The E2/m1 Ratio, a Status Report

Recent determinations of the E2/M1 ratio are reviewed. Many aspects of these studies have been questioned. Here we focus on the selection of a database, methods used in the extraction of this ratio, and interpretation(s) of the results. I. THE DATA: OLD VS NEW Soon after the Baryons ’92 conference, the VPI group wrote a short note [1] claiming a value of −1.5± 0.5% for the E2/M1 ratio a value in good agreement with the results of an independent analysis from the RPI group [2]. Also in this paper was a value for the E2/M1 ratio obtained in a ‘forced fit’ to preliminary Σ data from the LEGS group. This forced fit yielded a ratio of −2.9%, a value consistent with the most recent joint analysis of LEGS photoproduction and Compton scattering data (−2.85± 0.34± 0.21%) [3]. Through the 90’s, the database for pion photoproduction benefited from experimental programs at Bonn, BNL, Mainz, SAL and Yerevan. Inverse reaction data were also measured at LANL and TRIUMF. Recent analyses from the Mainz, LEGS, and RPI groups imply that a much larger (larger than −1.5%) E2/M1 ratio is required to fit these more recent measurements. The Mainz group(s) find values of about −2.5% [4,5] while the LEGS [3] and RPI [6] groups have found values closer to (and slightly above) −3%. In order to understand this data dependence, several groups (LEGS, Mainz, RPI, and VPI) have made fits using various subsets of the existing database. The results have been very interesting. The Mainz and LEGS groups have shown that an E2/M1 ratio close to the VPI value can be obtained, within their fitting schemes, if particular sets of data are used. The VPI and RPI groups have also (jointly) found that very similar E2/M1 ratios result when identical databases are fitted. This implies that much of the disagreement between previous analyses has been database dependent and not model dependent. However, one significant problem remains to be resolved. The effect of different cross section measurements, from Bonn, Mainz and BNL, is significant for both the E2/M1 ratio and the M1 amplitude itself. In particular, the LEGS cross sections [7] are considerably larger than the Bonn and Mainz values. Hopefully the reason for this difference can be understood in the near future.