Measurements of Born Cross Sections for $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2595{)}^{-}+c.c.$ and $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2625{)}^{-}+c.c.$ at $\sqrt{s}=$4918.0 and 4950.9 MeV

Abstract

Using $e^{+}{e}^{-}$ collision data collected with the BESIII detector operating at the BEPCII collider, the Born cross sections of $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2595{)}^{-}+c.c.$ and $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2625{)}^{-}+c.c.$ are measured for the first time at center-of-mass energies of $\sqrt{s}=4918.0$ and 4950.9 MeV. Non-zero cross sections are observed very close to the production threshold. The measured Born cross sections of $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2625{)}^{-}+c.c.$ are about $2\sim 3$ times greater than those of $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2595{)}^{-}+c.c.$, thereby indicating that the exotic structure potentially exists in the excited charmed baryons. The Born cross sections are $15.6\pm 3.1\pm 0.9$ pb and $29.4\pm 3.7\pm 2.7$ pb for $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2595{)}^{-}+c.c.$, and are $43.4\pm 4.0\pm 4.1$ pb and $76.8\pm 6.5\pm 4.2$ pb for $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2625{)}^{-}+c.c.$ at $\sqrt{s}=4918.0$ and 4950.9 MeV, respectively. Based on the polar angle distributions of the ${\bar{\Lambda }}_c(2625{)}^{-}$ and ${\Lambda }_c(2625{)}^{+}$, the form-factor ratios $\sqrt{|G_E{|}^2+3|G_M{|}^2}/|G_C|$ are determined for $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c(2625{)}^{-}+c.c.$ for the first time, which are $5.95\pm 4.07\pm 0.15$ and $0.94\pm 0.32\pm 0.02$ at $\sqrt{s}=4918.0$ and 4950.9 MeV, respectively. All of these first uncertainties are statistical and second systematic.

10 pages, 6 figures

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