Measurement of the production and lepton charge asymmetry of $\textit{W}$ bosons in Pb+Pb collisions at $\sqrt{s_{\mathrm{\mathbf{NN}}}}=$ 2.76 TeV with the ATLAS detector

TL;DR

This ATLAS study measures $W\rightarrow\ell\nu$ production and lepton charge asymmetry in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV, using track-based $p_T^{miss}$ in a high-multiplicity environment. The analysis combines muon and electron channels to extract differential yields as functions of $|\eta_\ell|$ and $\langle N_{part}\rangle$, and compares results to NLO QCD with and without EPS09 nuclear corrections. Yields per binary collision are largely centrality-independent and in agreement with theory, while the observed charge asymmetry reflects the neutron/proton composition of Pb and is consistent with nuclear PDFs within uncertainties. The results establish $W$ bosons as benchmarks for jet energy loss and as probes of PDFs in multi-nucleon systems, highlighting the need for more precise data to decisively confirm nuclear modifications.

Abstract

A measurement of $\textit{W}$ boson production in lead-lead collisions at $\sqrt{s_{\mathrm{NN}}}=$2.76 TeV is presented. It is based on the analysis of data collected with the ATLAS detector at the LHC in 2011 corresponding to an integrated luminosity of 0.14 $\mathrm{nb}^{-1}$ and 0.15 $\mathrm{nb}^{-1}$ in the muon and electron decay channels, respectively. The differential production yields and lepton charge asymmetry are each measured as a function of the average number of participating nucleons $< N_{\mathrm{part}} >$ and absolute pseudorapidity of the charged lepton. The results are compared to predictions based on next-to-leading-order QCD calculations. These measurements are, in principle, sensitive to possible nuclear modifications to the parton distribution functions and also provide information on scaling of $\textit{W}$ boson production in multi-nucleon systems.

Figures (11)

• Figure 1: Muon transverse momentum distribution in the data (points) before applying the signal selection requirements. The $p_{\mathrm{T}}$ distribution of QCD multi--jet processes from the MC simulation is also shown in the same figure. The shaded histogram is scaled to $\langle N_{\mathrm{coll}} \rangle$ and the solid histogram is rescaled to match the data in a control region $10<p_{\mathrm{T}}^{\mu}<20 \mathrm{\ Ge V} \textrm{Ge V}$. The background fraction from QCD multi--jet processes is determined from the number of muons in the MC surviving the final selection criteria.
• Figure 2: Measured muon absolute pseudorapidity (top) and transverse momentum (bottom) distributions for $W^{+}\rightarrow\mu^{+}\nu_{\mu}$ (left) and $W^{-}\rightarrow\mu^{-}\bar{\nu_{\mu}}$ (right) candidates after applying the complete set of selection requirements in the fiducial region, $p_{\mathrm{T}}^\mu>25\mathrm{\ Ge V} \textrm{Ge V}, p_{\mathrm{T}}^{\mathrm{miss}}>25\mathrm{\ Ge V} \textrm{Ge V}, m_{\mathrm{T}}>40\mathrm{\ Ge V} \textrm{Ge V}$ and $0.1<|\eta_\mu|<2.4$. The contributions from electroweak and QCD multi--jet processes are normalised according to their expected number of events. The $W\rightarrow\mu\nu_{\mu}$ MC events are normalised to the number of background--subtracted events in the data. The background and signal predictions are added sequentially.
• Figure 3: Measured missing transverse momentum (top) and transverse mass (bottom) distributions for $W^{+}\rightarrow\mu^{+}\nu_{\mu}$ (left) and $W^{-}\rightarrow\mu^{-}\bar{\nu_{\mu}}$ (right) candidates after applying the complete set of selection requirements in the fiducial region, $p_{\mathrm{T}}^\mu>25\mathrm{\ Ge V} \textrm{Ge V}, p_{\mathrm{T}}^{\mathrm{miss}}>25\mathrm{\ Ge V} \textrm{Ge V}, m_{\mathrm{T}}>40\mathrm{\ Ge V} \textrm{Ge V}$ and $0.1<|\eta_\mu|<2.4$. The contributions from electroweak and QCD multi--jet processes are normalised according to their expected number of events and added sequentially. The $W\rightarrow\mu\nu_{\mu}$ MC events are normalised to the number of background--subtracted events in the data. The background and signal predictions are added sequentially.
• Figure 4: Electron transverse momentum distribution in the data (points). The $p_{\mathrm{T}}$ distribution of multi--jet events from a data control sample (see text) and of simulated electroweak processes ($W\rightarrow\tau\nu_{\tau}$ and $Z\rightarrow e^+e^-$) are also shown. The total uncertainties from the fit are shown as solid grey bands.
• Figure 5: Measured electron absolute pseudorapidity (top) and transverse momentum (bottom) distributions for $W^{+}\rightarrow e^{+}\nu_{e}$ (left) and $W^{-}\rightarrow e^{-}\bar{\nu_{e}}$ (right) candidates after applying the complete set of selection requirements in the fiducial region, $p_{\mathrm{T}}^e>25\mathrm{\ Ge V} \textrm{Ge V}, p_{\mathrm{T}}^{\mathrm{miss}}>25\mathrm{\ Ge V} \textrm{Ge V}, m_{\mathrm{T}}>40\mathrm{\ Ge V} \textrm{Ge V}$ and $|\eta_e|<2.47$ excluding the transition region ($1.37<|\eta_e|<1.52$). The contributions from electroweak and QCD multi--jet processes are normalised according to their expected number of events. The $W\rightarrow e\nu_{e}$ MC events are normalised to the number of background--subtracted events in the data. The background and signal predictions are added sequentially.