Branching fraction measurements of $\eta$ and ${\eta }^{\prime }(958)$ anomalous decays

Abstract

Branching fractions of $\eta$ and ${\eta }^{\prime }(958)$ anomalous decays are measured using $pp$ collision data at a center-of-mass energy of $\sqrt{s}=13\text{TeV}$ collected by the LHCb experiment and corresponding to an integrated luminosity of $5.4{\text{fb}}^{-1}$. The measured values and upper limits are \begin{align*} \cal{B}(η\toμ^+μ^-) &= (5.02\pm0.14\pm0.18\pm0.26)\times10^{-6} , \\ \cal{B}(η'(958)\toμ^+μ^-) &<1.9\times10^{-7}\textrm{ at 90% CL} , \\ \cal{B}(η\toπ^+π^-μ^+μ^-) &<5.4\times10^{-7}\textrm{ at 90% CL}, \\ \cal{B}(η'(958)\toπ^+π^-μ^+μ^-) &= (2.73\pm0.14\pm0.13\pm0.14)\times10^{-5} , \end{align*} where the uncertainties are, respectively, statistical, systematic, and due to the external branching fraction of the $\varphi (1020)\to {\mu }^{+}{\mu }^{-}$ decay, which is used for normalization. This marks the first experimental search for the ${\eta }^{\prime }(958)$ decay, and the most precise measurement of the branching fraction $\mathcal{B}(\eta \to {\mu }^{+}{\mu }^{-})$ to date.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://lhcb-glance.cern.ch/alcm/public/analysis/full-details/4077/ (LHCb public pages)