Evidence of Final-State Suppression of High-p_T Hadrons in Au + Au Collisions Using d + Au Measurements at RHIC

Transverse momentum spectra of charged hadrons with $p_T<$ 6 GeV/c have been measured near mid-rapidity (0.2 $<\eta <$ 1.4) by the PHOBOS experiment at RHIC in Au + Au and d + Au collisions at ${\sqrt{s_{_{NN}}} = \rm {200 GeV}}$\mathrm{.}Thespectrafordifferentcollisioncentralitiesarecomparedto${p + \bar{p}}$ collisions at the same energy. The resulting nuclear modification factor for central Au + Au collisions shows evidence of strong suppression of charged hadrons in the high-$p_T$ region ($>2$ GeV/c). In contrast, the d + Au nuclear modification factor exhibits no suppression of the high-$p_T$ yields. These measurements suggest a large energy loss of the high-$p_T$ particles in the highly interacting medium created in the central Au + Au collisions. The lack of suppression in d + Au collisions suggests that it is unlikely that initial state effects can explain the suppression in the central Au + Au collisions.