Measuring pion beta decay with high-energy pion beams

摘要

Improved measurements of the pion beta decay rate are possible with an intense high-energy pion beam. The rate for the decay (pi)(sup +) (yields) (pi)(sup 0)e(sup +)v(epsilon) is predicted by the Standard Model (SM) to be R((pi)(sup +) (yields) (pi)(sup 0)e(sup +)v(epsilon)) = 0.3999(plus minus)0.0005 s(sup (minus)1). The best experimental number, obtained using in-flight decays, is R((pi)(sup +) (yields) (pi)(sup 0)e(sup +)v(epsilon)) = 0.394 (plus minus) 0.015 s(sup (minus)1). A precise measurement would test the SM by testing the unitarity of the Cabibbo-Kobayashi-Maskawa matrix for which one analysis of the nuclear beta decay data has shown a 0.4% discrepancy. Several nuclear correction factors, needed for nuclear decay, are not present for pion beta decay, so that an experiment at the 0.2% level would be a significant one. Detailed study of possible designs will be needed, as well as extensive testing of components. The reduction of systematic errors to the 0.1% level can only be done over a period of years with a highly stable apparatus and beam. At a minimum, three years of occupancy of a beam line, with 800 hours per year, would be required.