Far-forward neutral particle asymmetry measurements in the RHICf experiment

The RHICf experiment installed an electromagnetic calorimeter in front of the Zero-Degree Calorimeter (ZDC) of the RHIC-STAR experiment in 2017 to measure the transverse-spin asymmetries of the far-forward neutral particles produced from transversely polarized proton collisions at RHIC. It is known that the far-forward neutrons produced in transversely polarized proton collisions have a large transverse-spin asymmetry, and the ZDC at RHIC serves as a polarimeter to monitor the polarization in the collision experiments. In particular, it is an important tool for tuning the polarization direction at the collision point. The electromagnetic calorimeter installed in the RHICf experiment was able to obtain high-precision position information, and furthermore, by moving the detector position, the kinematic region that can be measured was greatly extended and improving the precision of the measurement. In addition to neutrons, we also measured the far-forward asymmetry of neutral $\pi$ mesons and collected new data on particle production in the far-forward region. Furthermore, we are conducting a combined analysis with the STAR detectors to advance our understanding of the particle production mechanism.