Improved measurement of $\eta /{\eta }^{\prime }$ mixing in $B_{(s)}^0\to J/\psi {\eta }^{(\prime )}$ decays

Abstract

Branching fraction ratios between the decays $B_{(s)}^0\to J/\psi {\eta }^{(\prime )}$ are measured using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of $7$, $8$ and $13\text{TeV}$, corresponding to an integrated luminosity of $9{\text{fb}}^{-1}$. The measured ratios of these branching fractions are $\frac{BF(B^0\to J/\psi {\eta }^{\prime })}{BF(B^0\to J/\psi \eta )}=0.48\pm 0.06\pm 0.02\pm 0.01$ and $\frac{BF(B_s^0\to J/\psi {\eta }^{\prime })}{BF(B_s^0\to J/\psi \eta )}=0.80\pm 0.02\pm 0.02\pm 0.01$, where the uncertainties are statistical, systematic and related to the precision of the ${\eta }^{(\prime )}$ branching fractions, respectively. They are used to constrain the $\eta /{\eta }^{\prime }$ mixing angle, ${\varphi }_P$, and to probe the presence of a possible glueball component in the ${\eta }^{\prime }$ meson, described by the gluonic mixing angle ${\varphi }_G$. The obtained results are ${\varphi }_P=(41.6_{-1.2}^{+1.0}{)}^{\circ }$ and ${\varphi }_G=(28.1_{-4.0}^{+3.9}{)}^{\circ }$, where the uncertainties are statistically dominated. While the value of ${\varphi }_P$ is compatible with existing experimental determinations and theoretical calculations, the angle ${\varphi }_G$ differs from zero by more than four standard deviations, which points to a substantial glueball component in the ${\eta }^{\prime }$ 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 $B_s^0\to J/\psi \varphi$ decay, which serves as the normalisation channel. These results supersede the previous LHCb measurements and are the most precise to date.

arXiv commentAll figures and tables, along with any supplementary material and additional information, are available at https://lbfence.cern.ch/alcm/public/analysis/full-details/4487

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