Measurement of Transverse Single-Spin Asymmetries in $π^0$ and $η$ Meson Production in $\sqrt{s}$ = 200 GeV $p^\uparrow+p$ Collisions with sPHENIX

TL;DR

This work measures transverse single-spin asymmetries A_N in inclusive π^0 and η production from p^↑+p collisions at √s = 200 GeV using the sPHENIX detector, leveraging Run-24 data to extend the explored kinematic range. The analysis reconstructs π^0 and η via di-photon decays in a high-rate electromagnetic calorimeter, applying rigorous event, cluster, and mass-based selections, and then extracts A_N with two complementary methods, correcting for polarization and background. Comprehensive systematic studies (calculation method, background modeling, and bunch shuffling) yield robust uncertainties, with results for forward production largely consistent with zero, and align with PHENIX measurements while probing lower p_T and broader η coverage. The findings constrain the role of Sivers-type and Collins-type mechanisms in this kinematic regime and provide essential input for CT3 and TMD-based descriptions of proton spin structure, informing future precision tests and theoretical modeling in spin physics.

Abstract

The sPHENIX experiment is a next-generation collider detector at the Relativistic Heavy Ion Collider (RHIC) designed for rare jet and heavy-flavor probes of Au + Au, $p$ + Au, and polarized $p+p$ collisions. The experiment includes a large acceptance, granular electromagnetic calorimeter and very high-rate data acquisition plus trigger system. In RHIC Run-24, sPHENIX sampled 107 $\mathrm{pb}^{-1}$ of collision data with transversely polarized protons at $\sqrt{s}=200$ GeV using an efficient high-$p_T$ photon trigger. This dissertation describes the extraction of transverse single-spin asymmetries in inclusive production of $π^0$ and $η$ mesons decaying into two photons. Such observables are sensitive to multi-parton correlations in the proton, which are related to transverse-momentum dependent (TMD) effects. The new sPHENIX data set allows for significant extension of the kinematic range covered by previous RHIC mid-rapidity measurements. The results are corrected for background contributions and three different sources of systematic uncertainties are considered: the calculation method, the method of background subtraction, and contributions from possible false asymmetries due to instrumental effects. The results are presented and compared to existing measurements from the PHENIX experiment.

Figures (96)

• Figure 1: Definition of the left-right asymmetry $A_N$ in $p^\uparrow+p$ collisions. A transversely polarized proton (spin $S_T$) colliding with an unpolarized proton can produce a hadron ($h$), jet, or electroweak boson (left). The distribution of these products is asymmetric to the left and right of the transverse spin direction (right). From Perdekamp2015TransverseSS.
• Figure 2: Overview of existing measurements of $A_N$ in the process $p^\uparrow + p \rightarrow \pi^0 + X$. The top panel shows measured $A_N$ as a function of $x_F = 2p_z/\sqrt{s}$. The bottom panel shows the average pion transverse momentum $p_T$ in each $x_F$ bin, indicating the kinematic region of each measurement. From STAR_jet_AN2.
• Figure 3: TMD PDFs organized according to quark and nucleon polarization. The labels U, L and T refer to unpolarized (i.e. integrated over spin), longitudinally polarized, and transversely polarized, respectively. A $\perp$ superscript in the PDF name indicates explicit dependence on quark transverse momentum $k_T$; the PDFs $f_1$, $g_1$ and $h_{1T}$ along the main diagonal are integrated over $k_T$. From Longo.
• Figure 4: Diagrammatic depiction of the nucleon spin, parton spin, and parton transverse momentum correlations encoded by TMD PDFs and FFs.
• Figure 5: Summary of CT3 correlators for unpolarized (U), longitudinally polarized (L) and transversely polarized (T) hadrons. From Pitonyak_collinear_TSAs.