240 research outputs found
Threshold Neutral Pion Photoproduction on the Proton
The neutral pion photoproduction on the proton near threshold has a very
small scattering cross section when compared to the charged channels, which in
ChPT is explained by strong cancellations between the lowest order pieces.
Therefore it is very sensitive to higher-order corrections of chiral
perturbation theory. We perform a fully covariant calculation up to chiral
order p^3 and we investigate the effect of the inclusion of the Delta(1232)
resonance as an explicit degree of freedom. We show that the convergence
improves, leading to a much better agreement with data at a wide range of
energies.Comment: Proceedings to the NSTAR2015 conference, which will be published in
JPS conference proceeding
Role of current quark mass dependent multi-quark interactions in low lying meson mass spectra
We call attention to a class of current-quark mass dependent multi-quark
interaction terms which break explicitly the chiral and
symmetries. They complete the set of effective quark interactions that
contribute at the same order in as the 't Hooft flavor determinant
interaction and the eight quark interactions in the phase of spontaneously
broken chiral symmetry. The classification scheme matches the counting
rules based on arguments set by the scale of spontaneous chiral symmetry
breaking. Together with the leading in four quark Nambu-Jona-Lasinio
Lagrangian and current quark mass matrix, the model is apt to account for the
correct empirical ordering and magnitude of the splitting of states in the low
lying mass spectra of spin zero mesons. The new terms turn out to be essential
for the ordering in the pseudoscalar sector and for the scalars.Comment: 6 pages, 1 figure, prepared for the proceedings of the workshop
"Excited QCD 2013", 03-09 February 2013 in Bjelasnica Mountain, Sarajevo,
Bosni
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