The Experiment

Experiment E838, measurement of the $\pi ^-p\rightarrow \gamma \gamma n$ reaction on liquid hydrogen, was carried out at the Tri University Meson Facility (TRIUMF) cyclotron. Negative pions from the M9A meson channel were stopped in a liquid hydrogen target at the center of the RMC detector (26,32). Prompt photon-pairs from the decay of at-rest pionic hydrogen atoms were converted into $e^+e^-$ pairs by a lead converter and were tracked in a multi-wire proportional chamber and a large volume drift chamber.

During May-June, 1997, $\pi^-A \rightarrow \gamma\gamma n$ data was collected on solid CH$_2$ and carbon targets for trigger and acceptance studies and during December 1998 commissioning pion capture data on liquid hydrogen was taken at a low beam intensity with the RMC protium target. Finally, data for the $\pi ^-p\rightarrow \gamma \gamma n$ reaction on liquid hydrogen was collected during the April-May, 1999 production run. A summary of the run schedule and the data collected is listed in Table 3.1.

Table 3.1: Summary of Runs for E838.

Year	Run Conditions	Objectives	Run No.
May-June, 1997	CH$_2$ and C target	Acceptance and trigger	8057-8068
		studies
December, 1998	liq. H$_2$ target,	Determination of optimum	8134-8423
	low beam $\sim$ 70 $\mu$A	run conditions for E838
April-May, 1999	$\pi ^-p\rightarrow \gamma \gamma n$	Final E838 Production	8437-8910
	data on liq. H$_2$	Run