Observation of e +e -→ηJ/ψ at center-of-mass energy √s=4.009GeV

Using a 478 pb(-1) data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider storage ring at a center-of-mass energy of root s = 4.009 GeV, the production of e(+)e(-) -> eta J/psi is observed for the first time with a statistical significance of greater than 10 sigma. The Born cross section is measured to be (32.1 +/- 2.8 +/- 1.3) pb, where the first error is statistical and the second systematic. Assuming the eta J/psi signal is from a hadronic transition of the psi(4040), the fractional transition rate is determined to be B(psi(4040) -> eta J/psi) = (5.2 +/- 0.5 +/- 0.2 +/- 0.5) x 10(-3), where the first, second, and third errors are statistical, systematic, and the uncertainty from the psi(4040) resonant parameters, respectively. The production of e(+)e(-) -> pi(0)J/psi is searched for, but no significant signal is observed, and B(psi(4040) -> pi(0)J/psi) < 2.8 x 10(-4) is obtained at the 90% confidence level.