The Origin of Large-p_T pi^0 Suppression at RHIC

We present results for inclusive $\pi^0$ production in proton-proton and in Au-Au at RHIC energy $\sqrt s=200$ GeV. We use next-to-leading order perturbative QCD calculation and we include nuclear effects such as parton energy loss and nuclear shadowing. We consider the ratio of $\pi^0$ distribution in Au-Au and p-p collisions for $p_T > 3$ GeV for three cases of parton energy loss: 1) constant parton energy loss per parton scattering, $\epsilon^a_n=const$, 2) Landau-Pomeranchuk-Migdal energy-dependent energy loss, $\epsilon^a_n \sim \sqrt E^a_n$ and 3) Bethe-Heitler energy-dependent energy loss, $\epsilon^a_n \sim E^a_n$. We show that recently observed suppression of $\pi^0$ production in Au-Au collisions at RHIC, which is found to increase with $p_T$ increasing from 3GeV to 8GeV, can be reproduced by $\epsilon^a_n=0.06 E^a_n$. We show that the ratio of prompt photons to neutral pions produced in Au-Au collisions at RHIC has a strong $p_T$ dependence approaching one at $p_T\sim 10$GeV.