Measurement of B(J/psi -> eta ' e(+)e(-)) and search for a dark photon

Abstract

Using a data sample of (1310.6 +/- 7.0) x 10(6) J/psi decay events collected with the BESIII detector at BEPCII, we study the electromagnetic Dalitz decay J/psi -> eta ' e(+)e(-) with two dominant eta' decay modes, eta' -> gamma pi(+)pi(-) and eta' -> pi(+)pi(-)eta . The branching fraction is determined to be B(J/psi -> eta ' e(+)e(-)) = (6.59 +/- 0.07 +/- 0.17) x 10(-5), which improves in precision by a factor of 2 over the previous BESIII measurement. A search for the dark photon (gamma') is performed via J/psi -> eta'gamma',gamma' -> e(+)e(-). Excluding the w and phi mass regions, no significant signal is observed in the mass range from 0.1 to 2.1 GeV/e2. We set upper limits at the 90% confidence level on B(J/psi -> eta 'gamma') x B(gamma' -> e(+)e(-)), B(J/psi -> eta 'gamma') and the mixing strength as a function of dark photon mass. This is among the first searches for dark photons in charmonium decays.