Recently measured large $A_N$ for forward neutrons in $p^{\uparrow }A$ collisions at $\sqrt{s_{NN}}=200$ GeV explained through simulations of ultraperipheral collisions and hadronic interactions

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider recently reported transverse single-spin asymmetry, $A_N$, for forward neutrons in $p^{\uparrow }A$ collisions at $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$. $A_N$ in $p^{\uparrow }\mathrm{Al}$ and $p^{\uparrow }\mathrm{Au}$ collisions were measured as $-0.015$ and 0.18, respectively. These values are clearly different from the measured $A_N=-0.08$ in $p^{\uparrow }p$ collisions. In this paper, I propose that a large $A_N$ for forward neutrons in ultraperipheral $p^{\uparrow }A$ collisions may explain the PHENIX measurements. The proposed model is demonstrated using two Monte Carlo simulations. In the ultraperipheral collision simulation, I use the starlight event generator for the simulation of the virtual photon flux and then use the maid2007 unitary isobar model for the simulation of neutron production in the interactions of a virtual photon with a polarized proton. In the $p^{\uparrow }A$ hadronic interaction simulation, the differential cross sections for forward neutron production are predicted by a simple one-pion exchange model and the Glauber model. The simulated $A_N$ values for both the contribution of ultraperipheral collisions and the hadronic interactions are in good agreement with the PHENIX results.

Figure 1

(a) Coordinate axes in the detector reference frame. (b) Kinematical variables and coordinate axes in the ${\gamma }^{*}{p}^{\uparrow }$ center-of-mass frame $\{0,2,3\}$.

Figure 2

Target asymmetry $T\left({\theta }_{\pi }\right)$ of the ${\gamma }^{*}{p}^{\uparrow }\to {\pi }^{+}n$ interaction as function of $W$. In the detector reference frame, the curves correspond to the rapidity of produced neutrons $\eta =6.8$ and 8.0.

Figure 3

Differential UPC cross sections as a function of $W$ for the $p^{\uparrow }\mathrm{Au}\to {\pi }^{+}n$ and $p^{\uparrow }\mathrm{Au}\to {\pi }^{+}{\pi }^{0}n$ interactions.

Figure 4

Differential cross sections of UPCs and hadronic interactions as a function of $x_{\mathrm{F}}$ and $\mathrm{\Phi }$.

Figure 5

Transverse single-spin asymmetry $A_N$ of forward neutron, comparing the PHENIX results with simulations.