arXiv subjects
Publications and source records attributed to Th. Feldmann.
We investigate proton mass effects in the handbag approach to wide-angle Compton scattering. We find that theoretical uncertainties due to the proton mass are significant for photon energies presently studied at Jefferson Lab. With the proposed energy upgrade such uncertainties will be clearly reduced.
In position space the interaction terms of soft-collinear effective theory must be multipole-expanded to obtain interaction terms with homogeneous scaling behaviour. In this note we provide a manifestly gauge-invariant formulation of the theory after this expansion in the presence of non-abelian gauge fields, extending our previous result. We give the effective Lagrangian (including the Yang-Mills Lagrangian for collinear and ultrasoft gluons) and heavy-to-light transition currents to second order in the power expansion, paying particular attention to the field redefinitions that lead to the gauge symmetries of the effective Lagrangian.
An important unresolved question in strong interaction physics concerns the parameterization of power-suppressed long-distance effects to hard processes that do not admit an operator product expansion (OPE). Recently Bauer et al.\ have developed an effective field theory framework that allows one to formulate the problem of soft-collinear factorization in terms of fields and operators. We extend the formulation of soft-collinear effective theory, previously worked out to leading order, to second order in a power series in the inverse of the hard scale. We give the effective Lagrangian and the expansion of ``currents'' that produce collinear particles in heavy quark decay. This is the first step towards a theory of power corrections to hard processes where the OPE cannot be used. We apply this framework to heavy-to-light meson transition form factors at large recoil energy.
Eta-eta' mixing is discussed in the quark-flavor basis with the hypothesis that the decay constants follow the pattern of particle state mixing. On exploiting the divergences of the axial vector currents - which embody the axial vector anomaly - all mixing parameters are fixed to first order of flavor symmetry breaking. An alternative set of parameters is obtained from a phenomenological analysis. We also discuss mixing in the octet-singlet basis and show how the relevant mixing parameters are related to those in the quark-flavor basis. The dependence of the mixing parameters on the strength of the anomaly and the amount of flavor symmetry breaking is investigated. Finally, we present a few applications of the quark-flavor mixing scheme, such as radiative decays of vector mesons, the photon-pseudoscalar meson transition form factors, the coupling constants of eta and eta' to nucleons, and the isospin-singlet admixtures to the pi^0 meson.
We show -- by explicit computation of first-order corrections -- that the QCD factorization approach previously applied to hadronic two-body decays and to form factor ratios also allows us to compute non-factorizable corrections to exclusive, radiative B meson decays in the heavy quark mass limit. This removes a major part of the theoretical uncertainty in the region of small invariant mass of the photon. We discuss in particular the decays B\to K^* γand B\to K^* l^+l^- and complete the calculation of corrections to the forward-backward asymmetry zero. The new correction shifts the asymmetry zero by 30%, but the result confirms our previous conclusion that the asymmetry zero provides a clean phenomenological determination of the Wilson coefficient C_9.
Within the framework of light-cone quantisation we derive the complete and exact overlap representation of skewed parton distributions for unpolarised and polarised quarks and gluons. Symmetry properties and phenomenological applications are discussed.
Recently it has been shown that symmetries emerging in the heavy quark and large recoil energy limit impose various relations on form factors that parametrise the decay of B mesons into light mesons. These symmetries are broken by perturbative effects. In this paper we discuss the structure of heavy-to-light form factors including such effects and compute symmetry-breaking corrections to first order in the strong coupling. As an application of our results we consider the forward-backward asymmetry zero in the rare decay B -> V l^+ l^- and the possibility to constrain potential new physics contributions to the Wilson coefficient C_9.
Recently it has been shown that in the large-recoil limit new symmetries emerge which impose various relations on form factors that parametrise the decay of heavy B mesons into light mesons. These symmetry relations are broken by radiative QCD corrections. We show that these corrections are perturbatively calculable, and present results to first order in the strong coupling constant alpha_s.
We argue that charmonium decays into meson pairs fall into two distinct classes: one that is under control of perturbative QCD and another one that is governed by a soft mechanism. We concentrate on a systematic analysis of J/Psi (Psi') decays into a light pseudoscalar and a light vector meson and eta(c) decays into a pair of light vector mesons. These processes belong to the second class and are characterized by non-conserved hadronic helicity. It is assumed that, in these cases, the charmonium state decays dominantly through a light-quark Fock component by a soft mechanism which is characteristic of OZI-rule allowed strong decays. Estimating the light-quark admixture by meson mixing, we obtain a reasonable description of the branching ratios for these processes.
We consider pion pair production in two-photon collisions, gamma* gamma -> pi+ pi-, at large photon virtuality Q^2 and center-of-mass energy \sqrt{s} in the hard-scattering picture. In the limit where s is large but s << Q^2 we derive an expression for the two-pion light-cone distribution in terms of the conventional pion distribution amplitudes. Our result reproduces the well-known scaling behavior, helicity selection rule and symmetry relations in this regime.
I discuss the progress in our theoretical understanding of the eta-gamma and eta'-gamma transition form factors, including the recent data from CLEO and L3 at large momentum transfer, Q^2. The experimental data above Q^2=1 GeV^2 can be well described by the hard scattering approach if the distribution amplitudes for eta and eta' mesons are taken close to the asymptotic one. Particular attention is paid to the interpretation of the data in terms of properly defined eta-eta' mixing parameters. I also comment on the use and misuse of interpolation formulas for P-gamma and P-gamma^* transition form factors.
We investigate the b-u skewed parton distributions (SPDs) for B->pi transitions and determine the contributions from several sources (overlaps of soft light-cone wave functions, quark-antiquark annihilations and meson resonances). The B->pi transition form factors, which are relevant in exclusive semi-leptonic and non-leptonic B-decays, are obtained by integrating the b-u SPDs over the momentum fraction x. A phenomenological determination of the relevant parameters allows us to predict the form factors and to obtain the branching ratios for semi-leptonic B->pi decays.
We derive the dimensional non-perturbative part of the QCD effective action for scalar and pseudoscalar meson fields by means of chiral and conformal bosonization. The related structural coupling constants L_5 and L_8 of the chiral lagrangian are estimated using general relations which are valid in a variety of chiral bosonization models without explicit reference to model parameters. The asymptotics for large scalar fields in QCD is elaborated, and model-independent constraints on dimensional coupling constants of the effective meson lagrangian are evaluated. We determine also the interaction between scalar quarkonium and the gluon density and obtain the scalar glueball-quarkonium potential.