# Transverse Spin Asymmetries in the $p^{\uparrow}p\to p\pi^0 X$ Process at STAR

## TL;DR

This paper reports the first measurement of transverse spin asymmetries in the process where a polarized proton collides with an unpolarized proton, producing a neutral pion and other particles, revealing new insights into spin-momentum correlations.

## Contribution

It introduces the first exploration of spin asymmetries in the $p^{}p o p\pi^0 X$ process at high energy, with detailed measurements of azimuthal dependencies.

## Key findings

- Observation of significant single-spin asymmetries.
- Correlation patterns between pion and proton azimuthal angles.
- Evidence suggesting spin-dependent fluctuations in proton structure.

## Abstract

A significant sample of $p^{\uparrow}p\to p\pi^0 X$ events has been observed at STAR in $\sqrt{s}=200$ GeV transversely polarized $pp$ collisions, where an isolated $\pi^0$ is detected in the forward pseudorapidity range $2.65<\eta<3.9$ along with the forward-going proton $p$, which scatters with a near-beam forward pseudorapidity into Roman Pot detectors. The sum of the $\pi^0$ and the scattered proton energies is consistent with the incident proton energy of 100 GeV, indicating that no further particles are produced in this direction. It is postulated that the forward incident proton may have fluctuated into a $p+\pi^0$ system, with an angular momentum correlated with the initial proton spin. The backward-going proton interacts with the $p+\pi^0$ system, which then separates such that the $\pi^0$ has a transverse momentum of ${\sim}2$ GeV/$c$ and the proton has a transverse momentum of ${\sim}0.2$ GeV/$c$, while the backward proton shatters into the remaining particles $X$. Correlations between the $\pi^0$ and scattered proton will be presented, along with single-spin asymmetries which depend on the azimuthal angles of both the pion and the proton. This is the first time that spin asymmetries have been explored for this process, and a model to explain their azimuthal dependence is needed.