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The Crystal Ball at BNL Until recently, the Crystal Ball has resided at BNL, where Dr. Briscoe has been strongly involved with the program from its inception and serves on the collaboration executive committee. The Crystal Ball Collaboration has thus far produced 12 refereed journal articles, papers in 11 international conference proceedings (including some by Briscoe [Bri00]), and 28 presentations at conferences, workshops, and meetings. The p-p ® gn Reaction (Shafi, Briscoe, Strakovsky) Aziz Shafi has just defended his Ph.D. thesis, The Radiative Capture of Pions on the Proton [Sha03] and is preparing an article for PRC. His preliminary results were presented at APS meetings in 1999 (Atlanta) and 2001 (Maui) and were shown in a poster at the 2001 Low-q Workshop in Halifax; his final results were presented at the 2003 Low-q Workshop there. Figure 1 shows examples of our results for radiative capture and our largest background reaction, charge exchange (CEX), which in itself is a publishable result. We obtained data at 18 pion momenta at 18 angles and these data represent the most significant contribution to the SAID database for the pion radiative-capture process. Our agreement with experiments measuring the time-reversed reaction gn ® p-p (see below) and with the SAID [Arn02] and MAID [Kam01] predictions that do not include our data is excellent. The difficulty in doing the CEX background subtraction is the most likely cause of the disagreement with previous experiments that attempted to measure radiative capture directly. Our agreement with the measurements of the inverse process leads us to have confidence that we can rely on photoproduction measurements using the deuteron as a neutron target. This is important in both our JLab and MAMI programs.
Figure 17. Differential cross section and existing data at p p = 238 MeV/c for radiative capture [Tra79, Com75, Bou73] (left) and charge exchange [Com75, Bor82] (right). |
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