Hadronic transitions Y (2S)→Y(1S)

Using a 73.6 pb^-1 data sample of Y(2S) events collected with the CLEO II detector at the Cornell Electron Storage Ring, we have investigated the hadronic transitions between the Y(2S) and the Y(1S). The dipion transition Y(2S)→Y(1S)π⁺π was studied using two different analysis techniques. Selecting events in which Y(1S)→e⁺e^-,μ⁺μ^- ("exclusive" analysis), and using the Y(1S) leptonic branching fractions world averages from the PDG review, we obtained B(Y(2S)→Y(1S)π⁺π^-) = 0.189±0.004±0.010, while using a method allowing Y(1S)→anything ("inclusive" analysis) we obtained B(Y(2S)→Y(1S)π⁺π^- = 0.196 ±0.002±0.010. The appropriate weighted average of the two measurements gives B(Y(2S) →Y(1S)μ⁺μ^-) = 0.192±0.002±0.010. Combining the exclusive and inclusive results we derive the Y(1S) leptonic branching fractions B_ee = 0.0229±0.0008±0.0011 and B_μμ = 0.0249±0.0008±0.0013. We also studied Y(2S)→Y(1S)π⁰π⁰ and obtained B(Y(2S)→Y(1S)π⁰π⁰) = 0.092±0.006±0.008. Parameters of the ππ system (dipion invariant mass spectra, angular distributions) were analyzed and found to be consistent with current theoretical models. Lastly, we searched for the η and single π⁰ transitions and obtained the 90% confidence level upper limits B(Y(2S)→Y(1S)η)<0.0028 and B(Y(2S)→Y(1S)π⁰)<0.0011.