Combination of CDF and D0 measurements of the W boson helicity in top quark decays - Cdf, T; Collaborations, D; Aaltonen, T; Abazov, V; Abbott, B; Acharya, B; Adams, M; Adams, T; Alexeev, G; Alkhazov, G; Alton, A; González, B; et al. (Physical Review D, 2012)