Observation of a near-threshold enhancement in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><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> mass spectrum from <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>ϕ</mml:mi><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:math> from 3.51 to 4.60 GeV
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, 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, Y. L. Fan, J. Fang, S. 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, Yang Gao, Y. Gao, I. Garzia, P. T. Ge, C. Geng, 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, C. Y. Guan, A. Q. Guo, L. B. Guo, R. P. Guo, Y. P. Guo, A. Guskov, T. T. Han, W. Y. Han
Abstract
The process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\phi}\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ is studied using data samples collected with the BESIII detector at the BEPCII Collider at center of mass energies $\sqrt{s}$ ranging from 3.51 GeV to 4.60 GeV. An enhancement is observed near the threshold of $\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$. The lineshape of this enhancement is studied in different approaches, including fit with a Breit-Wigner function or a reversed exponential function. The Breit-Wigner function has a mass of $(2262\ifmmode\pm\else\textpm\fi{}4\ifmmode\pm\else\textpm\fi{}28)\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and a width of $(72\ifmmode\pm\else\textpm\fi{}5\ifmmode\pm\else\textpm\fi{}43)\text{ }\text{ }\mathrm{MeV}$, where the quoted uncertainties are statistical and systematic, respectively. The rising rate of the reversed exponential function is measured as $33\ifmmode\pm\else\textpm\fi{}11\ifmmode\pm\else\textpm\fi{}6\text{ }\text{ }\mathrm{MeV}/{c}^{2}$. For the $\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ system, the ${J}^{PC}$ quantum numbers of ${0}^{\ensuremath{-}+}$ and ${0}^{++}$ are rejected, while other ${J}^{PC}$ hypotheses are possible, according to the helicity-angle study. The energy-dependent cross section of the ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\phi}\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ process is measured for the first time in this energy region, and contributions from excited $\ensuremath{\psi}$ states and vector charmoniumlike $Y$-states are investigated.