Study of associated charm production in W final states at sqrt s = 7 TeV

The cross section ratios $\sigma(W^+ \bar{c} + X)/\sigma(W^- c + X)$ and $\sigma(W + c + X)/\sigma(W + jets + X)$ at $\sqrt{s}=7$ TeV are measured with the CMS detector at the LHC, with a data sample corresponding to a total integrated luminosity of $36$ pb$^{-1}$. These ratios provide important information on the strange and anti-strange quark parton density functions of the proton at the electroweak scale. Using muonic decays of the W boson and lifetime tagging techniques to extract the charm fraction in the selected $W+jet$ sample, the following ratios are obtained: $\sigma(W^+ \bar{c} + X)/\sigma(W^- c + X) = 0.92 \pm 0.19~(stat.) \pm 0.04~(syst.)$ and $\sigma(W + c + X)/\sigma(W+ jet + X) = 0.143 \pm 0.015~(stat.) \pm 0.024~(syst.)$. The ratios are measured in the kinematic region $p_T^{jet}>20$ GeV, $|\eta^{jet}|<2.1$ for W decays into muons with $p_T^\mu>25$ GeV, $|\eta^{\mu}|<2.1$. Results are in agreement with theoretical predictions at next-to-leading order based on available parton distribution functions.