Nuclear Dependence of the Transverse-Single-Spin Asymmetry for Forward Neutron Production in Polarized $p+A$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

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 $p+p$ collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in $p+p$ collisions predicts only a moderate atomic-mass-number ($A$) dependence. In contrast, the asymmetries observed at RHIC in $p+A$ collisions showed a surprisingly strong $A$ dependence in inclusive forward neutron production. The observed asymmetry in $p+\mathrm{Al}$ collisions is much smaller, while the asymmetry in $p+\mathrm{Au}$ collisions is a factor of 3 larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed $A$ dependence.

Figure 1

ZDC location and beam orbits of a proton (blue) beam and a heavy-ion (yellow) beam in the special stores used for this analysis; the $z$ axis shows the nominal beam direction, and the dashed line represents the zero-degree neutron trajectory. DX and D0 are the RHIC beam bending dipole magnets.

Figure 2

$A_N^{\text{fit}}$ fit of ZDC inclusive samples.

Figure 3

The $r$ distribution of the (a) $\mathrm{ZDC}\otimes \mathrm{BBC}$-tag sample and (b) $\mathrm{ZDC}\otimes \mathrm{BBC}$-veto sample for three collision systems.

Figure 4

Forward neutron $A_N$ in $p+A$ collisions for $A=1$ ($p$), 27 (Al), and 197 (Au), for ZDC inclusive, $\mathrm{ZDC}\otimes \mathrm{BBC}$-tag, and $\mathrm{ZDC}\otimes \mathrm{BBC}$-veto triggered samples; color bars are systematic uncertainties, and statistical uncertainties are smaller than the marker size; the 3% scale uncertainty (not shown) is from the polarization normalization uncertainty. Data points are shifted horizontally for better visibility.