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arXiv · 1807.01201

Observation of $\Upsilon(2S)\to\gamma \eta_{b}(1S)$ decay

Belle Collaboration·B. G. Fulsom·T. K. Pedlar·I. Adachi·H. Aihara·S. Al Said·D. M. Asner·H. Atmacan·V. Aulchenko·T. Aushev·R. Ayad·V. Babu·I. Badhrees·A. M. Bakich·V. Bansal·P. Behera·C. Beleño·M. Berger·V. Bhardwaj·B. Bhuyan·T. Bilka·J. Biswal·A. Bondar·G. Bonvicini·A. Bozek·M. Bračko·T. E. Browder·L. Cao·D. Červenkov·V. Chekelian·A. Chen·B. G. Cheon·K. Chilikin·K. Cho·S. -K. Choi·Y. Choi·S. Choudhury·D. Cinabro·S. Cunliffe·N. Dash·S. Di Carlo·J. Dingfelder·Z. Doležal·T. V. Dong·Z. Drásal·S. Eidelman·D. Epifanov·J. E. Fast·T. Ferber·R. Garg·V. Gaur·N. Gabyshev·A. Garmash·M. Gelb·A. Giri·P. Goldenzweig·E. Guido·J. Haba·K. Hayasaka·H. Hayashii·S. Hirose·W. -S. Hou·T. Iijima·K. Inami·G. Inguglia·A. Ishikawa·R. Itoh·M. Iwasaki·Y. Iwasaki·W. W. Jacobs·H. B. Jeon·S. Jia·Y. Jin·D. Joffe·K. K. Joo·T. Julius·T. Kawasaki·H. Kichimi·C. Kiesling·D. Y. Kim·H. J. Kim·J. B. Kim·K. T. Kim·S. H. Kim·K. Kinoshita·P. Kodyš·S. Korpar·D. Kotchetkov·P. Križan·R. Kroeger·P. Krokovny·T. Kuhr·R. Kulasiri·A. Kuzmin·Y. -J. Kwon·J. S. Lange·I. S. Lee·S. C. Lee·L. K. Li·Y. B. Li
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Abstract

We report the observation of $\Upsilon(2S)\to\gamma\eta_{b}(1S)$ decay based on analysis of the inclusive photon spectrum of $24.7$ fb$^{-1}$ of $e^+ e^-$ collisions at the $\Upsilon(2S)$ center-of-mass energy collected with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We measure a branching fraction of $\mathcal{B}(\Upsilon(2S)\to\gamma\eta_{b}(1S))=(6.1^{+0.6+0.9}_{-0.7-0.6})\times 10^{-4}$, and derive an $\eta_{b}(1S)$ mass of $9394.8^{+2.7+4.5}_{-3.1-2.7}$ MeV/$c^{2}$, where the uncertainties are statistical and systematic, respectively. The significance of our measurement is greater than 7 standard deviations, constituting the first observation of this decay mode.

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Belle Collaboration, B. G. Fulsom, T. K. Pedlar, I. Adachi, H. Aihara, S. Al Said, D. M. Asner, H. Atmacan, V. Aulchenko, T. Aushev, R. Ayad, V. Babu, I. Badhrees, A. M. Bakich, V. Bansal, P. Behera, C. Beleño, M. Berger, V. Bhardwaj, B. Bhuyan, T. Bilka, J. Biswal, A. Bondar, G. Bonvicini, A. Bozek, M. Bračko, T. E. Browder, L. Cao, D. Červenkov, V. Chekelian, A. Chen, B. G. Cheon, K. Chilikin, K. Cho, S. -K. Choi, Y. Choi, S. Choudhury, D. Cinabro, S. Cunliffe, N. Dash, S. Di Carlo, J. Dingfelder, Z. Doležal, T. V. Dong, Z. Drásal, S. Eidelman, D. Epifanov, J. E. Fast, T. Ferber, R. Garg, V. Gaur, N. Gabyshev, A. Garmash, M. Gelb, A. Giri, P. Goldenzweig, E. Guido, J. Haba, K. Hayasaka, H. Hayashii, S. Hirose, W. -S. Hou, T. Iijima, K. Inami, G. Inguglia, A. Ishikawa, R. Itoh, M. Iwasaki, Y. Iwasaki, W. W. Jacobs, H. B. Jeon, S. Jia, Y. Jin, D. Joffe, K. K. Joo, T. Julius, T. Kawasaki, H. Kichimi, C. Kiesling, D. Y. Kim, H. J. Kim, J. B. Kim, K. T. Kim, S. H. Kim, K. Kinoshita, P. Kodyš, S. Korpar, D. Kotchetkov, P. Križan, R. Kroeger, P. Krokovny, T. Kuhr, R. Kulasiri, A. Kuzmin, Y. -J. Kwon, J. S. Lange, I. S. Lee, S. C. Lee, L. K. Li, Y. B. Li. 2018-07-03. Observation of $\Upsilon(2S)\to\gamma \eta_{b}(1S)$ decay. https://doi.org/10.1103/physrevlett.121.232001

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