Improved measurement of η/η′ mixing in Bs0→J/ψη′ decays

Branching fraction ratios between the decays Bs0?J/??' are measured using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb^-1. The measured ratios of these branching fractions are (Formula presented.) where the uncertainties are statistical, systematic and related to the precision of the ?^(') branching fractions, respectively. They are used to constrain the ?/?' mixing angle, ?<inf>P</inf>, and to probe the presence of a possible glueball component in the ?' meson, described by the gluonic mixing angle ?<inf>G</inf>. The obtained results are (Formula presented.) where the uncertainties are statistically dominated. While the value of ?<inf>P</inf> is compatible with existing experimental determinations and theoretical calculations, the angle ?<inf>G</inf> differs from zero by more than four standard deviations, which points to a substantial glueball component in the ?' meson and/or unexpectedly large contributions from gluon-mediated processes in these decays. The absolute branching fractions are also measured relative to that of the well-established Bs0?J/?? decay, which serves as the normalisation channel. These results supersede the previous LHCb measurements and are the most precise to date.