Identified hadron spectra at large transverse momentum in p+p and d+Au collisions at \sqrts_NN = 200 GeV

TL;DR

We investigate identified hadron production at high $p_T$ in $p$+$p$ and $d$+Au collisions at $\sqrt{s_{NN}}=200$ GeV to test perturbative QCD factorization and fragmentation universality, providing crucial reference spectra for Au+Au studies. The STAR analysis uses the TPC and TOF for midrapidity PID across $0.3<p_T<10$ GeV/$c$, and contrasts the results with NLO pQCD calculations using KKF/AKK fragmentation functions and with the EPOS model, exploring $x_T$-scaling and the nuclear modification factor $R_{\mathrm{dAu}}$. Key findings include a clear $R_{\mathrm{dAu}}>1$ for pions and an even larger enhancement for protons in the 2–5 GeV/$c$ range, and a flavor-dependent pattern in baryon/meson production that favors AKK fragmentation functions. Overall, the results reinforce the role of hard scattering plus fragmentation in high-$p_T$ hadron production and provide stringent constraints on jet-quenching and quark-recombination mechanisms relevant to heavy-ion physics.

Abstract

We present the transverse momentum (pT) spectra for identified charged pions, protons and anti-protons from p+p and d+Au collisions at \sqrts_NN = 200 GeV. The spectra are measured around midrapidity (|y| < 0.5) over the range of 0.3 < pT < 10 GeV/c with particle identification from the ionization energy loss and its relativistic rise in the Time Projection Chamber and Time-of-Flight in STAR. The charged pion and proton+anti-proton spectra at high pT in p+p and d+Au collisions are in good agreement with a phenomenological model (EPOS) and with the next-to-leading order perturbative quantum chromodynamic (NLO pQCD) calculations with a specific fragmentation scheme and factorization scale. We found that all proton, anti-proton and charged pion spectra in p+p collisions follow xT-scalings for the momentum range where particle production is dominated by hard processes (pT > 2 GeV/c). The nuclear modification factor around midrapidity are found to be greater than unity for charged pions and to be even larger for protons at 2 < pT < 5 GeV/c.

Figures (7)

• Figure 1: $dE/dx$ distribution normalized by pion $dE/dx$ at 4.5 $<$$p_{\mathrm T}$$<$ 5.0 GeV/$c$ and $\mid\eta\mid$$<$ 0.5, and shifted by $\pm$5 for positively and negatively charged particles, respectively. The distributions are for minimum bias $d$+Au collisions. The pion, proton and anti-proton peak positions are indicated by arrows.
• Figure 2: Midrapidity ($\mid$$y$$\mid$$<$ 0.5) transverse momentum spectra for charged pions, proton and anti-proton in $p$+$p$ and $d$+Au collisions for various event centrality classes. Minimum bias distributions are fit to Levy functions which are shown as dashed curves.
• Figure 3: Nuclear modification factors, $R_{\mathrm dAu}$, for charged pions $\pi^{+}+\pi^{-}$ and $p$+$\bar{p}$ at $\mid$$y$$\mid$$<$ 0.5 in minimum bias and 0-20% central $d$+Au collisions. For comparison results on inclusive charged hadrons (STAR) from Ref. star_rdau at $\mid$$\eta$$\mid$$<$ 0.5 are shown by dashed curves. The first two shaded bands around $1$ correspond to the error due to uncertainties in estimating the number of binary collisions in minimum bias and 0-20% central $d$+Au collisions respectively. The last shaded band is the normalization uncertainty from trigger and luminosity in $p$+$p$ and $d$+Au collisions.
• Figure 4: Ratio of $\pi^{-}/\pi^{+}$, $\bar{p}/p$, $p/\pi^{+}$, $\bar{p}/\pi^{-}$ at midrapidity ($\mid$$y$$\mid$$<$ 0.5) as a function of $p_{\mathrm T}$ in $p$+$p$ minimum bias collisions. For comparison the results from lower energies at ISR isr53 and FNAL fnal are also shown for $p/\pi^{+}$ and $\bar{p}/\pi^{-}$ ratios. The dotted curves are the results from PYTHIA. The shaded bands below the $\pi^{-}/\pi^{+}$ and $\bar{p}/p$ ratios are the point--to--point correlated errors in the yields associated with the ratio.
• Figure 5: Same as Fig. \ref{['fig3a']} for d+Au minimum bias collisions. For comparison the $p/\pi^{+}$-ratio from lower energies at FNAL fnal are shown.