Study of hadronic transitions between Y states and observation of Y(4S)-> eta Y(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\invfb 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 BR((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)->etaY(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)->etaY(1S))/Gamma(Y(2S)->pi+pi-Y(1S))<5.2 x10^{-3}$ and Gamma(Y(3S)->etaY(1S))/Gamma(Y(3S)->pi+pi-Y(1S))<1.9x10^{-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(2S))/Gamma(Y(3S)->pi+pi-Y(1S))=0.577± 0.026_{stat}± 0.060_{syst}.