We report measurements of double radiative capture in pionic hydrogen and
pionic deuterium. The measurements were performed with the RMC spectrometer at
the TRIUMF cyclotron by recording photon pairs from pion stops in liquid
hydrogen and deuterium targets. We obtained absolute branching ratios of (3.02±0.27(stat.)±0.31(syst.))×10−5 for hydrogen and (1.42±0.120.09(stat.)±0.11(syst.))×10−5 for deuterium, and
relative branching ratios of double radiative capture to single radiative
capture of (7.68±0.69(stat.)±0.79(syst.))×10−5 for hydrogen
and (5.44±0.460.34(stat.)±0.42(syst.))×10−5 for
deuterium. For hydrogen, the measured branching ratio and photon energy-angle
distributions are in fair agreement with a reaction mechanism involving the
annihilation of the incident π− on the π+ cloud of the target proton.
For deuterium, the measured branching ratio and energy-angle distributions are
qualitatively consistent with simple arguments for the expected role of the
spectator neutron. A comparison between our hydrogen and deuterium data and
earlier beryllium and carbon data reveals substantial changes in the relative
branching ratios and the energy-angle distributions and is in agreement with
the expected evolution of the reaction dynamics from an annihilation process in
S-state capture to a bremsstrahlung process in P-state capture. Lastly, we
comment on the relevance of the double radiative process to the investigation
of the charged pion polarizability and the in-medium pion field.Comment: 44 pages, 7 tables, 13 figures, submitted to Phys. Rev.