[Paper Review] STAR Results from Polarized Proton Collisions at RHIC

L. C. Bland|arXiv (Cornell University)|Mar 6, 2004

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper presents initial results from the STAR experiment at RHIC on polarized proton collisions at √s = 200 GeV, demonstrating large analyzing powers for forward neutral pion and charged hadron production, confirming the persistence of spin-dependent asymmetries from lower-energy experiments. The study establishes a high-precision local polarimeter using the BBC detector, enabling real-time monitoring of beam polarization and validating longitudinal polarization at the interaction point.

ABSTRACT

This talk reports on progress from the first two years of polarized proton collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). STAR is one of the two large experiments at RHIC. It features large acceptance spanning a broad range of rapidity. The long-term goals of the STAR spin program are to measure the gluon contribution to the proton's spin; determine the contribution of specific quark flavors to the spin of the proton through the study of spin observables for vector boson production, and to determine the transversity structure function. The study of polarized proton collisions at $\sqrt{s}$=200 and 500 GeV is expected to provide important insight into the spin structure of the proton, revealing the contributions to the spin sum rule either from gluons or from the orbital motion of the partons. Selected STAR results from the study of polarized proton collisions during the first two runs are presented.

Motivation & Objective

• To measure the gluon contribution to the proton’s spin through inclusive jet and photon production in polarized proton collisions.
• To determine the flavor-dependent quark and antiquark contributions to proton spin via W± and Z⁰ boson production.
• To probe the transversity structure function and orbital angular momentum contributions using transversely polarized proton collisions.
• To validate the performance of spin rotator magnets and establish a local polarimeter for real-time beam polarization monitoring at RHIC.
• To extend leading-order pQCD predictions to exclusive final states such as γ + jet and test next-to-leading-order corrections.

Proposed method

• Utilized the STAR detector at RHIC to collect data during two runs (2001–2002 and 2003) with longitudinally and vertically polarized proton beams at √s = 200 GeV.
• Measured single-spin asymmetries (A_N) for inclusive neutral pion and charged hadron production in high pseudorapidity regions (3.4 < η < 5.0) using the Barrel Calorimeter (BBC).
• Correlated BBC-based asymmetries with data from the CNI polarimeter to validate beam polarization and determine analyzing power (A_N) independently.
• Applied leading-order pQCD with parton shower models to compare theoretical predictions with measured cross sections for forward pion production.
• Used scaler-based readout systems to monitor beam polarization in real time, enabling continuous assessment of spin rotator performance.
• Analyzed time-dependent asymmetries to confirm stable longitudinal polarization at the STAR interaction region when spin rotator magnets were active.

Experimental results

Research questions

• RQ1 Do large analyzing powers for forward pion production observed at lower energies persist at RHIC’s higher collision energy of √s = 200 GeV?
• RQ2 Can the BBC detector serve as a high-precision local polarimeter for monitoring transverse polarization components in polarized proton beams?
• RQ3 What is the magnitude and pseudorapidity dependence of the analyzing power for inclusive charged hadron production in the forward region?
• RQ4 How well do leading-order pQCD calculations with parton showers describe the measured cross section for large-rapidity neutral pion production?
• RQ5 Can the spin rotator system at RHIC reliably produce and maintain longitudinal polarization at the STAR interaction point?

Key findings

• The analyzing power for inclusive neutral pion production at large rapidity (3.4 < η < 5.0) was found to be consistent with NLO pQCD calculations and leading-order pQCD with parton showers.
• A non-zero analyzing power of approximately 0.006 was measured for inclusive charged hadron production in the pseudorapidity range 3.4 < η < 5.0, with a figure of merit sufficient for local polarization monitoring.
• The BBC-based asymmetry measurement showed a strong pseudorapidity dependence, with non-zero analyzing power only when hits were recorded on the inner ring (3.9 < η < 5.0), indicating sensitivity to forward particle flux.
• Time-dependent asymmetry measurements confirmed that the spin rotator magnets successfully produced longitudinal polarization at the STAR interaction region, as evidenced by the drop in BBC asymmetry when rotators were turned on.
• The local polarimeter achieved a statistical accuracy of 3 standard deviations within 30 minutes at a luminosity of 5×10³⁰ cm⁻²s⁻¹ and 25% beam polarization, enabling 10° resolution in polarization direction.
• The observed analyzing power in the BBC was approximately half that measured by the CNI polarimeter (A_N^CNI = 0.013 ± 0.004), confirming consistency between independent polarization monitoring systems.