Study of Z boson production in PbPb collisions at nucleon-nucleon centre of mass energy = 2.76 TeV

Abstract

A search for Z bosons in the mu^+mu^- decay channel has been performed in PbPb collisions at a nucleon-nucleon centre of mass energy = 2.76 TeV with the CMS detector at the LHC, in a 7.2 inverse microbarn data sample. The number of opposite-sign muon pairs observed in the 60--120 GeV/c^2 invariant mass range is 39, corresponding to a yield per unit of rapidity (y) and per minimum bias event of (33.8 +/- 5.5 (stat) +/- 4.4 (syst)) 10^{-8}, in the |y|<2.0 range. Rapidity, transverse momentum, and centrality dependencies are also measured. The results agree with next-to-leading order QCD calculations, scaled by the number of incoherent nucleon-nucleon collisions.

Figures (2)

• Figure 1: Dimuon invariant mass spectra. Full squares are opposite-sign dimuons, while the empty circle shows a unique like-sign dimuon candidate. The histogram shows the corresponding distribution measured in pp collisions at 7 TeV within 60--120 GeV/$c^2$, scaled to the 39 PbPb candidates.
• Figure 2: The yields of ${Z} \rightarrow \mu\mu$ per event: a) $dN/dy$ divided by the expected nuclear overlap function $T_{\rm AB}$ and as a function of event centrality parameterised as the number of participating nucleons $N_{\rm part}$, b) $dN/dy$ versus the ${Z}$ boson $y$, c) $d^2N/dydp_{\rm T}$ versus the ${Z}$ boson $p_{\rm T}$. Data points are located horizontally at average values measured within a given bin. Vertical lines (bands) correspond to statistical (systematic) uncertainties. Theoretical predictions are computed within the same bins as the data, and are described in the text.