Inclusive pi^0, eta, and direct photon production at high transverse momentum in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV

TL;DR

The study addresses the baseline characterization of high-pT pi0, eta, and direct photon production in p+p and d+Au collisions at 200 GeV, using STAR's Barrel EM Calorimeter and associated detectors. It combines γγ decays, background subtraction, and NLO pQCD comparisons to deliver cross sections, η/π0 ratios, and nuclear modification factors, along with a direct-photon measurement. The results show good agreement with NLO pQCD and PHENIX measurements, unity-like R_dA and R_CP at high pT, and a direct-photon yield consistent with theoretical expectations, establishing crucial baselines for interpreting heavy-ion (Au+Au) results and spin-driven studies. Overall, the paper provides robust benchmarks for hot and cold nuclear matter effects and informs the understanding of parton dynamics and photon production in hadronic collisions.

Abstract

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 Electromagnetic 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.

Figures (30)

• Figure 1: (a) Distribution of $r{} = E_{\textrm{BEMC}}/(E_{\textrm{BEMC}} + p_{\textrm{TPC}})$ in $d + \textrm{Au}$ events, which shows beam background at $r{} >{} 0.8$. The curve corresponds to a second order polynomial fit, constrained to pass through zero at $r{} = 1$, used to estimate the false rejection rate. (b) Distribution of $r$ in $p + p$ events.
• Figure 2: Centrality selection in the $d + \textrm{Au}$ data, based on the FTPC multiplicity $N_{\textrm{FTPC}}$. Three centrality classes were defined, containing $0$--$$$20\%$ most central, $20$--$40\%$ mid-central, and $40$--$$$100\%$ most peripheral events, respectively.
• Figure 3: Electron energy measured in the BEMC after calibration, divided by the momentum measured in the TPC, in the $p + p$ data. Solid line is a Gaussian fit, which shows that the peak is centered at unity.
• Figure 4: Distribution of the distances $D$ between BEMC points and their closest tracks, obtained from $p + p$ HighTower-$$1 data in the bin $4 <{} p_{T}{} <{} 5 \textrm{G} \textrm{e} \textrm{V} / c$. The curve shows a fit to Eq. (\ref{['eq:cpv_distr']}), the vertical line indicates the CPV cut.
• Figure 5: (color online) The energy asymmetry $Z_{\gamma\gamma}$ of photon pairs reconstructed in $p + p$ data (symbols) and in Monte Carlo simulation (histograms) for various triggers, normalized to unity for each trigger.