First Observation of the Radiative Decay ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }\gamma$

The radiative decay ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }\gamma$ is observed for the first time using a data sample of proton-proton collisions corresponding to an integrated luminosity of $1.7{\mathrm{fb}}^{-1}$ collected by the LHCb experiment at a center-of-mass energy of 13 TeV. Its branching fraction is measured exploiting the $B^0\to K^{*0}\gamma$ decay as a normalization mode and is found to be $\mathcal{B}({\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }\gamma )=(7.1\pm 1.5\pm 0.6\pm 0.7)\times {10}^{-6}$, where the quoted uncertainties are statistical, systematic, and systematic from external inputs, respectively. This is the first observation of a radiative decay of a beauty baryon.

Figure 1

Simultaneous fit to the (top) ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }\gamma$ and (bottom) $B^0\to K^{*0}\gamma$ invariant-mass distributions of selected candidates. The data are represented by black dots and the result of the fit by a solid blue curve while individual contributions are represented in different line styles (see legend).