W and Z production in the forward region at LHCb

Abstract

Measurements of electroweak boson production in pp collisions at $\sqrt{s} = 7$ TeV are presented using the decays $W \rightarrow \mu\nu$, $Z \rightarrow \mu\mu$ and $Z \rightarrow \tau\tau$ recorded using the LHCb detector at the LHC. For muonic $W$ and $Z$ decays, the data sample corresponds to an integrated luminosity of 37.1 $\pm$ 1.3 pb$^{-1}$. Here the $W$ and $Z$ bosons are reconstructed from muons with transverse momenta, $p_{T}$, above 20 GeV and pseudorapidity, $\eta$, between 2 and 4.5, and, in the case of the $Z$, a dimuon invariant mass $M_{Z}$ between 60 GeV and 120 GeV. For $Z$ decays to tau lepton final states, 247 pb$^{-1}$ of data has been used. Here one tau is identified through its decay to a muon and neutrinos; the other through its decay to an electron or muon and neutrinos. The cross-sections are measured to be: $\sigma(W^{+}\rightarrow\mu^{+}\nu) = $ 808 $\pm$ 7 $\pm$ 28 $\pm$ 28 pb; $\sigma(W^{+}\rightarrow\mu^{+}\nu) = $ 634 $\pm$ 7 $\pm$ 21 $\pm$ 22 pb; $\sigma(Z\rightarrow\mu\mu) = $ 74.9 $\pm$ 1.6 $\pm$ 3.8 $\pm$ 2.6 pb; $\sigma(Z\rightarrow\tau\tau) = $ 82 $\pm$ 8 $\pm$ 7 $\pm$ 4 pb. Here the first error is statistical, the second is systematic and the third is due to the luminosity determination. For muonic final states, differential measurements, cross-section ratios and the $W$ charge asymmetry are also measured in the same kinematic region. The ratio of the $Z\rightarrow\tau\tau$ and $Z\rightarrow\mu\mu$ cross-sections has been measured to be 1.09 $\pm$ 0.17, consistent with lepton universality. Theoretical predictions, calculated at next-to-next-to-leading order (NNLO) in QCD using recent parton distribution functions, are found to be in agreement with the measurements.