Hans D. Dahmen

Kℓ3 form factors and the algebra of charges

Starting from the algebra of vector and axial vector charges, we obtain information about the Kl3 form factors by approximating the commutation relations with the one particle intermediate states. We assume a once subtracted dispersion relation for the weak form factors appearing in the calculation. The subtraction constants can then be uniquely determined as a function of the weak decay constants ƒπ, ƒK and ƒK of the mesons contributing in the intermediate states. The momentum transfer dependence of the Kl3 form factors is characterized by the particle masses, the above mentioned decay constants and the two additional constants gπKκ and ƒK∗gπKK∗, where gπKκ∗ and gπKK∗ are strong coupling constants, and ƒK∗ is the weak decay constant of the K∗ meson. Assuming unsubtracted dispersion relations for the various form factors, we are able to make some numerical predictions.

The photoneutron cross section in 63Cu AND 141Pr

Abstract The (γ, n) cross section in 63 Cu and 141 Pr was measured from threshold to 33 MeV using bremsstrahlung from a betatron and observing the induced 62 Cu and 140 Pr activities. Under adequately high resolution both cross sections show structure, which is well described by the collective theory of Huber and Greiner.

Interference effects in cross sections calculated by photon difference methods

Abstract The photon difference type calculation transforms the photo-nuclear yield into the corresponding cross section. For a cross section containing very narrow resonances approximations in the transformation calculation can simulate interference like effects in the derived cross section, which can be caused by truncation errors and by the use of an incorrect bremsstrahlung spectrum.

The (γ, p) cross section in 208Pb

Abstract The (γ, p) cross section in 208 Pb was measured from threshold to 33 MeV, using bremsstrahlung from a 33 MeV betatron and the induced activity in 207 T1 was observed. The cross section shows detailed structure above a continuous background, in particular a prominent peak at 25 MeV which may be interpreted as due to an E1, ΔT = 1 transition to an isobaric analogue state. The integrated cross section from threshold to 33 MeV was 26.8 ± 0.4 MeV · mb.

Photonuclear quadrupole resonance in 160Gd

Abstract The (γ, n) and (γ, p) cross sections in 160 Gd were measured from threshold to 33 MeV using bremsstrahlung from a 33 MeV betatron and observing the induced activities. Both cross sections contain a broad maximum at 23.5 MeV. It is argued that this maximum is the quadrupole resonance predicted at 1.6 times the E1 giant-resonance energy by the hydrodynamic model.

Subtractions and off-shell continuation of weak form factors

The dependence of the weak form factors on the mass variables is studied in the one-particle approximation, taking the Kl3 form factors as an example. The off-shell continuation considered is the same as in soft-pion calculations. The off-shell dependence is obtained in terms of the subtraction constants of the matrix elements of the axial vector current Aπo and strong coupling constants. Under the assumption of unsubtracted dispersion relations the correction to the soft-pion result is estimated to be small.

Covariant one-particle approximation to chiral algebra and the π ϱ A1 system

Abstract A direct saturation of the commutation relations of axial charges with isospin currents is shown to lead in the covariant one particle approximation to frame independent relations among weak form factors. Assuming the absence of I = 1 operator Schwinger terms we obtain values for the weak decay constants f A and f ϱ and the width Γ Aϱπ of the A 1 meson in reasonable agreement with experiment.

CURRENT ALGEBRA AND DISPERSION RELATIONS.

Abstract In this paper we obtain an extension of the soft pion results to arbitrary points on the light cone. As an illustration of the procedure we study two particular matrix elements of the equal time commutators of charge densities, which we rewrite in the form of Low representation. Under some extra but weak assumptions we obtain then the desired generalization.

Current algebra and ππ scattering lengths

Abstract Starting from the SU(2) × SU(2) commutation relations for the axial vector charges, we derive a Low representation for the combination 2a0−5a2 of the ππ scattering lengths, which involves explicitely the low-energy results plus correction terms. The corrections are estimated by considering in addition canonical commutation relations for the divergence of the axial vector current Aμπ, and assuming the integrals to be dominated by an isoscalar, s-wave ππ resonance. Our approximation is crossing symmetric and includes the effect of the unitarity cut.

On shell, subtracted dispersion relations and low energy theorems from current algebra

A method discussed in an earlier paper is applied to the case of ππ and πN scattering and to electroproduction. As an example we derive for the ππ scattering amplitude a subtracted, covariant on-shell dispersion relation in which the subtraction polynomial is determined by the corresponding low energy theorem plus possibly small corrections. We also show how this same method may be applied to the derivation of low energy theorems.