CMS-HIN-13-004

CMS-HIN-13-004 ; CERN-PH-EP-2014-235
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
17 October 2014
J. High Energy Phys. 03 (2015) 022
Abstract: The production of Z bosons is studied in the dimuon and dielectron decay channels in PbPb and pp collisions at $\sqrt{s_{\mathrm{NN}}}$ = 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 $\mu\mathrm{b}^{-1}$ , while the pp data sample collected in 2013 at the same nucleon-nucleon centre-of-mass energy has an integrated luminosity of 5.4 $\mathrm{pb}^{-1}$ . 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 $\pm$ 0.05 (stat) $\pm$ 0.08 (syst) in the dimuon channel and 1.02 $\pm$ 0.08 (stat) $\pm$ 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.
Links: e-print arXiv:1410.4825 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; Public twiki page ; HepData record ; CADI line (restricted) ;

Figures	CMS Publications

Figures
png pdf	Figure 1-a: Dimuon invariant mass spectra for muons with ${\| \eta ^ {{\mu }} \| } < 2.4$ and ${p_{\mathrm {T}}} ^\mu > 20$ GeV/ $c$ in PbPb (a) and in pp (c) collisions and dielectron invariant mass spectra for electrons with ${\| \eta ^ {\mathrm {e}} \| } < 1.44$ and ${p_{\mathrm {T}}} ^{ {\mathrm {e}}}>20$ GeV/ $c$ in PbPb (b) and in pp (d) collisions. Full black circles represent opposite-charge lepton pair events and open black squares represent same-charge lepton pair events. Superimposed and normalized to the number of Z boson candidates in data is the MC simulation from PYTHIA $\mathrm {NN}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ , where N is a nucleon from the proper mix of protons and neutrons, embedded in HYDJET simulated events for the PbPb case, and ${\mathrm {p}} {\mathrm {p}}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ for the pp case.
png pdf	Figure 1-b: Dimuon invariant mass spectra for muons with ${\| \eta ^ {{\mu }} \| } < 2.4$ and ${p_{\mathrm {T}}} ^\mu > 20$ GeV/ $c$ in PbPb (a) and in pp (c) collisions and dielectron invariant mass spectra for electrons with ${\| \eta ^ {\mathrm {e}} \| } < 1.44$ and ${p_{\mathrm {T}}} ^{ {\mathrm {e}}}>20$ GeV/ $c$ in PbPb (b) and in pp (d) collisions. Full black circles represent opposite-charge lepton pair events and open black squares represent same-charge lepton pair events. Superimposed and normalized to the number of Z boson candidates in data is the MC simulation from PYTHIA $\mathrm {NN}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ , where N is a nucleon from the proper mix of protons and neutrons, embedded in HYDJET simulated events for the PbPb case, and ${\mathrm {p}} {\mathrm {p}}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ for the pp case.
png pdf	Figure 1-c: Dimuon invariant mass spectra for muons with ${\| \eta ^ {{\mu }} \| } < 2.4$ and ${p_{\mathrm {T}}} ^\mu > 20$ GeV/ $c$ in PbPb (a) and in pp (c) collisions and dielectron invariant mass spectra for electrons with ${\| \eta ^ {\mathrm {e}} \| } < 1.44$ and ${p_{\mathrm {T}}} ^{ {\mathrm {e}}}>20$ GeV/ $c$ in PbPb (b) and in pp (d) collisions. Full black circles represent opposite-charge lepton pair events and open black squares represent same-charge lepton pair events. Superimposed and normalized to the number of Z boson candidates in data is the MC simulation from PYTHIA $\mathrm {NN}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ , where N is a nucleon from the proper mix of protons and neutrons, embedded in HYDJET simulated events for the PbPb case, and ${\mathrm {p}} {\mathrm {p}}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ for the pp case.
png pdf	Figure 1-d: Dimuon invariant mass spectra for muons with ${\| \eta ^ {{\mu }} \| } < 2.4$ and ${p_{\mathrm {T}}} ^\mu > 20$ GeV/ $c$ in PbPb (a) and in pp (c) collisions and dielectron invariant mass spectra for electrons with ${\| \eta ^ {\mathrm {e}} \| } < 1.44$ and ${p_{\mathrm {T}}} ^{ {\mathrm {e}}}>20$ GeV/ $c$ in PbPb (b) and in pp (d) collisions. Full black circles represent opposite-charge lepton pair events and open black squares represent same-charge lepton pair events. Superimposed and normalized to the number of Z boson candidates in data is the MC simulation from PYTHIA $\mathrm {NN}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ , where N is a nucleon from the proper mix of protons and neutrons, embedded in HYDJET simulated events for the PbPb case, and ${\mathrm {p}} {\mathrm {p}}\to {\mathrm {Z}}\to {{\mu ^+}} {{\mu ^-}}$ or ${\mathrm {e}^+} {\mathrm {e}^-}$ for the pp case.
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