Determination of Spin-Parity Quantum Numbers for the Narrow Structure near the p Λ ¯ Threshold in e+e- →pK- Λ ¯ + c. c.

Abstract

A narrow structure in the pΛ¯ system near the mass threshold, named as X(2085), is observed in the process e+e-→pK-Λ¯ with a statistical significance greater than 20σ. Its spin and parity are determined for the first time to be JP=1+ in an amplitude analysis, with a statistical significance greater than 5σ over other quantum numbers (0-,1- and 2+). The pole positions of X(2085) are measured to be Mpole=(2084-2+4±9) MeV and Γpole=(58-3+4±25) 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-→pK-Λ¯ and uses the data samples collected with the BESIII detector at the center-of-mass energies s=4.008, 4.178, 4.226, 4.258, 4.416, and 4.682 GeV with a total integrated luminosity of 8.35 fb-1.