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We report measurements of double radiative capture in pionic hydrogen andpionic deuterium. The measurements were performed with the RMC spectrometer atthe TRIUMF cyclotron by recording photon pairs from pion stops in liquidhydrogen and deuterium targets. We obtained absolute branching ratios of $(3.02\pm 0.27 (stat.) \pm 0.31 (syst.)) \times 10^{-5}$ for hydrogen and $(1.42 \pm^{0.09}_{0.12} (stat.) \pm 0.11 (syst.)) \times 10^{-5}$ for deuterium, andrelative branching ratios of double radiative capture to single radiativecapture of $(7.68 \pm 0.69(stat.) \pm 0.79(syst.)) \times 10^{-5}$ for hydrogenand $(5.44 \pm^{0.34}_{0.46}(stat.) \pm 0.42(syst.)) \times 10^{-5}$ fordeuterium. For hydrogen, the measured branching ratio and photon energy-angledistributions are in fair agreement with a reaction mechanism involving theannihilation of the incident $\pi^-$ on the $\pi^+$ cloud of the target proton.For deuterium, the measured branching ratio and energy-angle distributions arequalitatively consistent with simple arguments for the expected role of thespectator neutron. A comparison between our hydrogen and deuterium data andearlier beryllium and carbon data reveals substantial changes in the relativebranching ratios and the energy-angle distributions and is in agreement withthe expected evolution of the reaction dynamics from an annihilation process inS-state capture to a bremsstrahlung process in P-state capture. Lastly, wecomment on the relevance of the double radiative process to the investigationof the charged pion polarizability and the in-medium pion field.Comment: 44 pages, 7 tables, 13 figures, submitted to Phys. Rev.

Double radiative pion capture on hydrogen and deuterium and the nucleon's pion cloud