For PANIC 2014

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Probing Low x Gluons at STAR with Forward Asymmetry Measurements

Adam Gibson for the STAR Collaboration
Department of Physics and Astronomy. Valparaiso University, Valparaiso, IN 46383, USA
May 8, 2013

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 pi0 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.