Title: Measurement of the unpolarized cross section and double-helicity asymmetry for direct photon production in polarized proton-proton collisions at $\sqrt{s} = 200 \, \mathrm{GeV}$

Abstract: Ever since polarized deep inelastic scattering experiments revealed that the quark polarization is insuffient to completely describe the proton spin, experimental programs have diligently searched for sources of the remaining spin.  Polarized gluons offer a potential contribution, and one uniquely probed by direct photon final states.  In this paper we present the first direct photon cross section and double-helicity asymmetry with the STAR experiment.

Images:

Figure 1: The posterior extraction residuals, defined as the difference between the inferred signal yield and the true yield and represented here by the mean and 68% credibility interval, were consistent with zero even when the signal in the pseudo-data was reweighted by (a) zero, (b) one, (c) two, and (d) a $E_{T}$ -dependent weight that corrected the direct photon cross section assumed in Pythia to that seen in the Run 9 data. Because the same partition of the simulation was used and the reweighting does not change the effective sample size of the pseudo-data, the statistical fluctuations are indeed expected to be identical in each test, except for the case of no signal where the positivity constraint biases the fluctuations.

Figure 2: The inclusive direct photon cross section is in excellent agreement with the resummed NLO calculation, justifying the use of the perturbative theory in constraining the gluon polarization with the direct photon asymmetry.

Figure 3: The direct photon double-helicity asymmetry only mildly discriminates between different models of the gluon polarization due to the limited statistics of the measurement.