We report a measurement of high-p_T inclusive pi^0, eta, and direct photon production in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV at midrapidity (0 < eta < 1). Photons from the decay pi^0 -> gamma gamma were detected in the Barrel Electromagne
tic Calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross section measurement by STAR is also presented, the signal was extracted statistically by subtracting the pi^0, eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading order perturbative QCD calculations.
Charged-particle spectra associated with direct photon ($gamma_{dir} $) and $pi^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $sqrt{s_{_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to parti
ally discriminate between $gamma_{dir}$ and $pi^0$. Assuming no associated charged particles in the $gamma_{dir}$ direction (near side) and small contribution from fragmentation photons ($gamma_{frag}$), the associated charged-particle yields opposite to $gamma_{dir}$ (away side) are extracted. At mid-rapidity ($|eta|<0.9$) in central Au+Au collisions, charged-particle yields associated with $gamma_{dir}$ and $pi^0$ at high transverse momentum ($8< p_{T}^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $|eta|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $gamma_{dir}$ and $pi^0$, and independent of the $gamma_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.
We report a measurement of the longitudinal double-spin asymmetry A_LL and the differential cross section for inclusive Pi0 production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. The cross section was measured over a transvers
e momentum range of 1 < p_T < 17 GeV/c and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T < 11 GeV/c and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of Pi0s in their parent jets was found to be around 0.7 for electromagnetically triggered events.
