Transverse single spin asymmetries of forward neutrons in $p+p$, $p+$Al and $p+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV as a function of transverse and longitudinal momenta

The PHENIX collaboration
Acharya, U.A. , Aidala, C. , Akiba, Y. , Alfred, M. , Andrieux, V. , Apadula, N. , Asano, H. , Azmoun, B. , Babintsev, V. , Bandara, N.S.
Phys.Rev.D 105 (2022) 032004, 2022.

Abstract (data abstract)
BNL-RHIC. In 2015, the PHENIX collaboration has taken p+p, p+Al and p+Au collision data at center of mass energies of $\sqrt{s_{NN}}=200$ GeV with the proton beam(s) transversely polarized. At very forward rapidities $\eta>6.8$ relative to the polarized proton beam, neutrons were detected either inclusively or in (anti)correlation with detector activity related to hard collisions. The resulting single spin asymmetries, that were already previously reported, have now been extracted as a function of the transverse momentum of the neutron as well as its longitudinal momentum fraction $x_F$. The explicit kinematic dependence, combined with the correlation information allows for a closer look at the interplay of different mechanisms suggested to describe these asymmetries, such as hadronic interactions or electromagnetic interactions in ultra-peripheral collisions, UPC.

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