First simultaneous measurement of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi mathvariant="normal">Ξ</mml:mi><mml:mn>0</mml:mn></mml:msup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mover accent="true"><mml:mi mathvariant="normal">Ξ</mml:mi><mml:mo stretchy="false">¯</mml:mo></mml:mover><mml:mn>0</mml:mn></mml:msup></mml:math> asymmetry parameters in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>ψ</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mn>3686</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:math> decay
M. Ablikim, М. Н. Ачасов, P. Adlarson, R. Aliberti, A. Amoroso, M. R. An, Q. An, Y. Bai, O. Bakina, I. Balossino, Y. Ban, V. Batozskaya, K. Begzsuren, N. Berger, M. Bertani, D. Bettoni, F. Bianchi, E. Bianco, J. Bloms, A. Bortone, I. Boyko, R. A. Briere, A. Brueggemann, H. Cai, X. Cai, A. Calcaterra, G. F. Cao, N. Cao, S. A. Çetin, J. F. Chang, T. T. Chang, W. L. Chang, G. R., G. Chelkov, C. Chen, Chao Chen, G. Chen, Haifeng Chen, M. L. Chen, S. J. Chen, S. M. Chen, T. Chen, Xiaoyan Chen, X. T. Chen, Y. B. Chen, Y. Q. Chen, Z. J. Chen, W. S. Cheng, S. K. Choi, X. Chu, G. Cibinetto, S. C. Coen, F. Cossio, J. J. Cui, H. L. Dai, J. P. Dai, A. Dbeyssi, R. E. de Boer, D. Dedovich, Z. Y. Deng, A. Denig, I. Denysenko, M. Destefanis, F. De Mori, B. Ding, X. X. Ding, Y. Ding, Y. Ding, J. Dong, L. Y. Dong, M. Y. Dong, X. Dong, S. X. Du, Z. H. Duan, P. Egorov, Y. L. Fan, J. Fang, S. S. Fang, Wenxing Fang, Y. Fang, R. Farinelli, L. Fava, F. Feldbauer, G. Felici, C. Q. Feng, J. H. Feng, K. Fischer, M. Fritsch, C. Fritzsch, C. D. Fu, Y. W. Fu, He Gao, Yang Gao, Yang Gao, S. Garbolino, I. Garzia, P. Ge, Z. W. Ge, C. Geng, E. Gersabeck
Abstract
The ${\mathrm{\ensuremath{\Xi}}}^{0}$ asymmetry parameters are measured using entangled quantum ${\mathrm{\ensuremath{\Xi}}}^{0}\ensuremath{-}{\overline{\mathrm{\ensuremath{\Xi}}}}^{0}$ pairs from a sample of $(448.1\ifmmode\pm\else\textpm\fi{}2.9)\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\ensuremath{\psi}(3686)$ events collected with the BESIII detector at BEPCII. The relative phase between the transition amplitudes of the ${\mathrm{\ensuremath{\Xi}}}^{0}{\overline{\mathrm{\ensuremath{\Xi}}}}^{0}$ helicity states is measured to be $\mathrm{\ensuremath{\Delta}}\mathrm{\ensuremath{\Phi}}=\ensuremath{-}0.050\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0ex}{0ex}}0.150\ifmmode\pm\else\textpm\fi{}0.020\text{ }\text{ }\mathrm{rad}$, which implies that there is no obvious polarization at the current level of statistics. The decay parameters of the ${\mathrm{\ensuremath{\Xi}}}^{0}$ hyperon $({\ensuremath{\alpha}}_{{\mathrm{\ensuremath{\Xi}}}^{0}},{\ensuremath{\alpha}}_{{\overline{\mathrm{\ensuremath{\Xi}}}}^{0}},{\ensuremath{\phi}}_{{\mathrm{\ensuremath{\Xi}}}^{0}},{\ensuremath{\phi}}_{{\overline{\mathrm{\ensuremath{\Xi}}}}^{0}})$ and the angular distribution parameter $[{\ensuremath{\alpha}}_{\ensuremath{\psi}(3686)}]$ and $\mathrm{\ensuremath{\Delta}}\mathrm{\ensuremath{\Phi}}$ are measured simultaneously for the first time. In addition, the $CP$ asymmetry observables are determined to be ${A}_{CP}^{{\mathrm{\ensuremath{\Xi}}}^{0}}=({\ensuremath{\alpha}}_{{\mathrm{\ensuremath{\Xi}}}^{0}}+{\ensuremath{\alpha}}_{{\overline{\mathrm{\ensuremath{\Xi}}}}^{0}})/({\ensuremath{\alpha}}_{{\mathrm{\ensuremath{\Xi}}}^{0}}\ensuremath{-}{\ensuremath{\alpha}}_{{\overline{\mathrm{\ensuremath{\Xi}}}}^{0}})=\ensuremath{-}0.007\ifmmode\pm\else\textpm\fi{}0.082\ifmmode\pm\else\textpm\fi{}0.025$ and $\mathrm{\ensuremath{\Delta}}{\ensuremath{\phi}}_{CP}^{{\mathrm{\ensuremath{\Xi}}}^{0}}=({\ensuremath{\phi}}_{{\mathrm{\ensuremath{\Xi}}}^{0}}+{\ensuremath{\phi}}_{{\overline{\mathrm{\ensuremath{\Xi}}}}^{0}})/2=\ensuremath{-}0.079\ifmmode\pm\else\textpm\fi{}0.082\ifmmode\pm\else\textpm\fi{}0.010\text{ }\text{ }\mathrm{rad}$, which are consistent with $CP$ conservation.