Observation of an Isoscalar Resonance with Exotic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>J</mml:mi></mml:mrow><mml:mrow><mml:mi>P</mml:mi><mml:mi>C</mml:mi></mml:mrow></mml:msup><mml:mo>=</mml:mo><mml:msup><mml:mrow><mml:mn>1</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math> Quantum Numbers in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>J</mml:mi><mml:mo stretchy="false">/</mml:mo><mml:mi>ψ</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:mi>γ</mml:mi><mml:mi>η</mml:mi><mml:msup><mml:mrow><mml:mi>η</mml:mi></mml:mrow><mml:mrow><mml:mo>′</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>
M. Ablikim, М. Н. Ачасов, P. Adlarson, S. Ahmed, M. Albrecht, R. Aliberti, A. Amoroso, M. R. An, Q. An, X. H. Bai, Y. Bai, O. Bakina, R. Baldini Ferroli, I. Balossino, Y. Ban, V. Batozskaya, D. Becker, K. Begzsuren, N. Berger, M. Bertani, D. Bettoni, F. Bianchi, J. Bloms, A. Bortone, I. Boyko, R. A. Briere, H. Cai, X. Z. Cai, A. Calcaterra, G. F. Cao, N. Cao, S. A. Çetin, J. F. Chang, W. L. Chang, G. Chelkov, C. Chen, G. Chen, H. S. Chen, M. L. Chen, S. J. Chen, T. Chen, X. R. Chen, X. T. Chen, Y. B. Chen, Z. J. Chen, W. S. Cheng, G. Cibinetto, F. Cossio, J. J. Cui, X. F. Cui, H. L. Dai, J. P. Dai, X. Dai, A. Dbeyssi, R. E. de Boer, D. Dedovich, Z. Y. Deng, A. Denig, I. Denysenko, M. Destefanis, F. De Mori, Y. Ding, C. Dong, J. Dong, L. Y. Dong, M. Y. Dong, X. Dong, S. X. Du, P. Egorov, Y. L. Fan, J. Fang, S. S. Fang, Y. Fang, R. Farinelli, L. Fava, F. Feldbauer, G. Felici, C. Q. Feng, J. H. Feng, M. Fritsch, C. D. Fu, Y. Gao, Y. Gao, I. Garzia, P. T. Ge, C. Geng, E. Gersabeck, A. Gilman, K. Goetzen, L. Gong, W. X. Gong, W. Gradl, M. Greco, M. H. Gu, Y. T. Gu, C. Y. Guan, A. Q. Guo, A. Q. Guo, L. B. Guo, R. P. Guo
Abstract
Using a sample of $(10.09\ifmmode\pm\else\textpm\fi{}0.04)\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }J/\ensuremath{\psi}$ events collected with the BESIII detector operating at the BEPCII storage ring, a partial wave analysis of the decay $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\eta}{\ensuremath{\eta}}^{\ensuremath{'}}$ is performed. The first observation of an isoscalar state with exotic quantum numbers ${J}^{PC}={1}^{\ensuremath{-}+}$, denoted as ${\ensuremath{\eta}}_{1}(1855)$, is reported in the process $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}_{1}(1855)$ with ${\ensuremath{\eta}}_{1}(1855)\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\eta}}^{\ensuremath{'}}$. Its mass and width are measured to be $(1855\ifmmode\pm\else\textpm\fi{}{9}_{\ensuremath{-}1}^{+6})\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and $(188\ifmmode\pm\else\textpm\fi{}{18}_{\ensuremath{-}8}^{+3})\text{ }\text{ }\mathrm{MeV}$, respectively, where the first uncertainties are statistical and the second are systematic, and its statistical significance is estimated to be larger than $19\ensuremath{\sigma}$.