Projected statistical and systematic uncertainties for direct photon production in run 2015

- center-of-mass energy 200 GeV
- sampled luminosity 40 pb-1
- beam polarization 60%

Background from charged tracks is removed by matching FMS clusters with hits in the preshower. The position resolution in the FMS is much better than the single tower size, so the preshower granularity is defining the limit on the matching cut. The matching distribution for charged particles exhibits a distinct peak at about half of the width of the preshower channels and it falls off steeply towards larger values, making the veto condition very effective to remove this kind of background from the direct photon signal.

Other photons from meson decays have to be separated from the direct photon sample by their event topology. For this, a two-fold selection is applied to the clusters in the FMS. Any event that contains two or more clusters above a certain threshold (1.0 GeV) are discarded. The remaining clusters require an energy of 10.0 GeV or more. This initial cut is reducing the photon background more effectively with increasing pT (dashed line), but it needs further refinement to study the xF-dependence of the transverse single-spin asymmetries with good accuracy. For the background asymmetry, an upper limit of measured neutral pion asymmetries was used to estimate the systematic uncertainties (0.0<AN(xF)<0.18). An additional pT-cut of 2.0 GeV is used for the xF-distribution.


The current projections assume a high-tower trigger with a prescale factor of 500. While this is not a dedicated trigger direct photons, it is considered a conservative estimate especially in combination with the high prescale factor. More realistic studies are needed to define a more effective trigger for direct photons.