We present spectra of charged hadrons from Au+Au and d+Au collisions at
sNN=200 GeV measured with the BRAHMS experiment at RHIC. The
spectra for different collision centralities are compared to spectra from p+pˉ collisions at the same energy scaled by the number of binary
collisions. The resulting ratios (nuclear modification factors) for central
Au+Au collisions at η=0 and η=2.2 evidence a strong suppression in
the high pT region (>2 GeV/c). In contrast, the d+Au nuclear
modification factor (at η=0) exhibits an enhancement of the high pT
yields. These measurements indicate a high energy loss of the high pT
particles in the medium created in the central Au+Au collisions. The lack of
suppression in d+Au collisions makes it unlikely that initial state effects can
explain the suppression in the central Au+Au collisions.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let