Polarization and entanglement in baryon–antibaryon pair production in electron–positron annihilation

• Published in: Nature physics Vol. 15; no. 7; pp. 631 - 634
• Main Authors: Bai, Y, Begzsuren, K, Berger, N, Boyko, I, Chelkov, G, Chen, H S, Dai, H L, Deng, Z Y, Destefanis, M, Dong, J, Dong, M Y, Duan, P F, Fang, S S, Fegan, S, Gao, X L, Gao, Y, Greco, M, Gu, L M, Gu, M H, Gu, Y T, Guo, R P, Guo, Y P, Heng, Y K, Hu T, Huang, J S, Huang, Z L, Hussain, T, Julin, A, Kalantar-Nayestanaki, N, Kavatsyuk, M, Kolcu, O B, Kopf, B, Cheng, Li, Li, F Y, Li, Q Y, Li T, Li, X Q, Liu, Feng, Liu, H M, Liu, J Y, Liu, Ke, Liu, Z A, Long, Y F, Lu, J G, Luo, M X, Ma, X X, Maas, F E, Maldaner, S, Mao, Y J, Mao, Z P, Min, J, Mitchell, R E, Mo, X H, Mo, Y J, Yu, Muchnoi N, Muramatsu, H, Olsen, S L, Patteri, P, Qi, H R, Rosner Ch, Shan, W, Shao, M, Shen, P X, Sheng, H Y, Sosio, S, Sui, F F, Sun, Y Z, Wang, B L, Wang, L L, Wang, Y F, Wang, Z, Weidenkaff, P, Wen, S P, Xiao, D, Xiao, Z J, Xiu, Q L, Xu, Q J, Yan, F, Yan, W B, Yang, H X, Yang, S L, Yang, Y H, Yin, J H, You, Z Y, Yu, B X, Yuan, C Z, Zafar, A A, Zhang, B Y, Zhang, D H, Zhang, J L, Zhang, Y T, Zhang, Z Y, Zhao, J Z, Zhao, Q, Zhao, T C, Zhao, Z G, Zheng, B, Zheng, J P, Zhou, L, Zhu, S H
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.07.2019; Nature Publishing Group
• Subjects: 639/766/419/1131; 639/766/483/1139; Angular distribution; Atomic; Baryons; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; CP violation; Decay; Entanglement; Helicity; Letter; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Pair production; Parameters; Particle decay; Phase transitions; Physics; Physics and Astronomy; Polarization; Positron annihilation; Quarks; Theoretical
• ISSN: 1745-2473; 1745-2481; 1745-2481
• DOI: 10.1038/s41567-019-0494-8

Particles directly produced at electron–positron colliders, such as the J / ψ meson, decay with relatively high probability into a baryon–antibaryon pair 1 . For spin-1/2 baryons, the pair can have the same or opposite helicites. A non-vanishing phase Δ Φ between the transition amplitudes to these helicity states results in a transverse polarization of the baryons 2 – 4 . From the joint angular distribution of the decay products of the baryons, this phase as well as the parameters characterizing the baryon and the antibaryon decays can be determined. Here, we report the measurement of Δ Φ  = 42.4 ± 0.6 ± 0.5° using Λ  →  pπ − and Λ ¯ → p ¯ π + , n ¯ π 0 decays at BESIII. We find a value for the Λ  →  pπ − decay parameter of α −  = 0.750 ± 0.009 ± 0.004, 17 ± 3% higher than the current world average, which has been used as input for all Λ polarization measurements since 1978 5 , 6 . For Λ ¯ → p ¯ π + we find α +  = −0.758 ± 0.010 ± 0.007, giving A CP  = ( α −  +  α + )/( α −  −  α + ) = −0.006 ± 0.012 ± 0.007, a precise direct test of charge–parity symmetry (CP) violation in Λ decays. The decay asymmetry and helicity phase of polarized baryon–antibaryon pairs are measured at the BESIII experiment, testing charge–parity symmetry and revealing a discrepancy of the Λ  →  pπ − decay asymmetry with respect to the current world average.