Nuclear dependence of the transverse-single-spin asymmetry for forward neutron production in polarized collisions at GeV

GeVでの偏極陽子-原子核衝突における前方中性子のシングルスピン左右非対称性の原子核依存性

Aidala, C.*; 長谷川 勝一   ; 今井 憲一; 佐甲 博之   ; 佐藤 進  ; 谷田 聖   ; PHENIX Collaboration*; 他309名*

Aidala, C.*; Hasegawa, Shoichi; Imai, Kenichi; Sako, Hiroyuki; Sato, Susumu; Tanida, Kiyoshi; PHENIX Collaboration*; 309 of others*

During 2015 the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in collisions predicts only a moderate atomic-mass-number () dependence. In contrast, the asymmetries observed at RHIC in collisions showed a surprisingly strong dependence in inclusive forward neutron production. The observed asymmetry in +Al collisions is much smaller, while the asymmetry in + Au collisions is a factor of three larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed dependence.