Transverse single-spin asymmetries (TSSA) in forward and very forward neutral hadrons generated in transversely polarized p+p collisions at high energy provide us unique and rich testing grounds for deeper understanding of the production mechanism in terms of perturbative and nonperturbative strong interactions. Ever since the large TSSAs were discovered in FNAL four decades ago, the phenomena inspired remarkable developments in perturbative regime.
On the other hand, such a large TSSA was also discovered even in very forward neutron production at RHIC where nonperturbative mechanism suppose to play a key role. The origin of these observed TSSAs have been studied separately in different theoretical frameworks so far due to the applicable limits of these theoretical regimes.
In 2007, the RHICf collaboration installed an electromagnetic calorimeter in the zero-degree region of the STAR detector at the Relativistic Heavy Ion Collider (RHIC) and measured neutral $\pi^0$ and neutrons produced at pseudorapidity larger than 6 in transversely polarized p+p collisions at $\sqrt{s}=510$ GeV. This new experiment will bridge our understandings of TSSA for the meson in forward and the neutron in very forward productions.