Measurement of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="normal">Λ</mml:mi></mml:math> baryon polarization in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>e</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>e</mml:mi><mml:mo>−</mml:mo></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:mi mathvariant="normal">Λ</mml:mi><mml:mover accent="true"><mml:mi mathvariant="normal">Λ</mml:mi><mml:mo stretchy="false">¯</mml:mo></mml:mover></mml:math> at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>3.773</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>GeV</mml:mi></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, 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, D. Y. Chen, G. Chen, H. S. Chen, M. L. Chen, S. J. Chen, X. R. Chen, Y. B. Chen, Z. J. Chen, W. S. Cheng, G. Cibinetto, F. Cossio, 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. Fang, Y. Fang, R. Farinelli, L. Fava, F. Feldbauer, G. Felici, C. Q. Feng, J. H. Feng, M. Fritsch, C. D. Fu, Yang Gao, Yang Gao, Y. Gao, I. Garzia, P. Ge, C. Geng, E. Gersabeck, A. Gilman, K. Goetzen, L. Gong, W. X. Gong, W. Gradl, M. Greco, L. M. Gu, M. H. Gu, C. Y. Guan, A. Q. Guo, A. Q. Guo, L. B. Guo, R. P. Guo, Y. P. Guo, A. Guskov, T. T. Han, W. Y. Han
Abstract
Using a data sample of $\ensuremath{\psi}(3770)$ events collected with the BESIII detector at BEPCII corresponding to an integrated luminosity of $2.9\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, we report a measurement of $\mathrm{\ensuremath{\Lambda}}$ spin polarization in ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ at $\sqrt{s}=3.773\text{ }\text{ }\mathrm{GeV}$. The significance of polarization is found to be $2\ensuremath{\sigma}$ including the systematic uncertainty, which implies a zero phase between the transition amplitudes of the $\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ helicity states. This phase can be interpreted in terms of psionic form factors, and is determined to be $\mathrm{\ensuremath{\Delta}}{\mathrm{\ensuremath{\Phi}}}^{\mathrm{\ensuremath{\Psi}}}={\mathrm{\ensuremath{\Phi}}}_{E}^{\mathrm{\ensuremath{\Psi}}}\ensuremath{-}{\mathrm{\ensuremath{\Phi}}}_{M}^{\mathrm{\ensuremath{\Psi}}}=(7{1}_{\ensuremath{-}46}^{+66}\ifmmode\pm\else\textpm\fi{}5)\ifmmode^\circ\else\textdegree\fi{}$. Similarly, the ratio between the form factors is found to be ${R}^{\ensuremath{\psi}}=|{G}_{E}^{\mathrm{\ensuremath{\Psi}}}/{G}_{M}^{\mathrm{\ensuremath{\Psi}}}|=0.4{8}_{\ensuremath{-}0.35}^{+0.21}\ifmmode\pm\else\textpm\fi{}0.03$. The first uncertainties are statistical and the second systematic.