3,612 research outputs found
New insights into the spin structure of the nucleon
We analyze the low-energy spin structure of the nucleon in a covariant
effective field theory with explicit spin-3/2 degrees of freedom to third order
in the small scale expansion. Using the available data on the strong and
electromagnetic width of the Delta-resonance, we give parameter-free
predictions for various spin-polarizabilities and moments of spin structure
functions. We find an improved description of the nucleon spin structure at
finite photon virtualities for some observables and point out the necessity of
a fourth order calculation.Comment: 13 pages, 6 figure
Subleading contributions to the chiral three-nucleon force I: long-range terms
We derive the long-range contributions to the tree-nucleon force at
next-to-next-to-next-to-leading order in the chiral expansion. We give both
momentum and coordinate space representations.Comment: 15 pages, 4 figure
The reaction pi N -> pi pi N in chiral effective field theory with explicit Delta(1232) degrees of freedom
The reaction pi N -> pi pi N is studied at tree level up to next-to-leading
order in the framework of manifestly covariant baryon chiral perturbation
theory with explicit Delta(1232) degrees of freedom. Using total cross section
data to determine the relevant low-energy constants, predictions are made for
various differential as well as total cross sections at higher energies. A
detailed comparison of results based on the heavy-baryon and relativistic
formulations of chiral perturbation theory with and without explicit Delta
degrees of freedom is given.Comment: 30 pages, 13 figure
Elastic pion-nucleon scattering in chiral perturbation theory: A fresh look
Elastic pion-nucleon scattering is analyzed in the framework of chiral
perturbation theory up to fourth order within the heavy-baryon expansion and a
covariant approach based on an extended on-mass-shell renormalization scheme.
We discuss in detail the renormalization of the various low-energy constants
and provide explicit expressions for the relevant -functions and the
finite subtractions of the power-counting breaking terms within the covariant
formulation. To estimate the theoretical uncertainty from the truncation of the
chiral expansion, we employ an approach which has been successfully applied in
the most recent analysis of the nuclear forces. This allows us to reliably
extract the relevant low-energy constants from the available scattering data at
low energy. The obtained results provide a clear evidence that the breakdown
scale of the chiral expansion for this reaction is related to the
-resonance. The explicit inclusion of the leading contributions of the
-isobar is demonstrated to substantially increase the range of
applicability of the effective field theory. The resulting predictions for the
phase shifts are in an excellent agreement with the ones from the recent
Roy-Steiner-equation analysis of pion-nucleon scattering
Pion-nucleon scattering in covariant baryon chiral perturbation theory with explicit Delta resonances
We present the results of a third order calculation of the pion-nucleon
scattering amplitude in a chiral effective field theory with pions, nucleons
and delta resonances as explicit degrees of freedom. We work in a manifestly
Lorentz invariant formulation of baryon chiral perturbation theory using
dimensional regularization and the extended on-mass-shell renormalization
scheme. In the delta resonance sector, the on mass-shell renormalization is
realized as a complex-mass scheme. By fitting the low-energy constants of the
effective Lagrangian to the - and -partial waves a satisfactory
description of the phase shifts from the analysis of the Roy-Steiner equations
is obtained. We predict the phase shifts for the and waves and compare
them with the results of the analysis of the George Washington University
group. The threshold parameters are calculated both in the delta-less and
delta-full cases. Based on the determined low-energy constants, we discuss the
pion-nucleon sigma term. Additionally, in order to determine the strangeness
content of the nucleon, we calculate the octet baryon masses in the presence of
decuplet resonances up to next-to-next-to-leading order in SU(3) baryon chiral
perturbation theory. The octet baryon sigma terms are predicted as a byproduct
of this calculation.Comment: 41 pages, 12 figures, 7 table
Subleading contributions to the chiral three-nucleon force II: Short-range terms and relativistic corrections
We derive the short-range contributions and the leading relativistic
corrections to the three-nucleon force at next-to-next-to-next-to-leading order
in the chiral expansion.Comment: 14 pages, 4 figure
Spin Glass and ferromagnetism in disordered Cerium compounds
The competition between spin glass, ferromagnetism and Kondo effect is
analysed here in a Kondo lattice model with an inter-site random coupling
between the localized magnetic moments given by a generalization of
the Mattis model which represents an interpolation between ferromagnetism and a
highly disordered spin glass. Functional integral techniques with Grassmann
fields have been used to obtain the partition function. The static
approximation and the replica symmetric ansatz have also been used. The
solution of the problem is presented as a phase diagram giving {\it
versus} where is the temperature, and are the
strengths of the intrasite Kondo and the intersite random couplings,
respectively. If is small, when temperature is decreased, there is a
second order transition from a paramagnetic to a spin glass phase. For lower
, a first order transition appears between the spin glass phase and a
region where there are Mattis states which are thermodynamically equivalent to
the ferromagnetism. For very low , the Mattis states become stable. On
the other hand, it is found as solution a Kondo state for large
values. These results can improve the theoretical description of the well known
experimental phase diagram of .Comment: 17 pages, 5 figures, accepted Phys. Rev.
Pion production in nucleon-nucleon collisions in chiral effective field theory: next-to-next-to-leading order contributions
A complete calculation of the pion-nucleon loops that contribute to the
transition operator for up-to-and-including
next-to-next-to-leading order (NLO) in chiral effective field theory near
threshold is presented. The evaluation is based on the so-called momentum
counting scheme, which takes into account the relatively large momentum of the
initial nucleons inherent in pion-production reactions. We show that the
significant cancellations between the loops found at next-to-leading order
(NLO) in the earlier studies are also operative at NLO. In particular, the
corrections (with being the nucleon mass) to loop diagrams cancel
at NLO, as do the contributions of the pion loops involving the low-energy
constants , i=1...4. In contrast to the NLO calculation however, the
cancellation of loops at NLO is incomplete, yielding a non-vanishing
contribution to the transition amplitude. Together with the one-pion exchange
tree-level operators, the loop contributions provide the long-range part of the
production operator. Finally, we discuss the phenomenological implications of
these findings. In particular, we find that the amplitudes generated by the
NLO pion loops yield contributions comparable in size with the most
important phenomenological heavy-meson exchange amplitudes.Comment: 28 pages, 7 figures, 2 table
Threshold neutral pion photoproduction off the tri-nucleon to O(q^4)
We calculate electromagnetic neutral pion production off tri-nucleon bound
states (3H, 3He) at threshold in chiral nuclear effective field theory to
fourth order in the standard heavy baryon counting. We show that the fourth
order two-nucleon corrections to the S-wave multipoles at threshold are very
small. This implies that a precise measurement of the S-wave cross section for
neutral pion production off 3He allows for a stringent test of the chiral
perturbation theory prediction for the S-wave electric multipole E_{0+}^{pi0
n}.Comment: 17 pages, 5 figures, title changed, final version to appear in EPJA.
arXiv admin note: substantial text overlap with arXiv:1103.340
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