4,850 research outputs found
Compton scattering from the proton at NLO in the chiral expansion
We present calculations of differential cross sections for Compton scattering
from the proton, using amplitudes calculated to fourth order in heavy baryon
chiral perturbation theory. We compare with available data up to 200 MeV. We
find that the agreement for angles below 90 degree is acceptable over the whole
energy range, but that at more backward angles the agreement decreases above
about 100 MeV, and fails completely above the photoproduction threshold.Comment: 13 pages RevTeX, 7 eps figures. Revised version correcting some
errors, principally in the coding of 3rd order contributions; figures changed
slightly and contribution from diagram 1h revised. Conclusions are largely
unchanged, but the failure of the chiral expansion is visible at rather lower
angles than appeared previously to be the cas
Invalid Waivers of Counsel as Harmless Errors: Judicial Economy or a Return to Betts v. Brady
Compton scattering from the proton: An analysis using the delta expansion up to N3LO
We report on a chiral effective field theory calculation of Compton
scattering from the proton. Our calculation includes pions, nucleons, and the
Delta(1232) as explicit degrees of freedom. It uses the "delta expansion", and
so implements the hierarchy of scales m_pi < M_Delta-M_N < Lambda_chi. In this
expansion the power counting in the vicinity of the Delta peak changes, and
resummation of the loop graphs associated with the Delta width is indicated.
We have computed the nucleon Compton amplitude in the delta expansion up to
N3LO for photon energies of the order of m_pi. This is the first order at which
the proton Compton scattering amplitudes receive contributions from contact
operators which encode contributions to the spin-independent polarisabilities
from states with energies of the order of Lambda_chi. We fit the coefficients
of these two operators to the experimental proton Compton data that has been
taken in the relevant photon-energy domain, and are in a position to extract
new results for the proton polarisabilities alpha and beta.Comment: 6 pages. Proceeding of Sixth International Workshop on Chiral
Dynamics, Bern (Switzerland), 6th -- 10th July 2009. To be published in Po
A self-consistent approach to the Wigner-Seitz treatment of soliton matter
We propose a self-consistant approach to the treatment of nuclear matter as a
crystal of solitons in the Wigner-Seitz approximation. Specifically, we use a
Bloch-like boundary condition on the quarks at the edge of a spherical cell
which allows the dispersion relation for a given radius to be calculated
self-consistently along with the meson fields; in previous work some ansatz for
the dispersion relation has always been an input. Results in all models are
very sensitive to the form of the dispersion relation, so our approach
represents a significant advance. We apply the method to both the Friedberg Lee
model and the chiral quark-meson model of Birse and Banerjee. Only the latter
shows short range repulsion; in the former the transition to a quark plasma
occurs at unrealistically low densities.Comment: Revtex; 14 pages with 9 eps figure
Proton polarisabilities from Compton data using Covariant Chiral EFT
We present a fit of the spin-independent electromagnetic polarisabilities of
the proton to low-energy Compton scattering data in the framework of covariant
baryon chiral effective field theory. Using the Baldin sum rule to constrain
their sum, we obtain
(stat)(Baldin)(theory)fm
and (stat)(Baldin)(theory)fm, in
excellent agreement with other chiral extractions of the same quantities.Comment: 4 pages, 2 figure
Comment on "Nucleon spin-averaged forward virtual Compton tensor at large Q^2"
In recent work, Hill and Paz apply the operator product expansion to forward
doubly virtual Compton scattering. The resulting large- form of the
amplitude is compatible with the one we obtain by extrapolation of
low- results from a chiral effective field theory, providing support for
our approach. That paper also presents a result for the two-photon contribution
to the Lamb shift in muonic hydrogen that has a much larger uncertainty than in
previous work. We show that this an overestimate arising from the inclusion of
the proton pole term in the subtracted dispersion relation for .Comment: 3 pages; version accepted for publicatio
Impact of the Delta (1232) resonance on neutral pion photoproduction in chiral perturbation theory
We present an ongoing project to assess the importance of D-waves and the
resonance for descriptions of neutral pion photoproduction in
Heavy Baryon Chiral Perturbation Theory. This research has been motivated by
data published by the A2 and CB-TAPS collaborations at MAMI [1]. This data has
reached unprecedented levels of accuracy from threshold through to the
resonance. Accompanying the experimental work, there has also been a series of
publications studying the theory that show that, to go beyond an energy of
MeV, it is necessary to include other aspects, in particular the
as a degree of freedom [2] and possibly higher partial waves
[3].Comment: Proceedings to the 8th International Workshop on Chiral Dynamics 201
Predictions of covariant chiral perturbation theory for nucleon polarisabilities and polarised Compton scattering
We update the predictions of the SU(2) baryon chiral perturbation theory for
the dipole polarisabilities of the proton,
fm,
and obtain the corresponding predictions for the quadrupole, dispersive, and
spin polarisabilities:
fm,
fm,
and
fm.
The results for the scalar polarisabilities are in significant disagreement
with semi-empirical analyses based on dispersion relations, however the results
for the spin polarisabilities agree remarkably well. Results for proton
Compton-scattering multipoles and polarised observables up to the Delta(1232)
resonance region are presented too. The asymmetries and
reproduce the experimental data from LEGS and MAMI. Results for
agree with a recent sum rule evaluation in the forward
kinematics. The asymmetry near the pion production threshold
shows a large sensitivity to chiral dynamics, but no data is available for this
observable. We also provide the predictions for the polarisabilities of the
neutron:
fm,
fm,
fm,
and
fm.
The neutron dynamical polarisabilities and multipoles are examined too. We also
discuss subtleties related to matching dynamical and static polarisabilities.Comment: 34 pages, 18 figures, minor updates and corrections, published
versio
Using EFT to analyze low-energy Compton scattering from protons and light nuclei
We discuss the application of an effective field theory (EFT) which
incorporates the chiral symmetry of QCD to Compton scattering from the proton
and deuteron. We describe the chiral EFT analysis of the proton Compton
scattering database presented in our recent review (arXiv:1203.6834), which
gives: alpha^{(p)}=10.5 +/- 0.5(stat) +/- 0.8(theory); beta^{(p)}= 2.7 +/-
0.5(stat) +/- 0.8(theory), for the electric and magnetic dipole polarizability
of the proton. We also summarize the chiral EFT analysis of the world data on
coherent Compton scattering from deuterium presented in arXiv:1203.6834. That
yields: alpha^{(s)}=10.5 +/- 2.0(stat) +/- 0.8(theory); beta^{(s)}=3.6 +/-
1.0(stat) +/- 0.8(theory).Comment: 5 pages. Invited talk, presented by Phillips at the 11th Conference
on the Intersections of Nuclear and Particle Physics (CIPANP 2012), St.
Petersburg, FL, May 201
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