integrated luminosity of the PbPb collision sample.
  • 5.4 pb−1 was the integrated luminosity of the pp sample used to measure Z →µ+µ− and Z →e+e− cross sections at the same collision energy.

Yerevan Physics Institute, Yerevan, Armenia

This section lists contributors and affiliated researchers, including S. Chatrchyan, V. Khachatryan, and A.M. Sirunyan, as well as a large author group from the Institut für Hochenergiephysik der OeAW in Vienna.

  • S. Chatrchyan, V. Khachatryan, and A.M. Sirunyan are listed among the contributors.
  • The Institut für Hochenergiephysik der OeAW in Wien, Austria is represented by a large group of named researchers.
  • Additional listed contributors include V. Mossolov, N. Shumeiko, and J. Suarez Gonzalez.

P.N. Lebedev Physical Institute, Moscow, Russia

The paper includes contributors affiliated with the P.N. Lebedev Physical Institute and the Skobeltsyn Institute of Nuclear Physics in Moscow, Russia.

  • The P.N. Lebedev Physical Institute is represented by V. Andreev, M. Azarkin, I. Dremin, M. Kirakosyan, A. Leonidov, G. Mesyats, S.V. Rusakov, and A. Vinogradov.
  • The Skobeltsyn Institute of Nuclear Physics at Lomonosov Moscow State University is represented by A. Belyaev, E. Boos, A. Ershov, A. Gribushin, A. Kaminskiy32, O. Kodolova, V. Korotkikh, I. Lokhtin, S. Obraztsov, S. Petrushanko, V. Savrin, A. Snigirev, and I. Vardanyan.

State Research Center of Russian Federation, Institute for High Energy Physics, Protvino, Russia

The State Research Center of the Russian Federation, Institute for High Energy Physics, Protvino, Russia, is represented by the listed researchers.

  • I. Azhgirey, I. Bayshev, S. Bitioukov, V. Kachanov, and A. Kalinin are listed under the institution.
  • D. Konstantinov, V. Krychkine, V. Petrov, R. Ryutin, and A. Sobol are also listed.
  • L. Tourtchanovitch, S. Troshin, N. Tyurin, A. Uzunian, and A. Volkov complete the listed researchers.

Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain

The section identifies CIEMAT in Madrid, Spain, and lists its associated authors.

  • Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain: CIEMAT, Madrid, Spain, is associated with J. Alcaraz Maestre, C. Battilana, E. Calvo, M. Cerrada, and additional contributors.The passage lists 28 individuals affiliated with this institution.

Universidad Aut´onoma de Madrid, Madrid, Spain

The section lists four contributors affiliated with Universidad Autónoma de Madrid in Madrid, Spain.

  • The listed contributors are C. Albajar, J.F. de Trocóniz, M. Missiroli, and D. Moran.Their names appear under the Universidad Autónoma de Madrid, Madrid, Spain affiliation.

Universidad de Oviedo, Oviedo, Spain … University of Wisconsin, Madison, USA

This author-affiliation section lists CMS collaborators from universities and research institutes across Europe, Asia, and the United States. The affiliations span the institutions represented in the merged sections, including CERN, national laboratories, and numerous universities.

  • CERN, European Organization for Nuclear Research, Geneva, Switzerland; Paul Scherrer Institut, Villigen, Switzerland; Institute for Particle Physics, ETH Zurich, Zurich, Switzerland; University of Bristol, Bristol, United Kingdom: European affiliations include CERN, Paul Scherrer Institut, ETH Zurich, Universit¨at Z¨urich, and the University of Bristol with Rutherford Appleton Laboratory.
  • National Central University, Chung-Li, Taiwan; National Taiwan University (NTU), Taipei, Taiwan; Chulalongkorn University, Faculty of Science, Department of Physics, Bangkok, Thailand; Imperial College, London, United Kingdom; Brunel University, Uxbridge, United Kingdom: Additional European and Asian affiliations include Imperial College, Brunel University, National Central University, National Taiwan University, and Chulalongkorn University.
  • Chulalongkorn University, Faculty of Science, Department of Physics, Bangkok, Thailand: The collaboration also includes Turkish institutions such as Cukurova, Middle East Technical, Bogazici, and Istanbul Technical Universities.
  • Baylor University, Waco, USA; The University of Alabama, Tuscaloosa, USA; Boston University, Boston, USA; Brown University, Providence, USA; University of California, Davis, Davis, USA; University of California, Los Angeles, USA: United States affiliations span Baylor, Alabama, Boston, Brown, and California universities, including institutions in Davis, Los Angeles, Riverside, San Diego, and Santa Barbara.
  • University of California, Riverside, Riverside, USA; University of California, San Diego, La Jolla, USA; University of California, Santa Barbara, Santa Barbara, USA; California Institute of Technology, Pasadena, USA; Carnegie Mellon University, Pittsburgh, USA; University of Colorado at Boulder, Boulder, USA; Cornell University, Ithaca, USA; Fermi National Accelerator Laboratory, Batavia, USA: Other U.S. collaborators are listed at Caltech, Carnegie Mellon, Colorado, Cornell, Fermilab, Florida institutions, Illinois Chicago, Johns Hopkins, Kansas, and Kansas State.
  • The Rockefeller University, New York, USA; Rutgers, The State University of New Jersey, Piscataway, USA; University of Tennessee, Knoxville, USA; Texas Tech University, Lubbock, USA; Vanderbilt University, Nashville, USA; University of Virginia, Charlottesville, USA; Wayne State University, Detroit, USA; University of Wisconsin, Madison, USA: The remaining U.S. affiliations include Rockefeller, Rutgers, Tennessee, Texas Tech, Vanderbilt, Virginia, Wayne State, and Wisconsin, alongside associated additional-affiliation notes.
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