On the positivity of MSbar parton distributions

Abstract

We revisit our argument that shows that parton distribution Functions (PDFs) in the MSbar{ scheme are non-negative in the perturbative region, with the main goals of elucidating its domain of validity and clarifying its theoretical underpinnings. We specifically discuss recent results proving that PDFs can turn negative at sufficiently low scale, we clarify quantitatively various aspects of our derivation of positivity in the perturbative region, and we provide an estimate for the scale above which PDF positivity holds.

19 pages, 1 figure. Argument for the positivity of Eq.(44) corrected, with unchanged conclusion. Eqs (46-47) corrected (also no effect on conclusion). Plot of C_gg in fig.1 corrected (previously affected by bug), conclusions correspondingly strengthened