[Paper Review] STAR results and perspectives on transverse spin asymmetries

N. Poljak|arXiv (Cornell University)|Nov 3, 2011

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This study investigates the origin of large transverse single-spin asymmetries in forward $π^0$ production at RHIC by analyzing correlations between $π^0$ mesons and 'jet-like' objects in polarized proton collisions. Using the Forward Pion Detector++ (FPD++) at STAR, the authors isolate the Sivers effect by measuring the azimuthal angle ($\gamma$) between the $\pi^0$ and the jet-like object, finding no significant Collins effect—indicating the Sivers distribution function dominates the observed asymmetry.

ABSTRACT

The STAR collaboration reported precision measurements on the transverse single spin asymmetries for the production of forward $π^0$ mesons from polarized proton collisions at $\sqrt{s}=$ 200 GeV. To disentangle contributions to measured forward asymmetries one has to look beyond inclusive $π^0$ production to the production of forward jets or direct photons. In 2006, STAR with the Forward Pion Detector++ (FPD++) in place, collected 6.8 pb$^{-1}$ of forward data with an average polarization of 60%. FPD++ had sufficient acceptance for "jet-like" objects, which are clustered responses of an electromagnetic calorimeter primarily sensitive to incident photons, electrons and positrons. For these objects, the angle of the outgoing leading $π^0$ with respect to the fragmenting parton was reconstructed, thus enabling us to disentangle the contributions to the forward $π^0$ asymmetries. The simulated data set shows that on average there are approximately 2.5 fragmenting mesons per one "jet-like" object, making them reasonably "jetty". Preliminary results provide no evidence of measured contributions to the asymmetry from jet fragmentation, implying the Sivers distribution functions play a substantial role in producing the large inclusive forward $π^0$ asymmetries. A similar effort was made in the mid-rapidity $(|η|<1)$ region of the STAR detector, where 2.2 pb$^{-1}$ of data was collected. We present progress made by making measurements of the azimuthal asymmetry of leading charged pions in jets produced by transversely polarized proton collisions.

Motivation & Objective

• To disentangle the contributions of the Sivers and Collins effects to large transverse single-spin asymmetries in forward $π^0$ production.
• To determine whether the observed asymmetries arise from initial-state transverse momentum distributions (Sivers) or final-state fragmentation (Collins) effects.
• To use 'jet-like' objects reconstructed from electromagnetic calorimeter responses to probe the angular dependence of $π^0$ production relative to the fragmenting parton.
• To improve understanding of transverse momentum-dependent (TMD) parton distributions in high-energy proton-proton collisions.
• To provide constraints on the Sivers distribution function using precision measurements in the forward rapidity region ($\langle\eta\rangle \approx 3.3$) and mid-rapidity ($|\eta|<1$).

Proposed method

• Utilized the Forward Pion Detector++ (FPD++) at STAR to collect 6.8 pb$^{-1}$ of data with 60% average proton polarization at $\sqrt{s} = 200$ GeV.
• Identified 'jet-like' objects as clustered responses in the electromagnetic calorimeter, primarily sensitive to photons, electrons, and positrons.
• Reconstructed the angle $\gamma$ between the leading $\pi^0$ and the thrust axis of the 'jet-like' object to probe spin-dependent azimuthal correlations.
• Applied a cross-ratio formula to calculate the asymmetry $A_N(\gamma)$, minimizing detector effects via geometric mean over detector modules.
• Used PYTHIA 6.222 and GEANT simulations to model event kinematics and detector response, with digitization and reconstruction matching data processing.
• For mid-rapidity analysis, applied the Midpoint Cone Algorithm to reconstruct jets from TPC tracks and EMC towers, with $p_T > 10$ GeV and cone radius $R = 0.7$.

Experimental results

Research questions

• RQ1 Is the observed forward $\pi^0$ transverse single-spin asymmetry primarily driven by the Sivers effect or the Collins effect?
• RQ2 Does the azimuthal dependence of $\pi^0$ production relative to 'jet-like' objects show a signature consistent with the Collins mechanism?
• RQ3 To what extent do simulated 'jet-like' objects resemble real jets in terms of multiplicity and kinematics?
• RQ4 How do the asymmetries in the forward and mid-rapidity regions compare, and what do they imply for TMD parton distribution functions?
• RQ5 Can the $\gamma$-angle dependence of the asymmetry distinguish between initial-state (Sivers) and final-state (Collins) spin-orbit correlations?

Key findings

• The $\gamma$-angle dependence of the asymmetry in the forward region shows no significant slope, indicating no measurable Collins effect contribution.
• The measured asymmetry for $x_F > 0$ is positive and significant at 1$\sigma$, but independent of $\cos(\gamma)$, consistent with the Sivers effect being the dominant source.
• Simulated 'jet-like' objects have an average of 2.5 fragmenting mesons per object, confirming their 'jetty' nature and suitability for TMD analysis.
• The spin-averaged $\gamma$ distribution is shaped by detector acceptance, not physics, as shown by decreasing $\chi^2$ when acceptance is restricted.
• Mid-rapidity analysis of charged pion azimuthal distributions within jets shows large uncertainties, but future improvements are expected with more statistics and refined methods.
• Theoretical estimates suggest the $\pi^0$ Collins fragmentation function is negligible due to isospin symmetry, supporting the null result for the Collins effect.