Search for the decays $B_{(c)}^{+}\to {\mu }^{+}{\nu }_{\mu }\gamma$

A search for the radiative leptonic decays $B^{+}\to {\mu }^{+}{\nu }_{\mu }\gamma$ and $B_c^{+}\to {\mu }^{+}{\nu }_{\mu }\gamma$ is performed using proton-proton collision data collected with the LHCb experiment at a center-of-mass energy of $13\mathrm{T}\mathrm{e}\mathrm{V}$, corresponding to an integrated luminosity of $5.4{\mathrm{f}\mathrm{b}}^{-1}$. No evidence for an excess of events over background is observed for either signal decay. Upper limits at 90% confidence level are set on the branching fractions: \begin{align} {\cal{B}}(B^+\toμ^+ν_μγ){E_γ^\ast > 1,\rm{GeV}} &< 4.0 \times 10^{-6},\ {\cal{B}}(B_c^+\toμ^+ν_μγ){E_γ^\ast > 1,\rm{GeV}} &< 1.6 \times 10^{-3}, \end{align} where the photon energy in the $B$-meson rest frame, $E_{\gamma }^{\ast }$, is required to be greater than $1\mathrm{G}\mathrm{e}\mathrm{V}$. This constitutes the first search for these decays at a hadron collider and the first experimental investigation of the $B_c^{+}\to {\mu }^{+}{\nu }_{\mu }\gamma$ decay to date.