Observation of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi mathvariant="normal">Ξ</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math> hyperon transverse polarization 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:mo stretchy="false">→</mml:mo><mml:msup><mml:mi mathvariant="normal">Ξ</mml:mi><mml:mo>−</mml:mo></mml:msup><mml:msup><mml:mover accent="true"><mml:mi mathvariant="normal">Ξ</mml:mi><mml:mo stretchy="false">¯</mml:mo></mml:mover><mml:mo>+</mml:mo></mml:msup></mml:math>
M. Ablikim, М. Н. Ачасов, P. Adlarson, 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, A. Brueggemann, 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, Shili Chen, T. Chen, Xiyong Chen, X. T. Chen, Y. B. Chen, Z. J. Chen, W. S. Cheng, X. Chu, G. Cibinetto, 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, Y. Ding, J. Dong, L. Y. Dong, M. Y. Dong, X. Dong, S. X. Du, 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, He Gao, Yang Gao, Yang Gao, S. Garbolino, I. Garzia, P. T. Ge, Z. W. Ge, C. Geng, E. Gersabeck, A. Gilman, K. Goetzen, L. Gong, W. X. Gong, W. Gradl, M. Greco, L. M. Gu, M. H. Gu, Y. T. Gu
Abstract
Using a sample of $(448.1\ifmmode\pm\else\textpm\fi{}2.9)\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\ensuremath{\psi}(3686)$ decays collected with the BESIII detector at BEPCII, we report an observation of ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}$ transverse polarization with a significance of $7.3\ensuremath{\sigma}$ in the decay $\ensuremath{\psi}(3686)\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}$ (${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}{\ensuremath{\pi}}^{\ensuremath{-}}$, ${\overline{\mathrm{\ensuremath{\Xi}}}}^{+}\ensuremath{\rightarrow}\overline{\mathrm{\ensuremath{\Lambda}}}{\ensuremath{\pi}}^{+}$, $\mathrm{\ensuremath{\Lambda}}\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{\ensuremath{-}}$, $\overline{\mathrm{\ensuremath{\Lambda}}}\ensuremath{\rightarrow}\overline{p}{\ensuremath{\pi}}^{+}$). The relative phase of the electric and magnetic form factors is determined to be $\mathrm{\ensuremath{\Delta}}\mathrm{\ensuremath{\Phi}}=(0.667\ifmmode\pm\else\textpm\fi{}0.111\ifmmode\pm\else\textpm\fi{}0.058)\text{ }\text{ }\mathrm{rad}$. This is the first measurement of the relative phase for a $\ensuremath{\psi}(3686)$ decay into a pair of ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}$ hyperons. The ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}$ decay parameters (${\ensuremath{\alpha}}_{{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}}$, ${\ensuremath{\phi}}_{{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}}$) and their conjugates (${\ensuremath{\alpha}}_{{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}}$, ${\ensuremath{\phi}}_{{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}}$), the angular-distribution parameter ${\ensuremath{\alpha}}_{\ensuremath{\psi}}$, and the strong-phase difference ${\ensuremath{\delta}}_{p}\ensuremath{-}{\ensuremath{\delta}}_{s}$ for $\mathrm{\ensuremath{\Lambda}}{\ensuremath{\pi}}^{\ensuremath{-}}$ scattering are measured to be consistent with previous BESIII results.