Boson Expansion Methods in (1+1)-dimensional Light-Front QCD

We derive a bosonic Hamiltonian from two dimensional QCD on the light-front. To obtain the bosonic theory we find that it is useful to apply the boson expansion method which is the standard technique in quantum many-body physics. We introduce bilocal boson operators to represent the gauge-invariant quark bilinears and then local boson operators as the collective states of the bilocal bosons. If we adopt the Holstein-Primakoff type among various representations, we obtain a theory of infinitely many interacting bosons, whose masses are the eigenvalues of the 't Hooft equation. In the large $N$ limit, since the interaction disappears and the bosons are identified with mesons, we obtain a free Hamiltonian with infinite kinds of mesons.Comment: 20 pages, latex, no figures, journal version (no significant changes), to appear in Phys. Rev.

Three-Nucleon Forces from Chiral Effective Field Theory

We perform the first complete analysis of nd scattering at next-to-next-to-leading order in chiral effective field theory including the corresponding three-nucleon force and extending our previous work, where only the two-nucleon interaction has been taken into account. The three-nucleon force appears first at this order in the chiral expansion and depends on two unknown parameters. These two parameters are determined from the triton binding energy and the nd doublet scattering length. We find an improved description of various scattering observables in relation to the next-to-leading order results especially at moderate energies (E_lab = 65 MeV). It is demonstrated that the long-standing A_y-problem in nd elastic scattering is still not solved by the leading 3NF, although some visible improvement is observed. We discuss possibilities of solving this puzzle. The predicted binding energy for the alpha-particle agrees with the empirical value.Comment: 36 pp, 20 figure

In-medium operator product expansion for heavy-light-quark pseudoscalar mesons

The operator product expansion (OPE) for heavy-light-quark pseudoscalar mesons (D-mesons and B-mesons) in medium is determined, both for a moving meson with respect to the surrounding medium as well as for a meson at rest. First of all, the OPE is given in terms of normal-ordered operators up to mass dimension 5, and the mass of the heavy-quark and the mass of the light-quark are kept finite. The Wilson coefficients of such an expansion are infrared (IR) divergent in the limit of a vanishing light-quark mass. A consistent separation of scales necessitates an OPE in terms of non-normal-ordered operators, which implies operator mixing, where the IR-divergences are absorbed into the operators. It is shown that the Wilson coefficients of such an expansion are IR-stable, and the limit of a vanishing light-quark mass is perfomed. Details of the major steps for the calculation of the Wilson coefficients are presented. By a comparison with previous results obtained by other theoretical groups we have found serious disagreements.Comment: 51 pages, 3 figure