Non-zero transverse single spin asymmetry of very forward $\pi^0$ in polarized $p + p$ collisions at $\sqrt{s} = 510$ GeV

Abstract

The RHICf experiment measured transverse single spin asymmetry of very forward (\eta > 6η>6) \pi^0π0 from polarized p + pp+p collisions at \sqrt{s} = 510s=510 GeV. In order to measure it precisely, we installed a new electromagnetic calorimeter at zero-degree area of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) and measured the \pi^0π0s over the kinematic range of x_F > 0.25xF>0.25 and 0 < p_T < 10<pT<1 GeV/cc in June 2017. A clear non-zero asymmetry was observed even in low p_T < 1pT<1 GeV/cc showing a similar x_FxF dependence with that of the forward (2 < \eta < 42<η<4) \pi^0π0. A possible diffractive contribution may need to be taken into account to explain the very forward \pi^0π0 asymmetry. RHICf-STAR combined analysis and follow-up experiment will give a clue to understand it qualitatively.