A measurement of the inclusive $W \to \mu\nu$ production cross-section using data from $pp$ collisions at a centre-of-mass energy of $\sqrt{ s} = 7 { \rm \, TeV}$ is presented. The analysis is based on an integrated luminosity of about $1.0 { \rm \, fb}^{ -1}$ recorded with the LHCb detector. Results are reported for muons with a transverse momentum greater than $20 { \rm \, Ge\kern -0.1em V\!/}c$ and pseudorapidity between 2.0 and 4.5. The $W^{ +}$ and $W^{ -}$ production cross-sections are measured to be $$\sigma_{ W^{ +} \to \mu^{ +}\nu} = 861.0 \pm 2.0 \pm 11.2 \pm 14.7 { \rm \, pb},$$ $$\sigma_{ W^{ -} \to \mu^{ -}\overline{ \nu}} = 675.8 \pm 1.9 \pm 8.8 \pm 11.6 { \rm \, pb},$$ where the first uncertainty is statistical, the second is systematic and the third is due to the luminosity determination. Cross-section ratios and differential distributions as functions of the muon pseudorapidity are also presented. The ratio of $W^{ +}$ to $W^{ -}$ cross-sections in the same fiducial kinematic region is determined to be $$\frac{ \sigma_{ W^{ +} \to \mu^{ +}\nu}}{ \sigma_{ W^{ -} \to \mu^{ -}\overline{ \nu}}} = 1.274 \pm 0.005 \pm 0.009,$$ where the uncertainties are statistical and systematic, respectively. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics.
Categories: hep-ex