Thomas Becher

Baryon chiral perturbation theory in manifestly Lorentz invariant form

Abstract. We show that in the presence of massive particles, such as nucleons, the standard low-energy expansion, in powers of meson momenta and light-quark masses, in general converges in only part of the low-energy region. The expansion of the scalar form factor

On the Structure of Infrared Singularities of Gauge-Theory Amplitudes

\sigma(t)

Infrared singularities of scattering amplitudes in perturbative QCD

, for instance, breaks down in the vicinity of

Factorization and Momentum-Space Resummation in Deep-Inelastic Scattering

t=4M_\pi^2

Dynamical Threshold Enhancement and Resummation in Drell-Yan Production

. In the language of heavy baryon chiral perturbation theory, the proper behaviour in the threshold region results only if the multiple internal-line insertions generated by relativistic kinematics are summed up to all orders. We propose a method that yields a coherent representation throughout the low-energy region, while keeping Lorentz and chiral invariance explicit at all stages. The method is illustrated with a calculation of the nucleon mass and of the scalar form factor to order

Threshold resummation in momentum space from effective field theory

p^4

Toward a NNLO calculation of the anti-B ---> X(s) gamma decay rate with a cut on photon energy: I. Two-loop result for the soft function

.

Origin of the Large Perturbative Corrections to Higgs Production at Hadron Colliders

A closed formula is obtained for the infrared singularities of dimensionally regularized, massless gauge-theory scattering amplitudes with an arbitrary number of legs and loops. It follows from an all-order conjecture for the anomalous-dimension matrix of n-jet operators in soft-collinear effective theory. We show that the form of this anomalous dimension is severely constrained by soft-collinear factorization, non-abelian exponentiation, and the behavior of amplitudes in collinear limits. Using a diagrammatic analysis, we demonstrate that these constraints imply that to three-loop order the anomalous dimension involves only two-parton correlations, with the possible exception of a single color structure multiplying a function of conformal cross ratios depending on the momenta of four external partons, which would have to vanish in all two-particle collinear limits. We suggest that such a function does not appear at three-loop order, and that the same is true in higher orders. Our formula predicts Casimir scaling of the cusp anomalous dimension to all orders in perturbation theory, and we explicitly check that the constraints exclude the appearance of higher Casimir invariants at four loops. Using known results for the quark and gluon form factors, we derive the three-loop coefficients of the 1/n pole terms (with n = 1,...,6) for an arbitrary n-parton scattering amplitude in massless QCD. This generalizes Catanis two-loop formula proposed in 1998.

Comment on form-factor shape and extraction of |V(ub)| from B ---> pi l nu

An exact formula is derived for the infrared singularities of dimensionally regularized scattering amplitudes in massless QCD with an arbitrary number of loops and legs. It is based on the conjecture that the anomalous-dimension matrix of n-jet operators in soft-collinear effective theory is fully determined by three functions of alpha{s}, which can be extracted from known perturbative results for the quark and gluon form factors. This allows us to predict the three-loop coefficients of all 1/;{k} poles for arbitrary n-parton scattering amplitudes, generalizing existing two-loop results.

Direct photon production with effective field theory

Renormalization-group methods in soft-collinear effective theory are used to perform the resummation of large perturbative logarithms for deep-inelastic scattering in the threshold region x {yields} 1. The factorization theorem for the structure function F{sub 2}(x,Q{sup 2}) for x {yields} 1 is rederived in the effective theory, whereby contributions from the hard scale Q{sup 2} and the jet scale Q{sup 2}(1 - x) are encoded in Wilson coefficients of effective-theory operators. Resummation is achieved by solving the evolution equations for these operators. Simple analytic results for the resummed expressions are obtained directly in momentum space, and are free of the Landau-pole singularities inherent to the traditional moment-space results. We show analytically that the two methods are nonetheless equivalent order by order in the perturbative expansion, and perform a numerical comparison up to next-to-next-to-leading order in renormalization-group improved perturbation theory.