| Among all of the reaction channels described by Chiral Perturbation Theory (χPT), the decay process η → π0γγ stands out as a stringent test of the predictive power of this theory. A number of calculations of its partial width have been performed and in most cases, the central results are about half or less of the experimental result and agree only because of large theoretical and experimental uncertainties.; The tenuous agreement between the theory and the existing measurement motivated the measurement of BR(η → π0γγ) with the Crystal Ball. The Crystal Ball is a highly segmented, large solid angle, electromagnetic calorimeter constructed from 672 optically isolated, NaI crystals in a spherical arrangement that covers 93% of the total solid angle.; Over a two week period in 1998, 3 × 107 η mesons were produced via the reaction π−p → η n using the AGS at Brookhaven National Laboratory. From these, the branching ratio of the decay η → π0γγ was extracted: 2.7 ± 0.9(stat.) ± 0.8(syst.) × 10 −4. Experimental limitations (including crystal granularity) and background due to competing physical processes resulted in large uncertainties in this measurement.; This result differs from the existing measurement by nearly 2σ and is in considerably better agreement with most of the χPT calculations. It is possible to conclude that the χPT calculations appear to be largely correct, but large systematic and statistical errors in the measurement prevent selection between differing methodologies. |
