Improved measurement of η/η′ mixing in B⁰_(s) → J/ψη^(′) decays

Branching fraction ratios between the decays  B⁰_(s) → 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

 B(B⁰ → J/ψη′ ) / B(B⁰ → J/ψη) = 0.48 ± 0.06 ± 0.02 ± 0.01,
 B(B⁰_s → J/ψη′ ) / B(B⁰_s → J/ψη) = 0.80 ± 0.02 ± 0.02 ± 0.01,

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, ϕP, and to probe the presence of a possible glueball component in the η′ meson, described by the gluonic mixing angle ϕG. The obtained results are

 ϕP = (41.6^+1.0_−1.2 )^◦,
 ϕG = (28.1^+3.9_−4.0 )^◦,

where the uncertainties are statistically dominated. While the value of ϕP is compatible with existing experimental determinations and theoretical calculations, the angle ϕG 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 B⁰_(s) → J/ψϕ decay, which serves as the normalisation channel. These results supersede the previous LHCb measurements and are the most precise to date.