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Perturbative NLO and NNLO QCD evolutions of parton distributions are studied, in particular in the (very) small-x region, where they are in very good agreement with all recent precision measurements of F p 2 (x,Q 2 ). These predictions turn out to be also rather insensitive to the specific choice of the factorization scheme (MS or DIS). A characteristic feature of perturbative QCD evolutions is a positive curvature ofF p 2 which increases as x decreases. This perturbatively stable prediction provides a sensitive test of the range of validity of perturbative QCD. The curvature of DIS structure functions like F p 2 (x,Q 2 ), i.e., its second derivative with respect to the photon’s virtuality Q 2 at fixed values of x, plays a decisive role in probing the range of validity of perturbative QCD evolutions of parton distributions in the small-x region. This has been observed recently [1, 2, 3, 4] and it was demonstrated that NLO(MS) evolutions imply a positive curvature which increases as x decreases. Such rather unique predictions provide a check of the range of validity of perturbative QCD evolutions. However, the curvature is a rather subtle mathematical quantity which a priori may sensitively depend on the theoretical (non)perturbative assumptions made for calculating it. Our main purpose is to study the dependence and stability of the predicted curvature with respect to a dieren t choice of the factorization scheme (DIS versus MS) and to the perturbative order of the evolutions by extending the common NLO (2-loop) evolution [3] to the next-to-next-toleading 3-loop order (NNLO) [4]. The valence qv = uv, dv and sea w = ¯

The Curvature of F2p(x, Q2) as a Probe of Perturbative QCD Evolutions in the Small-x Region

The Curvature of F₂^p(x, Q²) as a Probe of Perturbative QCD Evolutions in the Small-x Region