First observation of the isospin violating decay j/psi -> lambda(sigma)over-bar(0) + c.c.

Abstract

Using a sample of (225.2 +/- 2.8) x 10(6) J/psi events collected with the BESIII detector, we present results of a study of J/psi -> gamma Lambda(Lambda) over bar and report the first observation of the isospin violating decay J/psi -> Lambda(Sigma) over bar (0) + c.c., in which (Sigma) over bar (0) decays to gamma (Lambda) over bar. The measured branching fractions are B(J/psi -> (Lambda) over bar Sigma(0)) = (1.46 +/- 0.11 +/- 0.12) x 10(-5) and B(J/psi -> Lambda(Sigma) over bar (0)) = (1.37 +/- 0.12 +/- 0.11) x 10(-5). We search for Lambda(1520) -> gamma Lambda decay, and find no evident signal, and an upper limit for the product branching fraction B(J/psi -> Lambda(Lambda) over bar (1520) + c.c) x B(Lambda(1520) -> gamma Lambda) < 4.1 x 10(-6) is set at the 90% confidence level. We also report the observation of eta(c) -> Lambda<(Lambda)over bar> in J/psi -> gamma eta(c), eta(c) -> Lambda(Lambda) over bar and measure the branching fraction B(eta(c) -> Lambda(Lambda) over bar) = (1.16 +/- 0.12(stat) +/- 0.19(syst) +/- 0.28(PDG)) x 10(-3).