Event Structure and Double Helicity Asymmetry in Jet Production from Polarized p+p Collisions at sqrt(s) = 200 GeV

TL;DR

This study uses jet production in longitudinally polarized p+p collisions at sqrt(s)=200 GeV to probe the gluon spin contribution to the proton, Delta G, via the double helicity asymmetry A_LL. The analysis reconstructs jets with a seed-cone algorithm in PHENIX Central Arms and validates event structure using PYTHIA with MPI and GEANT-based detector simulations, comparing to NLO pQCD predictions. The measured A_LL is consistent with zero within uncertainties, and the results constrain the integral of Delta G(x) over 0.02<x<0.3, providing 95% and 99% confidence limits. The work demonstrates that MPI-tuned simulations reproduce the event structure and underpins the feasibility of using jet-based measurements to access gluon polarization in the proton.

Abstract

We report on event structure and double helicity asymmetry ($A_LL$) of jet production in longitudinally polarized p+p collisions at $\sqrt{s}$=200 GeV. Photons and charged particles were measured at midrapidity $|η| < 0.35$ with the requirement of a high-momentum ($>2$ GeV/$c$) photon in each event. Measured event structure is compared with {\sc pythia} and {\sc geant} simulations. The shape of jets and the underlying event were well reproduced at this collision energy. For the measurement of jet $A_{LL}$, photons and charged particles were clustered with a seed-cone algorithm to obtain the cluster $p_T$ sum ($p_T^{\rm reco}$). The effect of detector response and the underlying events on $p_T^{\rm reco}$ was evaluated with the simulation. The production rate of reconstructed jets is satisfactorily reproduced with the NLO pQCD jet production cross section. For $4 < p_T^{\rm reco} < 12$ GeV/$c$ with an average beam polarization of $< P > = 49%$ we measured $A_{LL} = -0.0014 \pm 0.0037^{\rm stat}$ at the lowest $p_T^{\rm reco}$ bin (4-5 GeV/$c$) and $-0.0181 \pm 0.0282^{\rm stat}$ at the highest $p_T^{\rm reco}$ bin (10-12 GeV/$c$) with a beam polarization scale error of 9.4% and a $\pT$ scale error of 10%. Jets in the measured $p_T^{\rm reco}$ range arise primarily from hard-scattered gluons with momentum fraction $0.02 < x < 0.3$ according to {\sc pythia}. The measured $A_{LL}$ is compared with predictions that assume various $ΔG(x)$ distributions based on the GRSV parameterization. The present result imposes the limit $-1.1 < \int_{0.02}^{0.3}dx ΔG(x, μ^2 = 1 {\rm GeV}^2) < 0.4$ at 95% confidence level or $\int_{0.02}^{0.3}dx ΔG(x, μ^2 = 1 {\rm GeV}^2) < 0.5$ at 99% confidence level.

Figures (20)

• Figure 1: (color online) PHENIX detector.
• Figure 2: (color online) Relations between the jets defined in this measurement.
• Figure 3: (color online) Distributions of the ratio $p_T^{\rm reco}/p_T^{\rm PY}$ evaluated with (dashed black) pythia default and (solid green) pythia MPI.
• Figure 5: (color online) The relative yields of $q+q$, $q+g$ and $g+g$ subprocesses in the pythia+ geant simulation. The results with all the subprocesses combined are also shown.
• Figure 6: (color online) Subprocess fractions of reconstructed jets as functions of $p_{T}$. It was evaluated with the pythia MPI and geant simulation. It should be noted that the gluon-quark reaction is the dominant reaction in all the momentum region from 4 to 12 GeV/$c$.