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ISSN - 00319007, EISSN - 10797114, Physical Review Letters, 2008, vol. 101, p. -

Observation of the bottomonium ground state in the decay Υ(3S)→γηb

Aubert B., Bona M., Karyotakis Y., Lees J. P., Poireau V., Prencipe E., Prudent X., Tisserand V., Garra Tico J., Grauges E., Lopez L., Palano A., Pappagallo M., Eigen G., Stugu B., Sun L., Abrams G. S., Battaglia M., Brown D. N., Cahn R. N., Jacobsen R. G., Kerth L. T., Kolomensky Yu G., Lynch G., Osipenkov I. L., Ronan M. T., Tackmann K., Tanabe T., Hawkes C. M., Soni N., Watson A. T., Koch H., Schroeder T., Walker D., Asgeirsson D. J., Fulsom B. G., Hearty C., Mattison T. S., McKenna J. A., Barrett M., Khan A., Blinov V. E., Bukin A. D., Buzykaev A. R., Druzhinin V. P., Golubev V. B., Onuchin A. P., Serednyakov S. I., Skovpen Yu I., Solodov E. P., Todyshev K. Yu, Bondioli M., Curry S., Eschrich I., Kirkby D., Lankford A. J., Lund P., Mandelkern M., Martin E. C., Stoker D. P., Abachi S., Buchanan C., Gary J. W., Liu F., Long O., Shen B. C., Vitug G. M., Yasin Z., Zhang L., Sharma V., Campagnari C., Hong T. M., Kovalskyi D., Mazur M. A., Richman J. D., Beck T. W., Eisner A. M., Flacco C. J., Heusch C. A., Kroseberg J., Lockman W. S., Martinez A. J., Schalk T., Schumm B. A., Seiden A., Wilson M. G., Winstrom L. O., Cheng C. H., Doll D. A., Echenard B., Fang F., Hitlin D. G., Narsky I., Piatenko T., Porter F. C., Andreassen R., Mancinelli G., Meadows B. T., Mishra K., Sokoloff M. D.

We report the results of a search for the bottomonium ground state ηb(1S) in the photon energy spectrum with a sample of (109±1) million of Υ(3S) recorded at the Υ(3S) energy with the BABAR detector at the PEP-II B factory at SLAC. We observe a peak in the photon energy spectrum at Eγ=921.2-2.8+2.1(stat)±2.4(syst)MeV with a significance of 10 standard deviations. We interpret the observed peak as being due to monochromatic photons from the radiative transition Υ(3S) →γηb(1S). This photon energy corresponds to an ηb(1S) mass of 9388.9-2.3+3.1(stat)±2.7(syst)MeV/c2. The hyperfine Υ(1S)- ηb(1S) mass splitting is 71.4-3.1+2.3(stat)±2.7(syst)MeV/c2. The branching fraction for this radiative Υ(3S) decay is estimated to be [4.8±0.5(stat)±1.2(syst)]×10-4. © 2008 The American Physical Society.