Study of hadronic transitions between Y states and observation of Y(4S) → ηY(1S) decay

We present a study of hadronic transitions between Y(mS) (m = 4, 3, 2) and Y(nS) (n = 2, 1) resonances based on 347.5 fb(-1) of data taken with the BABAR detector at the PEP-II storage rings. We report the first observation of Y(4S) -> eta Y(1S) decay with a branching fraction B(Y(4S) -> eta Y(1S) = (1.96 +/- 0.06(stat) +/- 0.09(syst)) X 10(-4) and measure the ratio of partial widths Gamma(Y(4S) -> eta Y(1S))/Gamma(Y(4S) -> pi(+)pi Y-(1S)) = 2.41 +/- 0.40(stat) +/- 0.12(syst). We set 90% CL upper limits on the ratios Gamma(Y(2S) -> eta Y(1S))/Gamma(Y(2S) -> pi(+)pi Y-(1S) < 5.2 X 10(-3) and Gamma(Y(3S) -> eta Y(1S)/Gamma(Y(3S) -> pi(+)pi Y-(1S)) < 1.9 X 10(-2). We also present new measurements of the ratios Gamma(Y(4S) -> pi(+)pi Y-(2S)/Gamma(Y(4S) -> pi(+)pi Y-(1S)) = 1.16 +/- 0.16(stat) +/- 0.14(syst) and Gamma(Y(3S) -> pi(+)pi Y-(1S)) = 0.577 +/- 0.026(stat) +/- 0.060(syst).