Study of hadronic transitions between Υ states and observation of Υ(4S)→ηΥ(1S) decay

We present a study of hadronic transitions between Υ(mS) (m=4, 3, 2) and Υ(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 Υ(4S)→ηΥ(1S) decay with a branching fraction B(Υ(4S)→ηΥ(1S))=(1.96±0.06stat±0.09syst)×10-4 and measure the ratio of partial widths Γ(Υ(4S)→ηΥ(1S))/Γ(Υ(4S)→π+π-Υ(1S))=2.41±0.40stat±0.12syst. We set 90% CL upper limits on the ratios Γ(Υ(2S)→ηΥ(1S))/Γ(Υ(2S)→π+π-Υ(1S))<5.2×10-3 and Γ(Υ(3S)→ηΥ(1S))/Γ(Υ(3S)→π+π-Υ(1S))<1.9×10-2. We also present new measurements of the ratios Γ(Υ(4S)→π+π-Υ(2S))/Γ(Υ(4S)→π+π-Υ(1S))=1.16±0.16stat±0.14syst and Γ(Υ(3S)→π+π-Υ(2S))/Γ(Υ(3S)→π+π-Υ(1S))=0.577±0.026stat±0.060syst.