  We compare the transverse momentum (p_T) distribution of inclusive
light-charged-particle production measured by the CDF Collaboration at the
Fermilab Tevatron with the theoretical prediction evaluated at next-to-leading
order in quantum chromodynamics (QCD) using fragmentation functions recently
determined through a global data fit. While, in the lower p_T range, the data
agree with the prediction within the theoretical error or slightly undershoot
it, they significantly exceed it in the upper p_T range, by several orders of
magnitude at the largest values of p_T, where perturbation theory should be
most reliable. This disagreement is too large to be remedied by introducing
additional produced particles into the calculation, and potentially challenges
the validity of the factorization theorem on which the parton model of QCD
relies. Clearly, a breakdown of the factorization theorem, being a fundamental
property of QCD, would be extremely difficult to understand.
