Measurement of the Cross Section for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup><mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:msup><mml:mrow><mml:mi mathvariant="normal">Ξ</mml:mi></mml:mrow><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:msup><mml:msup><mml:mrow><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="normal">Ξ</mml:mi></mml:mrow><mml:mrow><mml:mo stretchy="false">¯</mml:mo></mml:mrow></mml:mover></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math> and Observation of an Excited <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="normal">Ξ</mml:mi></mml:math> Baryon
M. Ablikim, M. N. Achasov, P. Adlarson, S. Ahmed, M. Albrecht, M. Alekseev, A. Amoroso, F. F. An, Q. An, Y. Bai, O. Bakina, R. Baldini Ferroli, I. Balossino, Y. Ban, K. Begzsuren, J. V. Bennett, N. Berger, M. Bertani, D. Bettoni, F. Bianchi, J. Biernat, J. Bloms, I. Boyko, R. A. Briere, H. Cai, X. Cai, A. Calcaterra, G. F. Cao, N. Cao, S. A. Cetin, J. Chai, J. F. Chang, W. L. Chang, G. Chelkov, D. Y. Chen, G Chen, H. S. Chen, J. C. Chen, M. L. Chen, S. J. Chen, Y. B. Chen, W. Cheng, G. Cibinetto, F. Cossio, X. F. Cui, H. L. Dai, J. P. Dai, X. C. Dai, A. Dbeyssi, 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, Z. L. Dou, S. X. Du, J. Z. Fan, J. Fang, S. S. Fang, Y. Fang, R. Farinelli, L. Fava, F. Feldbauer, G. Felici, C. Q. Feng, M. Fritsch, C. D. Fu, Y. Fu, Q. Gao, X. L. Gao, Y. Gao, Yuan Gao, Yansheng Gao, Z. Gao, B. Garillon, I. Garzia, E. Gersabeck, A. Gilman, K. Goetzen, L. Gong, W. X. Gong, W. Gradl, M. Greco, L. M. Gu, M. H. Gu, S. Gu, Y. T. Gu, A. Q. Guo, L. B. Guo, R. P. Guo, Y. P. Guo, A. Guskov, S. Han, X. Q. Hao
Abstract
Using a total of $11.0\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of ${e}^{+}{e}^{\ensuremath{-}}$ collision data with center-of-mass energies between 4.009 and 4.6 GeV and collected with the BESIII detector at BEPCII, we measure fifteen exclusive cross sections and effective form factors for the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}$ by means of a single baryon-tag method. After performing a fit to the dressed cross section of ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}$, no significant $\ensuremath{\psi}(4230)$ or $\ensuremath{\psi}(4260)$ resonance is observed in the ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}$ final states, and upper limits at the 90% confidence level on ${\mathrm{\ensuremath{\Gamma}}}_{ee}\mathcal{B}$ for the processes $\ensuremath{\psi}(4230)/\ensuremath{\psi}(4260)\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}{\overline{\mathrm{\ensuremath{\Xi}}}}^{+}$ are determined. In addition, an excited $\mathrm{\ensuremath{\Xi}}$ baryon at $1820\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ is observed with a statistical significance of $6.2--6.5\ensuremath{\sigma}$ by including the systematic uncertainty, and the mass and width are measured to be $M=(1825.5\ifmmode\pm\else\textpm\fi{}4.7\ifmmode\pm\else\textpm\fi{}4.7)\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and $\mathrm{\ensuremath{\Gamma}}=(17.0\ifmmode\pm\else\textpm\fi{}15.0\ifmmode\pm\else\textpm\fi{}7.9)\text{ }\text{ }\mathrm{MeV}$, which confirms the existence of the ${J}^{P}={\frac{3}{2}}^{\ensuremath{-}}$ state $\mathrm{\ensuremath{\Xi}}(1820)$.