Light-quark loops in K ->pi nu(nu)over-bar

Physics Letters B

We calculate the O(p6) corrections to the anomalous form factors appearing in π+, K+ → e+νγ, μ+νγ and K14 decays in chiral perturbation theory. The relevant dimension 6 terms of the lagrangian are evaluated assuming their saturation by the vector meson contribution.

Physical Review D, 1998

We calculate long-distance contributions to the amplitudes A(K 0 → 2π, I) induced by the gluon and the electroweak penguin operators Q 6 and Q 8 , respectively. We use the 1/N c expansion within the effective chiral lagrangian for pseudoscalar mesons. In addition, we adopt a modified prescription for the identification of meson momenta in the chiral loop corrections in order to achieve a consistent matching to the short-distance part. Our approach leads to an explicit classification of the loop diagrams into non-factorizable and factorizable, the scale dependence of the latter being absorbed in the low-energy coefficients of the effective theory. Along these lines we calculate the one-loop corrections to the O(p 0) term in the chiral expansion of both operators. In the numerical results, we obtain moderate corrections to B (1/2) 6 and a substantial reduction of B (3/2) 8 .

1999

. The non-leptonic hyperon decays are analyzed up to one-loop order including all counterterms in the framework of heavy baryon chiral perturbation theory. We use the exchange of the spin- decuplet resonances as an indication of which low-energy constants contribute significantly to these investigated processes. We choose four independent decay amplitudes that are not related by isospin relations in order to perform a fit for the pertinent low-energy constants and find a satisfactory fit both for s- and p-waves. The chiral corrections to the lowest order forms for the s-waves are moderate whereas there are significant modifications of the p-wave amplitudes.

Nuclear Physics B, 2003

We present the ingredients necessary for the determination of physical K → ππ decay amplitudes for ∆I = 3/2 transitions, from lattice simulations at unphysical kinematics and the use of chiral perturbation theory at next-to-leading order. In particular we derive the expressions for the matrix elements I=2 ππ|O W |K , where O W is one of the operators appearing in the ∆S = 1 weak Hamiltonian, in terms of low-energy constants at next-toleading order in the chiral expansion. The one-loop chiral corrections are evaluated for arbitrary masses and momenta, both in full QCD and in the quenched approximation. We also investigate the finite-volume effects in this procedure.

Nuclear Physics A, 1997

The next-to-leading order chiral pion-nucleon Lagrangian contains seven finite lowenergy constants. Two can be fixed from the nucleon anomalous magnetic moments and another one from the quark mass contribution to the neutron-proton mass splitting. We find a set of nine observables, which to one loop order do only depend on the remaining four dimension two couplings. These are then determined from a best fit. We also show that their values can be understood in terms of resonance exchange related to ∆ excitation as well as vector and scalar meson exchange. In particular, we discuss the role of the fictitious scalar-isoscalar meson. We also investigate the chiral expansion of the two P-wave scattering volumes P - 1 and P + 2 as well as the isovector S-wave effective range parameter b -. The one-loop calculation is in good agreement with the data. The difference P - 1 -P + 2 signals chiral loop effects in the πN P-waves. The calculated D-and F-wave threshold parameters compare well with the empirical values.

The European Physical Journal C, 2017

A representation of the two-loop contribution to the pion decay constant in SU (3) chiral perturbation theory is presented. The result is analytic up to the contribution of the three (different) mass sunset integrals, for which an expansion in their external momentum has been taken. We also give an analytic expression for the two-loop contribution to the pion mass based on a renormalized representation and in terms of the physical eta mass. We find an expansion of F π and M 2 π in the strange-quark mass in the isospin limit, and we perform the matching of the chiral SU (2) and SU (3) lowenergy constants. A numerical analysis demonstrates the high accuracy of our representation, and the strong dependence of the pion decay constant upon the values of the low-energy constants, especially in the chiral limit. Finally, we present a simplified representation that is particularly suitable for fitting with available lattice data.

Physical Review D, 2000

We recalculate the leading nonanalytic contributions to the amplitudes for hyperon nonleptonic decays in chiral perturbation theory. Our results partially disagree with those calculated before, and include new terms previously omitted in the P-wave amplitudes. Although these modifications are numerically significant, they do not change the well-known fact that good agreement with experiment cannot be simultaneously achieved using one-loop S-and P-wave amplitudes.

Physics Letters B, 1998

Baryon chiral perturbation theory as conventionally applied using dimensional regularization has a well-known problem with the convergence of the SU(3) chiral expansion. One can reformulate the theory equally rigorously using a momentum-space cutoff and we show that the convergence is thereby greatly improved for reasonable values of the cutoff. In effect, this is accomplished because the cutoff formalism removes the spurious physics of propagation at distances much smaller than the baryon size.

Annals of Physics, 2013

We present a novel analysis of the πN scattering amplitude in covariant baryon chiral perturbation theory up to O(p 3) within the extended-on-mass-shell renormalization scheme and including the ∆(1232) explicitly in the δ-counting. We take the hadronic phase shifts provided by partial wave analyses as basic experimental information to fix the low-energy constants. Subsequently, we study in detail the various observables and low-energy theorems related to the πN scattering amplitude. In particular, we discuss the results and chiral expansion of the phase shifts, the threshold coefficients, the Goldberger-Treiman relation, the pion-nucleon sigma term and the extrapolation onto the subthreshold region. The chiral representation of the amplitude in the theory with the ∆ presents a good convergence from very low energies in the subthreshold region up to energies well above threshold, leading also to a phenomenological description perfectly consistent with the one reported by the respective partial wave analyses and independent determinations. We conclude that a model-independent and systematic framework to analyze πN-scattering observables using directly experimental data shall be possible in covariant baryon chiral perturbation theory.

2000

We extend our previous analysis of the unitarized pion-nucleon scattering amplitude including up to fourth order terms in Heavy Baryon Chiral Perturbation Theory. We pay special attention to the stability of the generated ∆(1232) resonance, the convergence problems and the power counting of the chiral parameters.