Next-to-Next-to-Leading Order Global Analysis of Polarized Parton Distribution Functions.

We present a next-to-next-to-leading order (NNLO) global QCD analysis of the proton's helicity parton distribution functions, the first of its kind. To obtain the distributions, we use data for longitudinal spin asymmetries in inclusive and semi-inclusive lepton-nucleon scattering as well as in weak-boson and hadron or jet production in proton-proton scattering. We analyze the data using QCD perturbation theory at NNLO accuracy, employing approximations provided by the threshold resummation formalism in cases where full NNLO results for partonic hard-scattering functions are not readily available.

Soft-gluon effective coupling and cusp anomalous dimension.

We consider the extension of the CMW soft-gluon effective coupling [1] in the context of soft-gluon resummation for QCD hard-scattering observables beyond the next-to-leading logarithmic accuracy. We present two proposals of a soft-gluon effective coupling that extend the CMW coupling to all perturbative orders in the coupling . Although both effective couplings are well-defined in the physical four-dimensional space time, we examine their behaviour in space time dimensions.

Erratum: Diphoton Production at Hadron Colliders: A Fully Differential QCD Calculation at Next-to-Next-to-Leading Order [Phys. Rev. Lett. 108, 072001 (2012)].

This corrects the article DOI: 10.1103/PhysRevLett.108.

Evidence for polarization of gluons in the proton.

We discuss the impact of recent high-statistics Relativistic Heavy Ion Collider data on the determination of the gluon polarization in the proton in the context of a global QCD analysis of polarized parton distributions. We find evidence for a nonvanishing polarization of gluons in the region of momentum fraction and at the scales mostly probed by the data. Although information from low momentum fractions is presently lacking, this finding is suggestive of a significant contribution of gluon spin to the proton spin, thereby limiting the amount of orbital angular momentum required to balance the proton spin budget.

Diphoton production at hadron colliders: a fully differential QCD calculation at next-to-next-to-leading order.

We consider direct diphoton production in hadron collisions, and we compute the next-to-next-to-leading order QCD radiative corrections at the fully differential level. Our calculation uses the q(T) subtraction formalism, and it is implemented in a parton-level Monte Carlo program. The program allows the user to apply arbitrary kinematical cuts on the final-state photons and the associated jet activity and to compute the corresponding distributions in the form of bin histograms.

Global analysis of helicity parton densities and their uncertainties.

We present a new analysis of the helicity parton distributions of the nucleon. The analysis takes into account the available data from inclusive and semi-inclusive polarized deep inelastic scattering, as well as from polarized proton-proton (p-p) scattering at RHIC. For the first time, all theoretical calculations are performed fully at next-to-leading order (NLO) of perturbative QCD, using a method that allows incorporation of the NLO corrections in a very fast and efficient way in the analysis.

Perturbative generation of a strange-quark asymmetry in the nucleon.

We point out that perturbative evolution in QCD at three loops generates a strange-antistrange asymmetry s(x) - s(x) in the nucleon's sea just from the fact that the nucleon has nonvanishing up and down quark valence densities. The recently computed three-loop splitting functions allow for an estimate of this effect. We find that a fairly sizable asymmetry may be generated.

Next-to-next-to-leading-order logarithmic corrections at small transverse momentum in hadronic collisions.

We study the region of small transverse momenta in q&qmacr;- and gg-initiated processes with no colored particle detected in the final state. We present the universal expression of the O(alpha(2)(s)) logarithmically enhanced contributions up to next-to-next-to-leading-order logarithmic accuracy. From there we extract the coefficients that allow the resummation of the large logarithmic contributions.