Partial wave analysis of J/ψ →γηη'

Based on a sample of (10.09 +/- 0.04 )x10(9) J /psi events collected with the BESIII detector operating at the BEPCII storage ring, a partial wave analysis of the decay J/psi -> gamma eta eta ' is performed. An isoscalar state with exotic quantum numbers J(P) (C)=1(-+), denoted as eta(1)(1855 ), has been observed for the first time with statistical significance larger than 19 sigma . Its mass and width are measured to be (1855 +/- 9(-1)(+6)) MeV /c(2) and (188 +/- 18(-8)(+3)) MeV , respectively. The first uncertainties are statistical and the second are systematic. The product branching fraction B (J /psi -> gamma eta(1)(1855 ))B (eta(1)(1855 )->eta eta ') is measured to be (2.70 +/- 0.41(-0.35)(+0.16))x10(-6) . In addition, an upper limit on the ratio of branching fractions B (f(0)(1710 )->eta eta ')/B (f(0)(1710 )->pi pi) is determined to be 1.61 x10(-3) at 90% confidence level, which lends support to the hypothesis that the f(0)(1710 ) has a large glueball component.