Results 1 - 10 of 17
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[en] Effects of isospin breaking at the level of the Goldberger-Treiman discrepancies involving the neutral isotriplet axial and pion-nucleon couplings are analyzed to leading non-trivial order in chiral perturbation theory
[en] The pi 0 → gamma gamma width is determined to next to leading order in the combined chiral and 1/Nc expansions. It is shown that corrections driven by chiral symmetry breaking produce an enhancement of about 4.5% with respect to the width calculated in terms of the chiral-limit amplitude leading to Gammaπ0 → γγ = 8.1 +/- 0.08 MeV. This theoretical prediction will be tested via pi 0 Primakoff production by the PRIMEX experiment at Jefferson Lab
[en] In the chiral limit, the amplitude of the decay π0 → γγ is determined by the anomaly induced on the neutral iso-triplet axial current by the EM field. The amplitude is thus given in terms of the only available quantities in that limit, namely, the fine structure and the pion-decay constants.
[en] The decays of non-strange negative parity baryons via the emission of single π and η mesons are analyzed in the framework of the 1/Nc expansion. A basis of spin-flavor operators is established to that order, and with this basis the different partial wave decay amplitudes are obtained. The unknown effective coefficients are determined by fitting to the S- and D-wave partial widths as provided by the PDG. A full set of relations between widths that result at the leading order, i.e. order Nc0, is given and tested with the data. The rather large errors of the input partial widths, that result from the often discrepant results for the resonance parameters from different analyses of the data, lead to a rather good fit at the leading order Nc0. The next to leading order fit fails for that reason to pin down with satisfactory accuracy the effective sub leading effective constants
[en] Baryon Regge trajectories are discussed in the light of the 1/Nc expansion. The approximate dynamical symmetry SU(6)x O(3) is used to identify the spin-flavor singlet component of baryon masses. By fitting to the known baryon spectrum, this component is shown to produce distinct Regge trajectories for SU(6) 56- and 70-plets which are remarkably linear.
[en] The decays of non-strange positive parity baryons via emission of a pseudo-scalar meson are studied in the framework of the 1/Nc expansion to order 1/Nc. In particular, the pionic decays of the l=0 Roper baryons and of the l=2 baryons in the mass interval 1680-1950 MeV are analyzed using the available partial decay widths. Decay widths by emission of an eta meson are shown to be suppressed by a factor 1/Nc2 with respect to the pionic ones
[en] Nuclear electromagnetic currents derived in a chiral-effective-field-theory framework including explicit nucleons, Δ isobars, and pions up to N2LO, i.e. ignoring loop corrections, are used in a study of neutron radiative captures on protons and deuterons at thermal energies, and of A$=2 and 3 nuclei magnetic moments. With the strengths of the Δ-excitation currents determined to reproduce the n$-p$ cross section and isovector combination of the trinucleon magnetic moments, we find that the cross section and photon circular polarization parameter, measured respectively in n$-d$ and (rvec n)-$d processes, are significantly underpredicted by theory.
[en] Five dimensional dilaton models are considered as possible holographic duals of the pure gauge QCD vacuum. In the framework of these models, the QCD trace anomaly equation is considered. Each quantity appearing in that equation is computed by holographic means. Two exact solutions for different dilaton potentials corresponding to perturbative and non-perturbative β-functions are studied. It is shown that in the perturbative case, where the β-function is the QCD one at leading order, the resulting space is not asymptotically AdS. In the non-perturbative case, the model considered presents confinement of static quarks and leads to a non-vanishing gluon condensate, although it does not correspond to an asymptotically free theory. In both cases analyses based on the trace anomaly and on Wilson loops are carried out.
[en] The 1/Nc expansion of QCD provides a useful framework for phenomenological studies of both ground state and excited baryons. Here, we focus on its application to the excited baryon strong decays via emission of single pseudoscalar meson. The S- and D-wave decay amplitudes of the negative parity baryons in the [70,1-] of SU(6) are analyzed to subleading order in 1/Nc and to first order in SU(3) symmetry breaking. In particular, the SU(3) symmetry breaking is studied in detail through SU(3) breaking decay operators and state mixing.