Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at with the ATLAS experiment

This paper describes an analysis of the angular distribution of W→eν and W→μν decays, using data from pp collisions at $$\sqrt{s} = 7\ \mbox{TeV}$$ recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb−1. Using the decay lepton transverse momentum and the missing transverse momentum, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum $$(p_{\mathrm{T}}^{W} ): 35 < p_{\mathrm{T}}^{W}< 50\ \mbox{GeV}$$ and $$p_{\mathrm{T}}^{W}> 50\ \mbox{GeV}$$. Good agreement is found with theoretical predictions. For $$p_{\mathrm{T}}^{W}> 50\ \mbox{GeV}$$, the values of f0 and fL−fR, averaged over charge and lepton flavour, are measured to be: f0=0.127±0.030±0.108 and fL−fR=0.252±0.017±0.030, where the first uncertainties are statistical, and the second include all systematic effects.