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We report the first observation of the radiative transitionh_b(1P)->eta_b(1S)gamma, where the h_b(1P) is produced inUpsilon(5S)->h_b(1P)pi+pi- dipion transitions. We measure the eta_b(1S) mass tobe (9401.0+-1.9+1.4-2.4)MeV/c^2 with a width of (12.4+5.5-4.6+11.5-3.4)MeV anda decay branching fraction ofBF[h_b(1P)->eta_b(1S)gamma]=(49.8+-6.8+10.9-5.2)%. The measured eta_b(1S) masscorresponds to a hyperfine splitting of (59.3+-1.9+2.4-1.4)MeV/c^2. This valuedeviates significantly from the current world average obtained frommeasurements of Upsilon(3S)->eta_b(1S)gamma and Upsilon(2S)->eta_b(1S)gammareactions. We also report updated results for the h_b(1P) mass(9899.0+-0.4+-1.0)MeV/c^2 and its hyperfine splitting (0.8+-1.1)MeV/c^2. Thesemeasurements are performed using a 121.4fb^-1 data sample collected at the peakof the Upsilon(5S) resonance with the Belle detector at the KEKBasymmetric-energy e+e- collider.

Observation of h_b(1P)->eta_b(1S)gamma