  We analyze the scaling properties of inclusive hadron production in
proton-proton and in heavy ion collisions from fixed target to collider
energies. At large transverse momentum p_T, the invariant cross section
exhibits a power-like behavior Ed^3\sigma/d^3p\propto p_T^{-n} at fixed
transverse x, x_T=2p_T/\sqrt{s}, and fixed center-of-mass scattering angle
\theta_{cm}. Knowledge of the exponent n allows one to draw conclusions about
the production mechanisms of hadrons, which are poorly known, even at high p_T.
We find that high-p_T hadrons are produced by different mechanisms at
fixed-target and collider energies. For pions, higher-twist subprocesses where
the pion is produced directly dominate at fixed target energy, while
leading-twist partonic scattering plus fragmentation is the most important
mechanism at collider energies. High-p_T baryons on the other hand appear to be
produced by higher-twist mechanisms at all available energies. The higher-twist
mechanism of direct proton production can be verified experimentally by testing
whether high p_T protons are produced as single hadrons without accompanying
secondaries. In addition, we find that medium-induced gluon radiation in heavy
ion collisions can violate scaling.
