Nonplanar Cusp and Transcendental Anomalous Dimension at Four Loops in N=4 Supersymmetric Yang-Mills Theory.

We compute the nonplanar contribution to the universal anomalous dimension of the SU(4)-singlet twist-two operators in N=4 supersymmetric Yang-Mills theory at four loops through Lorentz spin 18. From this, we numerically evaluate the nonplanar contribution to the four-loop lightlike cusp anomalous dimension and derive the transcendental ζ_{3} and ζ_{5} parts of the universal anomalous dimension for arbitrary Lorentz spin in analytic form. As for the lightlike cusp anomalous dimension and the ζ_{5} part of the universal anomalous dimension, we confirm previous results.

Stability of the Electroweak Vacuum: Gauge Independence and Advanced Precision.

We perform a manifestly gauge-independent analysis of the vacuum stability in the standard model including two-loop matching, three-loop renormalization group evolution, and pure QCD corrections through four loops. All these ingredients are exact, except that light-fermion masses are neglected. We in turn apply the criterion of nullifying the Higgs self-coupling and its beta function in the modified minimal-subtraction scheme and a recently proposed consistent method for determining the true minimum of the effective Higgs potential that also avoids gauge dependence. Exploiting our knowledge of the Higgs-boson mass, we derive an upper bound on the pole mass of the top quark by requiring that the standard model be stable all the way up to the Planck mass scale and conservatively estimate the theoretical uncertainty. This bound is compatible with the Monte Carlo mass quoted by the Particle Data Group at the 1.3σ level.

Gluon- and quark-jet multiplicities with NNNLO and NNLL accuracy.

We present a new approach to consider and include both the perturbative and the nonperturbative contributions to the multiplicities of gluon and quark jets. Thanks to this new method, we have included for the first time new contributions to these quantities obtaining next-to-next-to-leading-logarithmic resummed formulas. Our analytic expressions depend on two nonperturbative parameters with a clear and simple physical interpretation. A global fit of these two quantities shows how our results solve a long-standing discrepancy in the theoretical description of the data.

Factorization breaking in high-transverse-momentum charged-hadron production at the Tevatron?

We compare the transverse-momentum (pT) 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 using fragmentation functions recently determined through a global data fit. While in the lower pT range the data agree with the prediction within the theoretical error or slightly undershoot it, they significantly exceed it in the upper pT range, by several orders of magnitude at the largest values of pT, potentially challenging the factorization theorem.

Large-x resummation in Q2 evolution.

The standard analytic solution to the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equation in Mellin space is improved by resumming the large-x divergences. Explicit results are given to next-to-leading order and next-to-leading logarithmic accuracy, which significantly reduce the parton density functions' theoretical uncertainties, more than the inclusion of next-to-next-to-leading order corrections in some cases, and is, therefore, of paramount importance for the reliable interpretation of ongoing and future experiments with hadron beams or targets, including those at the CERN Large Hadron Collider.

Strong-coupling constant with flavor thresholds at five loops in the modified minimal-subtraction scheme.

We present in analytic form the matching conditions for the strong-coupling constant alphas(nf) (mu) at the flavor thresholds to four loops in the modified minimal-subtraction scheme. Taking into account the present knowledge on the coefficient beta4 of the Callan-Symanzik beta function of quantum chromodynamics, we thus derive a five-loop formula for alphas(nf) (mu) together with appropriate relationships between the asymptotic scale parameters Lamda(nf) for different numbers of flavors nf.

Reconciling open-charm production at the Fermilab Tevatron with QCD.

We study the inclusive hadroproduction of D0, D+, D*+, and D(s)+ mesons at next-to-leading order in the parton model of quantum chromodynamics endowed with universal nonperturbative fragmentation functions fitted to e+e- annihilation data from CERN LEP1. Working in the general-mass variable-flavor-number scheme, we resum the large logarithms through the evolution of the fragmentation functions and, at the same time, retain the full dependence on the charm-quark mass without additional theoretical assumptions. In this way, the cross section distributions in transverse momentum recently measured by the CDF Collaboration in run II at the Fermilab Tevatron are described within errors.

Generalizing the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution of fragmentation functions to the smallest values.

An approach valid to any order which unifies the fixed order Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution of fragmentation functions at large x with soft gluon logarithmic resummation at small x is proposed. At lowest order, this approach, implemented with the double logarithmic approximation, reproduces exactly the modified leading logarithm approximation but is more complete due to the degrees of freedom given to the quark sector and the inclusion of the fixed order terms. We find that data from the largest x values to the peak region can be better fitted than with other approaches.

Width and partial widths of unstable particles.

In the gauge theory context, a definition of branching ratios and partial widths of unstable particles is proposed that satisfies the basic principles of additivity and gauge independence. A simpler definition, similar to the conventional one, is examined in the Z0-boson case. In order to establish contact with experiment, we show that it leads to a peak cross section that justifies the expression used by the LEP Electroweak Working Group through next-to-next-to-leading order, provided that the pole rather than the on-shell mass and width of the Z0 boson is employed.

Strong coupling constant from scaling violations in fragmentation functions.

We present a new determination of the strong coupling constant alpha(s) through the scaling violations in the fragmentation functions for charged pions, charged kaons, and protons. In our fit we include the latest e+e- annihilation data from CERN LEP1 and SLAC SLC on the Z-boson resonance and older, yet very precise, data from SLAC PEP at center-of-mass energy sqaure root of s = 29 GeV. At next-to-leading order, we find alpha(s)(5)(M(Z)) = 0.1170+/-0.0073. A new world average of alpha(s) is given.