Measurement of the W boson helicity using \ttbar\ events in the dilepton final state at sqrt(s) = 8 TeV

A measurement of the W boson helicity fractions in top quark decays is presented. The measurement is performed using a sample of top quark pair events with two opposite-sign leptons (electron or muon) in the final state. The sample was recorded in proton-proton collisions at a center-of-mass energy of 8 TeV, and corresponds to an integrated luminosity of 19.7fb$^{-1}$. The longitudinal, left-handed and right-handed helicity fractions are measured to be $F_{\rm 0}=0.653\pm 0.016 (\text{stat}) \pm 0.024 (\text{syst})$, $F_{\rm L}=0.329\pm 0.009 (\text{stat}) \pm 0.025 (\text{syst})$ and $F_{\rm R}=0.018\pm 0.008 (\text{stat}) \pm 0.026 (\text{syst})$, respectively. The results are found to be in agreement with predictions of the standard model.