100 research outputs found
Improved description of the pion-nucleon scattering phenomenology in covariant baryon chiral perturbation theory
We highlight some of the recent advances in the application of chiral
effective field theory (chiral EFT) with baryons to the scattering
process. We recall some problems that cast doubt on the applicability of chiral
EFT to and show how the relativistic formalism, once the
-resonance is included as an explicit degree of freedom, solves
these issues. Finally it is shown how this approach can be used to extract the
-terms from phenomenological information.Comment: Contribution to the 13th International Conference on Meson-Nucleon
Physics and the Structure of the Nucleon (MENU 2013), Rome, Italy, 30
September - October 4, 2013. 4 pages, 1 figur
Chiral perturbation theory of muonic hydrogen Lamb shift: polarizability contribution
The proton polarizability effect in the muonic-hydrogen Lamb shift comes out
as a prediction of baryon chiral perturbation theory at leading order and our
calculation yields for it: eV. This result is consistent with most of evaluations based on dispersive
sum rules, but is about a factor of two smaller than the recent result obtained
in {\em heavy-baryon} chiral perturbation theory. We also find that the effect
of -resonance excitation on the Lamb-shift is suppressed, as is
the entire contribution of the magnetic polarizability; the electric
polarizability dominates. Our results reaffirm the point of view that the
proton structure effects, beyond the charge radius, are too small to resolve
the `proton radius puzzle'.Comment: 16 pages, 5 figure
Forward doubly-virtual Compton scattering off the nucleon in chiral perturbation theory: the subtraction function and moments of unpolarized structure functions
The forward doubly-virtual Compton scattering (VVCS) off the nucleon contains
a wealth of information on nucleon structure, relevant to the calculation of
the two-photon-exchange effects in atomic spectroscopy and electron scattering.
We report on a complete next-to-leading-order (NLO) calculation of low-energy
VVCS in chiral perturbation theory (PT). Here we focus on the unpolarized
VVCS amplitudes and , and the corresponding
structure functions and . Our results are confronted,
where possible, with "data-driven" dispersive evaluations of low-energy
structure quantities, such as nucleon polarizabilities. We find significant
disagreements with dispersive evaluations at very low momentum-transfer ;
for example, in the slope of polarizabilities at zero momentum-transfer. By
expanding the results in powers of the inverse nucleon mass, we reproduce the
known "heavy-baryon" expressions. This serves as a check of our calculation, as
well as demonstrates the differences between the manifestly Lorentz-invariant
(BPT) and heavy-baryon (HBPT) frameworks.Comment: 31 pages, 11 figures, 1 table; supplemented material: 1 Mathematica
notebook; minor modifications, published versio
Forward doubly-virtual Compton scattering off the nucleon in chiral perturbation theory: II. Spin polarizabilities and moments of polarized structure functions
We examine the polarized doubly-virtual Compton scattering (VVCS) off the
nucleon using chiral perturbation theory (PT). The polarized VVCS
contains a wealth of information on the spin structure of the nucleon which is
relevant to the calculation of the two-photon-exchange effects in atomic
spectroscopy and electron scattering. We report on a complete
next-to-leading-order (NLO) calculation of the polarized VVCS amplitudes
and , and the corresponding polarized spin
structure functions and . Our results for the moments
of polarized structure functions, partially related to different spin
polarizabilities, are compared to other theoretical predictions and
"data-driven" evaluations, as well as to the recent Jefferson Lab measurements.
By expanding the results in powers of the inverse nucleon mass, we reproduce
the known "heavy-baryon" expressions. This serves as a check of our
calculation, as well as demonstrates the differences between the manifestly
Lorentz-invariant baryon PT (BPT) and heavy-baryon (HBPT)
frameworks.Comment: 47 pages, 11 figures, 2 tables. Includes Mathematica notebook with
the spin polarizabilities and moments of polarized structure functions as
supplemental material. Replaced to match the published versio
Neutron-proton scattering at next-to-next-to-leading order in Nuclear Lattice Effective Field Theory
We present a systematic study of neutron-proton scattering in Nuclear Lattice
Effective Field Theory (NLEFT), in terms of the computationally efficient
radial Hamiltonian method. Our leading-order (LO) interaction consists of
smeared, local contact terms and static one-pion exchange. We show results for
a fully non-perturbative analysis up to next-to-next-to-leading order (NNLO),
followed by a perturbative treatment of contributions beyond LO. The latter
analysis anticipates practical Monte Carlo simulations of heavier nuclei. We
explore how our results depend on the lattice spacing a, and estimate sources
of uncertainty in the determination of the low-energy constants of the
next-to-leading-order (NLO) two-nucleon force. We give results for lattice
spacings ranging from a = 1.97 fm down to a = 0.98 fm, and discuss the effects
of lattice artifacts on the scattering observables. At a = 0.98 fm, lattice
artifacts appear small, and our NNLO results agree well with the Nijmegen
partial-wave analysis for S-wave and P-wave channels. We expect the peripheral
partial waves to be equally well described once the lattice momenta in the
pion-nucleon coupling are taken to coincide with the continuum dispersion
relation, and higher-order (N3LO) contributions are included. We stress that
for center-of-mass momenta below 100 MeV, the physics of the two-nucleon system
is independent of the lattice spacing.Comment: 22 pages, 8 figure
Longitudinal evaluation of sEMG of masticatory muscles and kinematics of mandible changes in children treated for unilateral cross-bite
a b s t r a c t The aim of this study was to evaluate masticatory muscle activity and kinematics of mandible changes in children with unilateral posterior cross-bite (UPXB) after orthodontic treatment, and one year after retention. Twenty-five children with UPXB and functional mandibular shift were evaluated before treatment (mean age 12.5 years), after treatment (mean age 14.9 years), and one year after retention (mean age 16.8 years). The same data were collected in a control group of thirty age-matched normocclusive children. Simultaneous bilateral surface electromyographic (sEMG) activity from anterior temporalis (AT), posterior temporalis (PT), masseter (MA), and supra-hyoid (SH) muscle areas were evaluated at rest, during swallowing, mastication and clenching. Kinematic records of rest position, mandibular lateral shift, swallowing and mastication were analyzed. Results showed a lateral shift of the mandible present at rest. During swallowing, sEMG activity of SH predominated before and post-treatment and retention. High frequency of immature swallowing was maintained post-treatment and retention. During mastication, MA activity increased significantly and its asymmetry was corrected post-treatment. During clenching, cross-bite side AT and MA activity increased significantly posttreatment and remained stable after retention, and MA/AT ratio reversed. These findings reinforce the advantages of treating children with UPXB and functional shift as early as possible
Proton charge radius extraction from electron scattering data using dispersively improved chiral effective field theory
We extract the proton charge radius from the elastic form factor (FF) data using a novel theoretical framework combining chiral effective field theory and dispersion analysis. Complex analyticity in the momentum transfer correlates the behavior of the spacelike FF at finite Q 2 with the derivative at Q 2 = 0 . The FF calculated in the predictive theory contains the radius as a free parameter. We determine its value by comparing the predictions with a descriptive global fit of the spacelike FF data, taking into account the theoretical and experimental uncertainties. Our method allows us to use the finite- Q 2 FF data for constraining the radius (up to Q 2 ≈ 0.5 GeV 2 and larger) and avoids the difficulties arising in methods relying on the Q 2 → 0 extrapolation. We obtain a radius of 0.844(7) fm, consistent with the high-precision muonic hydrogen results
Nuclear binding near a quantum phase transition
How do protons and neutrons bind to form nuclei? This is the central question
of ab initio nuclear structure theory. While the answer may seem as simple as
the fact that nuclear forces are attractive, the full story is more complex and
interesting. In this work we present numerical evidence from ab initio lattice
simulations showing that nature is near a quantum phase transition, a
zero-temperature transition driven by quantum fluctuations. Using lattice
effective field theory, we perform Monte Carlo simulations for systems with up
to twenty nucleons. For even and equal numbers of protons and neutrons, we
discover a first-order transition at zero temperature from a Bose-condensed gas
of alpha particles (4He nuclei) to a nuclear liquid. Whether one has an
alpha-particle gas or nuclear liquid is determined by the strength of the
alpha-alpha interactions, and we show that the alpha-alpha interactions depend
on the strength and locality of the nucleon-nucleon interactions. This insight
should be useful in improving calculations of nuclear structure and important
astrophysical reactions involving alpha capture on nuclei. Our findings also
provide a tool to probe the structure of alpha cluster states such as the Hoyle
state responsible for the production of carbon in red giant stars and point to
a connection between nuclear states and the universal physics of bosons at
large scattering length.Comment: Published version to appear in Physical Review Letters. Main: 5
pages, 3 figures. Supplemental material: 13 pages, 6 figure
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