Radiative Amplitudes Research Articles

Scaling properties of the dielectronic recombination amplitudes

The dielectronic recombination amplitude is given in the resonance approximation by a product of the radiationless capture amplitude $S$ to one of the resonant intermediate states and the radiative decay amplitude $R$ of that resonant state. For highly charged ionic targets, both $S$ and $R$ are expected to scale approximately in the nuclear core charge ${Z}_{C}$ for each isoelectronic sequence, and we study the scaling property and its breaking as the number of electrons in the ion increases. The effects of Coulombic and exchange correlations as well as the relativistic correction are considered. Based on the explicit evaluation of the $S$ and $R$ amplitudes using Hartree-Fock wave functions, we have determined an effective scaling function, which has been used to predict the radiation and Auger-emission probabilities ${A}_{r}$ and ${A}_{a}$ for Ar, Mo, and W from the available values for Fe.

Weak radiative decays of baryons in SU(4)

Various sum rules for the weak radiative decay amplitudes of charmed and charm-zero baryons are derived in the framework of SU(3) and SU(4) with and without octet/sextet and 20 dominance, respectively. The effects of the inclusion of the right-handed currents and short-distance contributions are discussed.

Radiative transitions induced in heavy-ion collisions

Amplitudes for radiative transitions in fast-rotating molecules and heavy-ion collisions are derived. The effect of the Coriolis force on the electron motion and on the radiative transition amplitude is formulated. The term in the transition amplitude dependent on the angular velocity reflects the influence of the rotation of the states.

A study of the distribution of partial radiative widths and amplitudes for 149Sm(n, γ) 150Sm

A new approach to the study of width distributions is developed. The approach makes use of level-level interference effects in (n,γ) reactions to produce a set of radiative width amplitudes. This set is compared to the width distribution and to the hypothesis of Porter and Thomas for the case of 149Sm(n, γ) 150Sm. Both width and amplitude distributions show small but significant deviations from the hypothesis for transitions to low-lying 3 + and 4 + states in 150Sm. No significant width correlations are observed, nor is there apparent any violation of the assumption of random phases for the amplitudes.

Weak radiative decays of Σ and Λ hyperons

Unitarity and dispersion theory are used to calculate the contribution of the πN intermediate state to the weak radiative decays of Σ and Λ hyperons. The contribution from the octet baryon poles is also calculated. The current × current form is assumed for the weak Hamiltonian so that the weak parity-violating two-body vertex vanishes in the limit of SU (3) symmetry and the parity-violating octet baryon pole terms are zero. Consequently the parity-violating weak radiative decay amplitude comes entirely from the πN intermediate state. Our main results are: (i) the contribution of the πN intermediate state is comparable to the contribution of the octet baryon pole terms and (ii) the correct sign and order of magnitude for the asymmetry parameter α and a branching ratio in moderate agreement with the recent experiment of Gershwin et al. are obtained for the decay Σ + → P γ.

Internal bremsstrahlung and the mechanism of direct nuclear reactions: The triangle mechanism

The IBS spectrum accompanying a direct nuclear reaction is studied in a model using the non-relativistic Feynman-Shapiro diagram technique. The main attention is paid to the triangle diagrams. The threshold singularities and the magnitudes of angular momenta of particles interacting in vertices are investigated with regard to their influence on the shape of the IBS spectrum. The pole and the triangle diagram contributions to the total radiative amplitude are compared. The possibility of the experimental identification of the nuclear reaction mechanism is discussed.

Derivation of Relations of the Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin Type

An alternative method for deriving Kawarabayashi-Suzuki-Riazuddin-Fayyazuddin- (KSRF-) type relations, which uses a low-energy theorem in quantum electrodynamics, is presented. The method is applied to processes of the kind $V\ensuremath{\rightarrow}P+{P}^{\ensuremath{'}}+\ensuremath{\gamma}$. The amplitude for this process is exactly calculable to order zero in the photon momentum $k$ in terms of the strong vertex ${f}_{\mathrm{VP}{P}^{\ensuremath{'}}}({{m}_{V}}^{2}, {{m}_{P}}^{2}, {{m}_{P}}^{2})$, with all particles on the mass shell. KSRF-type relations are obtained when the momentum-independent term from the series expansion (in powers of the photon momentum) of the radiative amplitude is equated with the parallel term obtained from an explicit calculation of the amplitude. The latter calculation involves the use of current-algebra, partial conservation of axial-vector current, and hard-pion techniques. The derivation is worked out in detail for the $\ensuremath{\rho}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\gamma}$ process, and the method is further considered in connection with the appropriate radiative decays of ${\ensuremath{\varphi}}^{0}$ and ${K}^{*0}$ mesons.

Current-Algebra Calculation of theKe5Vector Form Factors

From considerations of the current-commutation relations of Gell-Mann and the assumption of a partially conserved axial-vector current, the vector form factors for the rare ${K}_{e5}$ decays are calculated. The presence of $K$-meson and $\ensuremath{\pi}$-meson pole terms in certain radiative amplitudes is responsible for the complicated momentum dependence of these form factors. The decay rates for the various ${K}_{e5}$ modes are evaluated, and the results are compared to previous estimates based on various models.

Chirality Conservation and Soft Pion Production

A formally ${\ensuremath{\gamma}}_{5}$-invariant system consisting of a Dirac field and a massless pseudoscalar field allows chirality conservation in the sense that its expectation value is a constant of motion. This leads to the consequence that in any reaction a change in the fermion chirality (\ensuremath{\sim}helicity\ifmmode\times\else\texttimes\fi{}velocity) is compensated for by the emission of a massless boson at zero energy, which can be expressed by a simple formula relating the radiative amplitude to the elastic amplitude. Assuming the pion-nucleon system to be ${\ensuremath{\gamma}}_{5}$-invariant when the pion mass can be neglected, the formula is applied to the processes $N+\ensuremath{\pi}\ensuremath{\rightarrow}N+\ensuremath{\pi} \mathrm{and} N+2\ensuremath{\pi}$. A reasonable agreement with experiment is obtained in a case dominated by the 3-3 resonance.