W and Z production at $\sqrt{s}$ = 7 TeV with the LHCb experiment

We have measured the $W$ and $Z$ boson production cross-sections in proton-proton collisions at a centre-of-mass energy of 7 TeV, using approximately 16.5 pb$^{-1}$ of data. The $W$ and $Z$ bosons are reconstructed from muons with transverse momentum $p_T$ exceeding 20 GeV, pseudorapidity $\eta_{\mu}$ within $2.0 - 4.5$., and, in the case of the \zed, a dimuon invariant mass $m_{Z}$ in the range 81 $\leq m_{Z} \leq$ 101 GeV. We obtain: \begin{align} \sigma_{Z} (2.0 \leq \eta_{\mu} \leq4.5, p_T \geq 20, 81 \leq m_{Z} \leq 101) = 73\pm4\pm7.3 \textrm{ pb} ; &\\ \sigma_{W+} (2.0 \leq \eta_{\mu} \leq 4.5, p_T \geq 20) = 1007\pm48\pm101 \textrm{ pb}; &\\ \sigma_{W-} (2.0 \leq \eta_{\mu} \leq 4.5, p_T\geq 20) = 680\pm40\pm68 \textrm{ pb,} \end{align} where the first error is statistical and systematic combined, and the second arises from the luminosity uncertainty. The $W$ and $Z$ cross-section ratios ($\sigma_{W+}+\sigma_{W-}$) / $\sigma_{Z}$ and $\sigma_{W+} / \sigma_{W-}$, and the $W$ charge asymmetry \begin{align} {\sigma_{W+}- \sigma_{W-} \over \sigma_{W+}+ \sigma_{W-}}, \end{align} have also been measured within the same kinematic region. Part of this region ($\eta \geq 2.5$) is unique to LHCb, allowing these measurements to test the Standard Model and provide constraints to parton density functions in a new kinematic region. All results are consistent with next-to-leading order theoretical predictions.