Systematic Study of High p_T Hadron Spectra in pp, pA and AA Collisions from SPS to RHIC Energies

TL;DR

This work develops a phenomenological LO QCD framework incorporating intrinsic transverse momentum, its nuclear broadening, and jet quenching to describe high-p_T hadron production across pp, pA, and AB collisions from SPS to RHIC. It demonstrates that intrinsic k_T is essential at low energies, and that K-factor- and regulator-enabled LO calculations can describe data, including Cronin-like enhancements, without invoking jet quenching at SPS. The study places strong limits on parton energy loss at SPS and provides predictions for RHIC that underscore the need for p+A benchmarks to disentangle initial-state effects from true medium-induced energy loss. Overall, the results argue that high-p_T spectra at SPS are dominated by semi-hard scattering with limited or no jet quenching, while RHIC-era measurements require careful control of shadowing and Cronin effects to identify possible dense-matter signatures.

Abstract

High-$p_T$ particle spectra in $p+p$ ($\bar p + p$), $p+A$ and $A+B$ collisions are calculated within a QCD parton model in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically. The intrinsic $k_T$ and its broadening in $p+A$ and $A+B$ collisions due to initial multiple parton scattering are found to be very important at low energies ($\sqrt{s}<50$ GeV). Comparisons with $S+S$, $S+Au$ and $Pb+Pb$ data with different centrality cuts show that the differential cross sections of large transverse momentum pion production ($p_T>1$ GeV/$c$) in $A+B$ collisions scale very well with the number of binary nucleon-nucleon collisions (modulo effects of multiple initial scattering). This indicates that semi-hard parton scattering is the dominant particle production mechanism underlying the hadron spectra at moderate $p_T \stackrel{>}{\sim} 1$ GeV/$c$. However, there is no evidence of jet quenching or parton energy loss. Within the parton model, one can exclude an effective parton energy loss $dE_q/dx>0.01$ GeV/fm and a mean free path $λ_q< 7$ fm from the experimental data of $A+B$ collisions at the SPS energies. Predictions for high $p_T$ particle spectra in $p+A$ and $A+A$ collisions with and without jet quenching at the RHIC energy are also given. Uncertainties due to initial multiple scattering and nuclear shadowing of parton distributions are also discussed.

Figures (14)

• Figure 1: Single-inclusive pion spectra in $p+p$ collisions at $E_{\rm lab}=200$ GeV. The solid lines are QCD parton model calculations with $Q$-dependent intrinsic $k_T$ and the dot-dashed line is without. The dashed line is for a constant intrinsic $\langle k^2_T\rangle_N=1.5$ GeV$^2$. Experimental data are from Ref. cronin-ex1. The inserted figure shows the corresponding $\pi^-/\pi^+$ ratio.
• Figure 2: Single-inclusive pion spectra in $p+p$ collisions at $E_{\rm lab}=300$ GeV. The solid lines are QCD parton model calculations with $Q$-dependent intrinsic $k_T$ and the dot-dashed line is without. Experimental data are from Ref. cronin-ex1. The inserted figure shows the corresponding $\pi^-/\pi^+$ ratio.
• Figure 3: The same as Fig. \ref{['figsps2']}, except at $E_{\rm lab}=400$ GeV
• Figure 4: The same as Fig. \ref{['figsps2']}, except at $E_{\rm lab}=800$ GeV and the experimental data are from Ref. ppex2
• Figure 5: Single-inclusive spectra of charged hadrons in $p+\bar{p}$ collisions at $\sqrt{s}=200, 900, 1800$ GeV. The solid lines are QCD parton model calculations with intrinsic $k_T$ and the dot-dashed lines are without. Experimental data are from Refs. ua1ptcdfpt.