Towards a Global QCD Analysis of Fragmentation Functions at Next-to-Next-to-Leading Order Accuracy.

We carry out a global QCD analysis of parton-to-pion fragmentation functions at next-to-next-to-leading order (NNLO) accuracy by performing a fit to the combined set of single-inclusive electron-positron annihilation and, for the first time, semi-inclusive deep-inelastic scattering multiplicity data. For the latter, we utilize the approximate NNLO QCD corrections that were derived recently within the threshold resummation formalism. We explore the impact of the NNLO corrections on the description of the semi-inclusive deep-inelastic scattering datasets in various kinematic regimes and on the resulting pion fragmentation functions.

Jet Production in Polarized Deep Inelastic Scattering at Next-to-Next-to-Leading Order.

We present the next-to-next-to-leading order (NNLO) calculation for single-inclusive jet production in polarized deep inelastic lepton-nucleon scattering e[over →]p[over →]→jet+X. We perform the computation based on the projection-to-Born method by combining our recent next-to-leading order result for di-jet production in polarized deep inelastic lepton-nucleon scattering along with the NNLO coefficients for the inclusive cross section. In this way, we achieve NNLO accuracy in a fully exclusive way for single-jet observables, the first time for a polarized cross section. We study the perturbative stability and phenomenological consequences of the QCD corrections for Electron-Ion Collider kinematics.

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 MS ¯ coupling α S . Although both effective couplings are well-defined in the physical four-dimensional space time, we examine their behaviour in d = 4 - 2 ϵ space time dimensions. We uncover an all-order perturbative relation with the cusp anomalous dimension: the (four dimensional) cusp anomalous dimension is equal to the d-dimensional soft-gluon effective coupling at the conformal point ϵ = ß ( α S ) , where the d-dimensional QCD ß -function, ß ( α S ) - ϵ , vanishes. We present the explicit expressions of the two soft-gluon couplings up to O ( α S 2 ) in d dimensions. In the four-dimensional case we compute the two soft couplings up to O ( α S 3 ) . For one of the two couplings, we confirm the O ( α S 3 ) result previously presented by other authors. For the other coupling, we obtain the explicit relation with the cusp anomalous dimension up to O ( α S 4 ) . We comment on Casimir scaling at O ( α S 4 ) .

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.108.072001.

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.

Higgs Boson pair production at next-to-next-to-leading order in QCD.

We compute the next-to-next-to-leading order QCD corrections for standard model Higgs boson pair production inclusive cross section at hadron colliders within the large top-mass approximation. We provide numerical results for the LHC, finding that the corrections are large, resulting in an increase of O(20%) with respect to the next-to-leading order result at c.m. energy sqrt[sH]=14 TeV. We observe a substantial reduction in the scale dependence, with overlap between the current and previous order prediction. All our results are normalized using the full top- and bottom-mass dependence at leading order. We also provide analytical expressions for the K factors as a function of sH.

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. We present selected numerical results related to Higgs boson searches at the LHC and corresponding results at the Tevatron.

Vector boson production at hadron colliders: a fully exclusive QCD calculation at next-to-next-to-leading order.

We consider QCD radiative corrections to the production of W and Z bosons in hadron collisions. We present a fully exclusive calculation up to next-to-next-to-leading order (NNLO) in QCD perturbation theory. To perform this NNLO computation, we use a recently proposed version of the subtraction formalism. The calculation includes the gamma-Z interference, finite-width effects, the leptonic decay of the vector bosons, and the corresponding spin correlations. Our calculation is implemented in a parton level Monte Carlo program. The program allows the user to apply arbitrary kinematical cuts on the final-state leptons and the associated jet activity and to compute the corresponding distributions in the form of bin histograms. We show selected numerical results at the Fermilab Tevatron and the LHC.

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. We find evidence for a rather small gluon polarization in the nucleon, over a limited region of momentum fraction, and for interesting flavor patterns in the polarized sea.

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. Results for analogous asymmetries in the heavy-quark sector are also presented.