Angular analysis of the $B^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ decay

This paper presents the first measurement of the forward-backward asymmetry, $A_{\mathrm{F}\mathrm{B}}$, and the flat term, $F_H$, that parameterise the angular distribution of the $B^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ decay. The proton-proton collision dataset used in the analysis corresponds to an integrated luminosity of 9 fb${}^{-1}$, collected with the LHCb experiment between 2011 and 2018. The analysis is performed in two intervals of dimuon mass squared, one above and one below the region containing the $J\text{\hspace{0.17em}⁣}\mathrm{/}\psi$ and $\psi (2S)$ narrow charmonium resonances. The Standard Model predictions lie within the obtained $68\mathrm{\%}$ confidence level interval in the high-mass and within the $99\mathrm{\%}$ interval in the low-mass region.