Observation of double radiative capture on pionic hydrogen

We report the first observation of double radiative capture on pionic hydrogen. The experiment was conducted at the TRIUMF cyclotron using the RMC spectrometer, and detected $\gamma$–ray coincidences following ${\pi }^{-}$ stops in liquid hydrogen. We found the branching ratio for double radiative capture to be $(3.05\pm 0.27(stat\mathrm{.})\pm 0.31(syst\mathrm{.}))\times {10}^{-5}$. The measured branching ratio and angle-energy distributions support the theoretical prediction of a dominant contribution from the $\pi \pi \to \gamma \gamma$ annihilation mechanism.