Precise Measurements of Decay Parameters and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi><mml:mi>P</mml:mi></mml:math> Asymmetry with Entangled <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Λ</mml:mi><mml:mtext>−</mml:mtext><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="normal">Λ</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="false">¯</mml:mo></mml:mrow></mml:mover></mml:mrow></mml:math> Pairs
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, Jin Chang, W. L. Chang, G. Chelkov, C. Chen, Chao Chen, G. Chen, H. S. Chen, M. L. Chen, S. J. Chen, S. M. Chen, T. Chen, X. R. Chen, X. T. Chen, Y. B. Chen, Z. J. Chen, W. S. Cheng, S. K. Choi, 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. Ge, Z. W. Ge, C. Geng, E. Gersabeck, A. Gilman, K. Goetzen, L. Gong, W. X. Gong, W. Gradl, M. Greco
Abstract
Based on 10 billion $J/\ensuremath{\psi}$ events collected at the BESIII experiment, a search for $CP$ violation in $\mathrm{\ensuremath{\Lambda}}$ decay is performed in the difference between $CP$-odd decay parameters ${\ensuremath{\alpha}}_{\ensuremath{-}}$ for $\mathrm{\ensuremath{\Lambda}}\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{\ensuremath{-}}$ and ${\ensuremath{\alpha}}_{+}$ for $\overline{\mathrm{\ensuremath{\Lambda}}}\ensuremath{\rightarrow}\overline{p}{\ensuremath{\pi}}^{+}$ by using the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}J/\ensuremath{\psi}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$. With a five-dimensional fit to the full angular distributions of the daughter baryon, the most precise values for the decay parameters are determined to be ${\ensuremath{\alpha}}_{\ensuremath{-}}=0.7519\ifmmode\pm\else\textpm\fi{}0.0036\ifmmode\pm\else\textpm\fi{}0.0024$ and ${\ensuremath{\alpha}}_{+}=\ensuremath{-}0.7559\ifmmode\pm\else\textpm\fi{}0.0036\ifmmode\pm\else\textpm\fi{}0.0030$, respectively. The $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ averaged value of the decay parameter is extracted to be ${\ensuremath{\alpha}}_{\mathrm{avg}}=0.7542\ifmmode\pm\else\textpm\fi{}0.0010\ifmmode\pm\else\textpm\fi{}0.0024$ with unprecedented accuracy. The $CP$ asymmetry ${A}_{CP}=({\ensuremath{\alpha}}_{\ensuremath{-}}+{\ensuremath{\alpha}}_{+})/({\ensuremath{\alpha}}_{\ensuremath{-}}\ensuremath{-}{\ensuremath{\alpha}}_{+})$ is determined to be $\ensuremath{-}0.0025\ifmmode\pm\else\textpm\fi{}0.0046\ifmmode\pm\else\textpm\fi{}0.0012$, which is one of the most precise measurements in the baryon sector. The reported results for the decay parameter will play an important role in the studies of the polarizations and $CP$ violations for the strange, charmed and beauty baryons.