Malcolm N. Butler

Strong and electromagnetic decays of the baryon decuplet

Abstract To explore further the role that QCD symmetries play on hadron dynamics, we discuss the radiative decay of the baryon decuplet to the baryon octet and compute the leading non-analytic SU(3) violating corrections induced at the one-loop level. In the limit of exact flavour SU(3) symmetry two of the possible decay modes are forbidden and the rates for these two decays are dominated by loop corrections. There is only one SU(3) conserving contact term at leading order in chiral perturbation theory for the radiative decays. We determine its coefficient from the observed branching ratio for Δ → Nγ and from the present upper limit on Ξ ∗0 → Ξ 0 γ . We then predict the branching fractions for Σ ∗− → Σ − γ, Σ ∗0 → Λγ, Σ ∗0 → Σ 0 γ, Σ ∗+ → Σ + γ, Ξ ∗0 γ → Ξ 0 γ and Ξ ∗− → Ξ − γ . Some of the decay modes are predicted to have branching fractions near the current experimental upper limit and could possibly be observed in the near future. The leading corrections to the octet-decuplet-meson strong coupling constant, C , are computed at one-loop and the strong decay rates of the Δ, Σ ∗ and Ξ ∗ allow us to determine | C | = 1.2 ± 0.1 and H = −2.2 ± 0.6 , in remarkable agreement with predictions based on spin-flavour symmetry. We calculate the Δ pole graph contribution to the polarisability of the nucleon to order 1 Λ χ 2 in chiral perturbation theory.

Electromagnetic moments of the baryon decuplet.

We compute the leading contributions to the magnetic dipole and electric quadrupole moments of the baryon decuplet in chiral perturbation theory. The measured value for the magnetic moment of the [Omega][sup [minus]] is used to determine the local counterterm for the magnetic moments. We compare the chiral perturbation theory predictions for the magnetic moments of the decuplet with those of the baryon octet and find reasonable agreement with the predictions of the large-[ital N][sub [ital c]] limit of QCD. The leading contribution to the quadrupole moment of the [Delta] and other members of the decuplet comes from one-loop graphs. The pionic contribution is shown to be proportional to [ital I][sub [ital z]] (and so will not contribute to the quadrupole moment of [ital I]=0 nuclei), while the contribution from kaons has both isovector and isoscalar components. The chiral logarithmic enhancement of both pion and kaon loops has a coefficient that vanishes in the SU(6) limit. The third allowed moment, the magnetic octupole, is shown to be dominated by a local counterterm with corrections arising at two loops. We briefly mention the strange counterparts of these moments.

Electromagnetic polarisability of the nucleon in chiral perturbation theory

Abstract We compute the polarisability of the nucleon to leading order in chiral perturbation theory. The contributions from kaons and baryon resonances as intermediate states are included in addition to the contribution from pions and nucleons that had been previously computed. The isoscalar operators are dominated by the infrared behaviour of pion loops giving rise to a 1 m π coefficient. In contrast, the isovector operators are dominated by loops involving kaons, giving a 1 m K coefficient, and further demonstrates that the strange quark is an important component of the nucleon. In addition, the inclusion of the decuplet of baryon resonances as intermediate states substantially modifies the result found from the octet baryons alone for the isoscalar polarisability.

E2M1 mixing ratio of Δ→Nγ and hyperon resonance radiative decay

Abstract We complete the leading contribution to the E 2 M 1 mixing ratio of the decay Δ → Nγ in heavy baryon chiral perturbation theory. We find the mixing ratio to be 4%⪅|δ E 2 M 1 |⪅11% , much larger than estimates based on the quark model and other hadronic models. We also compute the mixing ratio for the radiative decay of the hyperon resonances. The decays ⩞ ∗+ →⩞ + γ and Ξ ∗− →Ξ − γ provide a particularly sensitive probe of deviations from heavy baryon spin-flavour SU(6).

Evolution of cosmic rays in the early Galaxy

We calculate the variation of the cosmic-ray spectra at early Galactic epochs arising from dynamical evolution of the halo. In particular, we highlight the maximum effect such modified spectra can have on light isotope production in the early Galaxy, assuming a widely adopted spallation model. We find that spectra evolution alone can only maintain the linear Be versus Z relationship, observed at low Z, over the limited metallicity range Z i /4−Z i , where Z i marks the onset of linearity. At the very least, this indicates that nonlinearity in the Be abundance should be apparent at [Fe/H]≤−3, providing an observational test within the grasp of current techniques

Erratum and addendum to E2/M1 mixing ratio of Δ→Nγ and hyperon resonance radiative decay

Abstract We correct an error in the definition of our amplitudes, and then discuss using γ−γ correlations to find the ratio of electric quadrupole to magnetic dipole transitions in the electromagnetic production and decay of the Δ.

Probing hadronic structure with the decay Delta -->Nl+l-

We compute the branching ratio for

Probing hadronic structure with the decay [Delta][r arrow][ital Nl][sup +][ital l[minus]]

\Delta\rightarrow Ne^+e^-

Probing Hadronic Structure with The Decay

and

\Delta\rightarrow Nl^+l^-

\Delta\rightarrow N\mu^+\mu^-