Determination of Spin-parity Quantum Numbers for the Narrow Structure Near the $p\bar{\Lambda }$ Threshold in $e^{+}{e}^{-}\to p{K}^{-}\bar{\Lambda }+c.c.$

Abstract

A narrow structure in the $p\bar{\Lambda }$ system near the mass threshold, named as $X(2085)$, is observed in the process $e^{+}{e}^{-}\to p{K}^{-}\bar{\Lambda }$ with a statistical significance greater than $20\sigma$. Its spin and parity are determined for the first time to be $J^P=1^{+}$ in an amplitude analysis, with statistical significance greater than $5\sigma$ over other quantum numbers. The pole positions of $X(2085)$ are measured to be $M_{\mathrm{pole}}=(2086\pm 4\pm 6)$~MeV and ${\Gamma }_{\mathrm{pole}}=(56\pm 5\pm 16)$ MeV, where the first uncertainties are statistical and the second ones are systematic. The analysis is based on the study of the process $e^{+}{e}^{-}\to p{K}^{-}\bar{\Lambda }$ and uses the data samples collected with the BESIII detector at the center-of-mass energies $\sqrt{s}=4.008$, $4.178$, $4.226$, $4.258$, $4.416$, and $4.682$ GeV with a total integrated luminosity of $8.35{\text{fb}}^{-1}$.

arXiv comment8 pages, 2 figures

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