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

Branching fraction ratios between the decays Bs0→J/ψη′ 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 η^(′) branching fractions, respectively. They are used to constrain the η/η′ mixing angle, ϕ_P, and to probe the presence of a possible glueball component in the η′ meson, described by the gluonic mixing angle ϕ_G. The obtained results are (Formula presented.) where the uncertainties are statistically dominated. While the value of ϕ_P is compatible with existing experimental determinations and theoretical calculations, the angle ϕ_G differs from zero by more than four standard deviations, which points to a substantial glueball component in the η′ 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/ψϕ decay, which serves as the normalisation channel. These results supersede the previous LHCb measurements and are the most precise to date.