CMS collaboration (2258 authors)Calpas, BettyNayak, ArunaBargassa, PedrameBeirão Da Cruz E Silva, CristóvãoDi Francesco, AgostinoFaccioli, PietroFerreira Parracho, Pedro GuilhermeGallinaro, MicheleHollar, JonathanLeonardo, NunoLloret Iglesias, LaraRodrigues Antunes, JoaoSeixas, JoaoToldaiev, OleksiiVadruccio, DanieleVarela, JoaoVischia, PietroDavid Tinoco Mendes, AndreSilva, PedroMusella, PasqualePela, Joao2019-02-042019-02-042016-11-10http://dx.doi.org/10.1016/j.physletb.2016.10.007http://hdl.handle.net/10400.26/27095The W boson helicity fractions from top quark decays in tt‾ events are measured using data from proton–proton collisions at a centre-of-mass energy of 8TeV . The data were collected in 2012 with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.8fb−1 . Events are reconstructed with either one muon or one electron, along with four jets in the final state, with two of the jets being identified as originating from b quarks. The measured helicity fractions from both channels are combined, yielding F0=0.681±0.012(stat)±0.023(syst) , FL=0.323±0.008(stat)±0.014(syst) , and FR=−0.004±0.005(stat)±0.014(syst) for the longitudinal, left-, and right-handed components of the helicity, respectively. These measurements of the W boson helicity fractions are the most accurate to date and they agree with the predictions from the standard model.engjournal article2019-02-04