On the slope of the scalar Kℓ3 decay form factor

We discuss a mechanism, based on exact threshold theorems in the limit of SU(2) × SU(2) symmetry, which can be used to estimate a certain suppression of the scalar K_?3 decay form factor at low momentum transfers, while maintaining the validity of the Callan-Treiman relation.     

Chiral-limit theorems and corrections to PCAC in a model for the scalar Kℓ3 divergence form factor

Three sum rules on vertex functions are derived by applying a model on two-particle Kπ channels. SU(3) × SU(3) breaking is introduced by corrections to the PCAC forms of the axial currents. Taking the m_π → 0 limit the sum rules reduce to the Callan-Treiman theorem, a recent theorem by Dashen, Li, Pagels and Weinstein (DLPW) and a third relation. We encounter non-analytic correction terms in the two first sum rules. In the DLPW theorem we find, contrary to a previous suggestion, non-negligible ε and ε²ln ε terms which are due to the large K-mass. These terms are responsible for a curvature of the scalar K_?3 form factor, however negative values of the slope at the origin require also substantial SU(2) × SU(2) violations. The numerical results taking into account physical masses compare favourably with K_?3 decay and also with s-wave Kπ scattring data.     

Large transverse momenta from statistical bootstrap with spin

We show that statistical bootstrap models (SBM) with their prediction of exponentially decreasing p distributions (equivalently: constant maximal temperature τ₀ ≈ m_π) are not ruled out by the large p_⊥ observed above ≈ 100 GeV lab energy in hadron collisions. Usual SBM sum over all spin angular momenta of the decaying fireball; if a particular spin is fixed, SBM leads to large p_⊥. We discuss in this paper (after having justified a classical approach) several mechanisms yielding high-mass, high-spin fireballs, select arbitrarily one of these (single fireball excitation) and calculate in SBM with fixed spin the decay structure. The cross section $\text{d}\text{σ}\text{d}\text{M}$ is taken to be an empirical function, fitted to existing p_⊥ data at one energy and then calculable at other energies. We can easily reproduce the p_⊥ distribution. Further results concern multiplicities rising with p_⊥, anisotropy of the single-particle inclusive distribution and various correlations. Unexpectedly, but a posteriori rather naturally a jet-like structure is found in the spin-orthogonal plane.     

Origin of low-energy quadrupole collectivity in vibrational nuclei

The coupling of the giant quadrupole resonance to valence-space configurations is shown to be the origin of the formation of low-lying quadrupole-collective structures in vibrational nuclei with symmetric and mixed-symmetric character with respect to the proton-neutron degree of freedom. For the first time experimental evidence for this picture is obtained from electron- and proton scattering experiments on the nucleus ^{92}Zr that are sensitive to the relative phase of valence-space amplitudes by quantum interference.     

Thermal effects on neutrino-nucleus inelastic scattering in stellar environments

Thermal effects for inelastic neutrino-nucleus scattering off even-even nuclei in the iron region are studied. Allowed and first-forbidden contributions to the cross sections are calculated within the quasiparticle random-phase approximation, extended to finite temperatures within the Thermo-Field-Dynamics formalism. The GT₀ strength distribution at finite temperatures is calculated for the sample nucleus ⁵⁴Fe. The neutral-current neutrino-nucleus inelastic cross section is calculated for relevant temperatures during the supernova core collapse. The thermal population of the excited states significantly enhances the cross section at low neutrino energies. In agreement with studies using a large scale shell-model approach the enhancement is mainly due to neutrino up-scattering at finite temperatures.     

Hyperon-nucleon single-particle potentials with low-momentum interactions

Single-particle potentials in Hartree-Fock approximation for different hyperon-nucleon (YN channels are calculated in the framework of the effective low-momentum YN interaction . In contrast to the nucleon-nucleon interaction, the available experimental data for the YN interaction are scarce. As a consequence, no unique YN low-momentum potential can be predicted from the various bare potentials. The resulting momentum- and density-dependent single-particle potentials for several different bare OBE models and for chiral effective field theory are compared to each other.