First Determination of the Spin-Parity of Ξ_c(3055)^+,0 Baryons

The Ξ^0(−)⁢𝑏→Ξ_𝑐⁢(3055)⁺⁽⁰⁾⁢(→𝐷⁺⁽⁰)⁢Λ)⁢𝜋⁻ decay chains are observed, and the spin-parity of Ξ_𝑐⁢(3055)⁺⁽⁰⁾ baryons is determined for the first time. The measurement is performed using proton-proton collision data at a center-of-mass energy of √𝑠 =13  TeV, corresponding to an integrated luminosity of 5.4  fb⁻¹, recorded by the LHCb experiment between 2016 and 2018. The spin-parity of the Ξ_𝑐⁢(3055)⁺⁽⁰⁾ baryons is determined to be 3/2⁺ with a significance of more than 6.5⁢𝜎 (3.5⁢𝜎) compared to all other tested hypotheses. The up-down asymmetries of the Ξ^0(−)⁢𝑏→Ξ_𝑐⁢(3055)⁺⁽⁰⁾⁢(→𝐷⁺⁽⁰)⁢Λ)⁢𝜋⁻ transitions are measured to be −0.92±0.10±0.05 (−0.92±0.16±0.22), consistent with maximal parity violation, where the first uncertainty is statistical and the second is systematic. These results support the hypothesis that the Ξ_𝑐⁢(3055)⁺⁽⁰⁾ baryons correspond to the first 𝐷-wave 𝜆-mode excitation of the Ξ_𝑐 flavor triplet.