Study of W boson production in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV

TL;DR

This CMS study measures $W$-boson production via $W\rightarrow\mu\nu$ in PbPb and pp collisions at $\sqrt{s_{NN}}=2.76\ \text{TeV}$ to probe initial-state effects and nuclear PDFs. Using a PbPb minimum-bias sample ($N_{MB}=55.7\times10^{6}$) and a pp reference ($231\ \text{nb}^{-1}$), the authors extract $W^+$ and $W^-$ yields in $|\eta^{\mu}|<2.1$ and $p_T^{\mu}>25\ \text{GeV}/c$, with background subtraction and efficiency corrections. They report centrality-independent yields when normalised by the nuclear overlap function $T_{AA}$, observe an isospin-driven difference between $W^+$ and $W^-$ in PbPb, and find agreement with NLO/NNLO QCD predictions using MSTW2008 PDFs with EPS09 nuclear corrections (yield asymmetry $A$ and $R_{AA}$ values are provided). The results demonstrate the robustness of $W$ production as a probe of initial-state effects and neutron PDFs, with implications for constraining nuclear parton densities and future $W$+jets studies.

Abstract

A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon-nucleon (NN) centre-of-mass energy sqrt(s[NN]) of 2.76 TeV at the LHC using the CMS detector. In data corresponding to an integrated luminosity of 7.3 inverse microbarns, the number of W to mu mu-neutrino decays is extracted in the region of muon pseudorapidity abs(eta[mu])<2.1 and transverse momentum pt[mu]>25 GeV. Yields of muons found per unit of pseudorapidity correspond to (159 +/- 10 (stat.) +/- 12 (syst.)) 10E-8 W(plus) and (154 +/- 10 (stat.) +/- 12 (syst.)) 10E-8 W(minus) bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at sqrt(s)= 2.76 TeV corresponding to an integrated luminosity of 231 inverse nanobarns. The individual W(plus) and W(minus) yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading order perturbative QCD calculations.

Figures (3)

• Figure 1: (a) Single-muon transverse-momentum spectrum for $\lvert \eta^{\mu} \rvert<$2.1 in PbPb data (red-filled circles). Signal (green-hatched histogram) and background (blue-dashed histogram) contributions are fitted (black solid line) to the data. (b) Mean value of ${p}_\mathrm{T}\space/$ for charged tracks as a function of centrality, before any event selection is applied on the muon-triggered data (black squares) and after it (red-filled circles), together with predictions from the pythia+hydjet samples (green triangles). (c) Transverse mass distribution for selected events in PbPb (red-filled circles) and $\mathrm{p}$$\mathrm{p}$ (blue open squares) data, compared to simulation (green-hatched histogram). The error bars represent statistical uncertainties. (See the text for more details).
• Figure 2: Centrality dependence of normalised $\mathrm{W}$$\rightarrow \mu \nu_{\mu}$ cross sections $(1/T_\mathrm{AA}) (N_\mathrm{W}/\Delta\eta)$ in PbPb collisions, for all $\mathrm{W}$ candidates (red-filled points) and, separated by charge, $\mathrm{W^+}$ (violet-filled squares) and $\mathrm{W^-}$ (green-filled stars). The open symbols at $N_\text{part} \approx 120$ represent the MB events. At $N_\text{part} = 2$, the corresponding cross sections are displayed for $\mathrm{p}$$\mathrm{p}$ collisions divided by $\Delta \eta$, for the same $\sqrt{s}$. For clarity, both $\mathrm{W^+}$ and $\mathrm{W^-}$ points are slightly shifted on the horizontal axis. The cross sections are given for the phase space region $|\eta^{\mu}|<2.1$ and $p_{\mathrm{T}}\xspace^{\mu} > 25$${\,\text{Ge\spaceV\space/\space}c}$. The error bars represent the statistical, and the horizontal lines the systematic, uncertainties.
• Figure 3: Charge asymmetry $(N_{\mathrm{W^+}} - N_{\mathrm{W^-}}) / (N_{\mathrm{W^+}} + N_{\mathrm{W^-}})$ as a function of muon pseudorapidity for PbPb (red-filled circles) and $\mathrm{p}$$\mathrm{p}$ (blue open squares) collisions at $\sqrt{s} = 2.76\,\text{Te\spaceV}\xspace$. Overlaid are predictions for $\mathrm{p}$$\mathrm{p}$ collisions from mcfm calculations with MSTW2008 PDF (blue solid curve), as well as expectations for PbPb collisions from mcfm with MSTW2008 and EPS09 PDF, that include isospin and nuclear effects (red dashed curve). Each prediction has an additional uncertainty of 5%, estimated by the uncertainty from the choice of PDF. The experimental points have an additional 1.1% systematic uncertainty that is not shown in the figure. The dot-dashed horizontal line is drawn only for reference.