Abstract: A first measurement of the polarization of W bosons with large transverse momenta in pp collisions is resented. The measurement is based on 36pb?1 of data recorded at ?s=7?TeV by the CMS detector at the LHC. The left-handed, right-handed, and longitudinal polarization fractions (fL, fR, and f0, respectively) of W bosons with transverse momenta larger than 50 GeV are determined by using decays to both electrons and muons. The muon final state yields the most precise measurement: (fL?fR)?=0.240±0.036(stat)±0.031(syst) and f?0=0.183± 0.087(stat)±0.123(syst) for negatively charged W bosons and (fL?fR)+=0.310±0.036(stat)±0.017(syst) and f+0=0.171±0.085(stat)±0.099(syst) for positively charged W bosons. This establishes, for the first time, that W bosons produced in pp collisions with large transverse momenta are predominantly left-handed, as expected in the standard model.

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