Evidence for Parton kT Effects in High pT Particle Production

Abstract

Inclusive pizero and direct-photon cross sections in the kinematic range 3.5 < pT < 12 GeV/c with central rapidities are presented for 530 and 800 GeV/c proton beams and a 515 GeV/c pi- beam incident on beryllium targets. Current Next-to-Leading-Order perturbative QCD calculations fail to adequately describe the data for conventional choices of scales. Kinematic distributions from these hard scattering events provide evidence that the interacting partons carry significant initial-state parton transverse momentum (kT). Incorporating these kT effects phenomenologically greatly improves the agreement between calculations and the measured cross sections.

Figures (6)

• Figure 1: The $\pi^0$ and direct-photon inclusive cross sections as functions of $p_T$ for 515 GeV/$c$$\pi^-$-nucleon interactions compared to NLO PQCD results for several choices of scales. The solid curves show the NLO PQCD results for $Q=p_{T}/2$ scales adjusted for supplemental $\langle k_T \rangle$. (Note that the units for the $\pi^0$ and $\gamma$ results differ by a factor of 1000.)
• Figure 2: The $\pi^0$ and direct-photon inclusive cross sections as functions of $p_T$ for 530 GeV/$c$ proton-nucleon interactions compared to NLO PQCD results for several choices of PDF. The solid curves show the NLO result (using the CTEQ4M PDF) adjusted for supplemental $\langle k_T \rangle$. (Note that the units for the $\pi^0$ and $\gamma$ results differ by a factor of 1000.)
• Figure 3: The $\pi^0$ and direct-photon cross sections as functions of $p_T$ for 800 GeV/$c$ proton-nucleon interactions compared to LO and NLO PQCD results. The solid curves show NLO results adjusted for supplemental $\langle k_T \rangle$. (Note that the units for the $\pi^0$ and $\gamma$ results differ by a factor of 1000.)
• Figure 4: The $\Delta\phi$ distribution for high-mass $\pi^0$ pairs produced in 515 GeV/$c$$\pi^-$-nucleon interactions compared to curves showing LO PQCD results using various $\langle k_T \rangle$ values. The inset shows the pair-$p_T$ distribution for such pairs and the corresponding results of LO PQCD calculations.
• Figure 5: The out-of-plane momentum distributions for high-mass pairs produced in proton-nucleon interactions at 530 and 800 GeV/$c$ compared to results of LO PQCD calculations (using CTEQ4L PDF) for several $\langle k_T \rangle$ values.