The $(π^-,γγ)$ program at TRIUMF

Abstract

We report the first observation of the doubly-radiative decay mode of pionic hydrogen (pi- p --> gamma gamma n) and and deuterium (pi- d --> gamma gamma X) using the RMC pair spectrometer at TRIUMF. The process is interesting in the context of the pi-Compton scattering and the chiPT predictions of the pion polarizability We present our preliminary values for the B.R.(pi- p --> gamma gamma n)=3.8x10^-5 and B.R.(pi- d --> gamma gamma X)=1.6x10^-5. Our pi- p data indicates the dominance of pi pi --> gamma gamma mechanism. The pi- d data shows no evidence for the d1* dibaryon.