371 research outputs found
Reply to Comment on ``Determination of pion-baryon coupling constants from QCD sum rules''
In response to Kim's comment (nucl-th/9903040) on the sum rules for
pion-baryon coupling constants obtained in hep-ph/9512259 and hep-ph/9606471,
we point out that our treatment of the continuum is consistent with duality and
with the fact that the correlator in the presence of an external meson or field
should be represented by a double dispersion relation.Comment: 5 pages (RevTeX), reply to nucl-th/990304
Renormalisation-group analysis of electromagnetic couplings in the pionless effective field theory
The Wilsonian renormalisation group is applied to a system of two
nonrelativistic particles interacting via short-range forces and coupled to an
external EM field. By demanding that a fully off-shell one-particle-irreducible
5-point amplitude is independent of the cutoff, a renormalisation group
equation is derived for the interaction current density. This is solved to
obtain the fixed point corresponding to the unitary limit. The scaling
behaviour of perturbations around this point is analysed. Some of these terms
are related by gauge invariance to terms in the effective-range expansion;
others describe short-range physics that is not included in the two-body
potential. We construct observables including the bound-state form factor and
show how the scaling of the terms in the interaction current is reflected in
the power counting for their contributions to observables.Comment: 14 pages; comments and references adde
Exact renormalization group and many-fermion systems
The exact renormalization group methods is applied to many fermion systems
with short-range attractive force. The strength of the attractive
fermion-fermion interaction is determined from the vacuum scattering length. A
set of approximate flow equations is derived including fermionic and bosonic
fluctuations. The numerical solutions show a phase transition to a gapped
phase. The inclusion of bosonic fluctuations is found to be significant only in
the small-gap regime.Comment: Talk, given by B. Krippa on the International Workshop "Meson2004",
Cracow, Poland, 3 page
Proton polarisability contribution to the Lamb shift in muonic hydrogen at fourth order in chiral perturbation theory
We calculate the amplitude T_1 for forward doubly-virtual Compton scattering
in heavy-baryon chiral perturbation theory, to fourth order in the chiral
expansion and with the leading contribution of the gammaNDelta form factor.
This provides a model-independent expression for the amplitude in the
low-momentum region, which is the dominant one for its contribution to the Lamb
shift. It allows us to significantly reduce the theoretical uncertainty in the
proton polarisability contributions to the Lamb shift in muonic hydrogen. We
also stress the importance of consistency between the definitions of the Born
and structure parts of the amplitude. Our result leaves no room for any effect
large enough to explain the discrepancy between proton charge radii as
determined from muonic and normal hydrogen.Comment: 9 pages, 2 figures; added discussion of matching ChPT form factor
onto dipole form at non-zero Q^2; no significant changes in result
Comment on soft-pion emission in DVCS
The soft-pion theorem for pion production in deeply virtual Compton
scattering, derived by Guichon, Mosse and Vanderhaegen, is shown to be
consistent with chiral perturbation theory. Chiral symmetry requires that the
nonsinglet operators corresponding to spin-independent and spin-dependent
parton distributions have the same anomalous dimensions in cases where those
operators are related by chiral transformations. In chiral perturbation theory,
their scale-dependences can thus be absorbed in the coefficents of the
corresponding effective operators, without affecting their chiral structures.Comment: 2 pages, RevTe
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