Search for the C\!P-violating strong decays η→ π^+π^- and η^′(958) → π^+π^- - Aaij, R; Adeva, B; Adinolfi, M; Cheung, S; Evans, T; Gandini, P; Cazon, B; Hadavizadeh, T; Harnew, N; Hill, D; Hussain, N; Jalocha, J; et al. (Physics Letters B, 2016)