Speaker
Mr
Adam Gibson
(Valparaiso University)
Description
One of the major goals of the proton spin physics program at RHIC at BNL is to constrain the gluon polarization distribution $\Delta g(x)$ and thus determine the contribution of gluons to the spin of the proton. Measurements of spin asymmetries with jets and neutral pions at central pseudorapidities at STAR and PHENIX have been critical to this effort. A recent global analysis has, for the first time, yielded evidence of a positive $\Delta g$ in the region $0.05 < x < 0.2$. But, the gluon polarization remains under-constrained for the region $x < 0.05$. A variety of measurements at STAR are planned and underway to better constrain the low-$x$ gluon distribution with asymmetry measurements at non-central, intermediate and forward, pseudorapidities using neutral pion and dijet probes.
The STAR endcap electromagnetic calorimeter (EEMC) allows measurements of photonic channels in an intermediate pseudorapidity range, $0.8 < \eta < 2.0$. Results of a recent publication of the $\pi^{0}$ double longitudinal spin asymmetry, $A_{LL}$, at $\sqrt{s} = 200$ GeV will be presented. The status of measurements in the EEMC with much larger datasets, at $\sqrt{s} = 510$ GeV, will be discussed, with the prospect of probing significantly lower $x$ gluons. In an even more forward placement, the STAR forward electromagnetic calorimeter (called FMS) covers $2.65 < \eta < 4.0$. Prospects of, and progress toward, measuring $A_{LL}$ for neutral pions in the FMS with the large 2013 dataset at $\sqrt{s} = 510$ GeV will be discussed. Finally, proposed forward upgrades at STAR envision both electromagnetic and hadronic calorimetry in a pseudorapidity range similar to that currently covered by the FMS. The case of forward dijet production, and $A_{LL}$, at $\sqrt{s} = 500$ GeV with an upgraded detector will be presented, with the possibility of probing $\Delta g$ at values as low as $10^{-3}$ in $x$.
Primary author
Mr
Adam Gibson
(Valparaiso University)