Recent results from PHENIX on double helicity asymmetry ($A_{LL}$) measurement at $\sqrt{s}$ = 510 GeV

TL;DR

This paper reports recent measurements of double helicity asymmetry ($A_{LL}$) in $ ext{pi}^0$ and $J/ ext{psi}$ production at $ ext{510 GeV}$ from PHENIX, providing new insights into the gluon contribution to proton spin at low Bjorken x.

Contribution

It presents new preliminary $A_{LL}$ measurements at $ ext{510 GeV}$ with increased luminosity, extending the x-range coverage for gluon polarization studies.

Findings

$A_{LL}$ measurements in $ ext{pi}^0$ and $J/ ext{psi}$ at $ ext{510 GeV}$

Coverage of Bjorken x down to 0.01 for $ ext{pi}^0$ and 0.002 for $J/ ext{psi}$

Impact on constraining $ ext{Delta}G$

Abstract

One of the major objectives of the RHIC spin program at BNL is the measurement of the gluon helicity contribution, $\Delta G$, to the proton spin via measuring the double longitudinal spin asymmetry ($A_{LL}$) in various channels. In PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) the $A_{LL}$ in $\pi^{0}$, $\eta$, $J/\psi$ etc. are measured in wide rapidity range. In this talk, $A_{LL}$ in $\pi^{0}$ production in central rapidity and $J/\psi$ production in forward rapidity will be discussed. The $\pi^{0}$ is reconstructed through its diphoton decay channel within the rapidity range of $\left| \eta \right|$ $<$ 0.35 and azimuthal angle of $180\,^{\circ}$. Similarly, $J/\psi$ is reconstructed via dimuon decay channel within the rapidity range of 1.2 $<$ $\left| \eta \right|$ $<$ 2.2. Preliminary results for $A_{LL}$ in $\pi^{0}$ and $J/\psi$ production from the data collected in the year 2013 at center of mass energy ($\sqrt{s}$) = 510 GeV will be presented. Also, their impact on $\Delta G$ constraint will be discussed. In year 2013, the total integrated luminosity was 150 $pb^{-1}$ which is almost ten times the total luminosity recorded in the year 2009 at $\sqrt{s}$ = 200 GeV. Due to increase in the center of mass energy and integrated luminosity, the new measurements can cover the Bjorken x range down to 0.01 for $\pi^{0}$ and 0.002 for $J/\psi$.