The pi- p --> gamma gamma n Reaction and Pion Compton Scattering

Abstract

We propose to measure the two--photon capture mode of pionic hydrogen using the RMC pair spectrometer at the TRIUMF cyclotron. Currently, only an experimental upper limit of B.R.<= 5.5E-4 is available for the two-gamma capture mode. Our new data will critically test the theoretical calculations and the intriguing predicted dominance of the pi pi --> gamma gamma annihilation graph. More speculatively, since crossing symmetry relates pi pi --> gamma gamma to gamma pi --> gamma pi, this threshold reaction may be a novel probe of the pion&#39;s electric polarizability. We show results of our 1997 engineering run and Monte Carlo studies for the two photon capture mode of pionic hydrogen and carbon. We also present our preliminary upper limit for the pi- p --> gamma gamma n B.R. <= 2.8E-4.