First observation and branching fraction and decay parameter measurements of the weak radiative decay Xi0 --> Lambda e+e-

Abstract

The weak radiative decay Xi0 --> Lambda e+e- has been detected for the first time. We find 412 candidates in the signal region, with an estimated background of 15 +/- 5 events. We determine the branching fraction B(Xi0 --> Lambda e+e-) = [7.6 +/- 0.4(stat) +/- 0.4(syst) +/- 0.2(norm)] x 10^{-6}, consistent with an internal bremsstrahlung process, and the decay asymmetry parameter alpha_{XiLambdaee} = -0.8 +/- 0.2, consistent with that of Xi0 --> Lambda gamma. The charge conjugate reaction Xi0_bar --> Lambda_bar e+e- has also been observed.Comment: 20 pages, 5 figures, 4 tables; revised: 19 pages, 4 figures, 4 tables, after reviewers' comments: 1 figure removed, 1 figure corrected, minor editorial changes; to be published in Phys. Lett.