3,633 research outputs found
Low energy analysis of pi N --> pi N
We derive a representation for the pion nucleon scattering amplitude that is
valid to the fourth order of the chiral expansion. To obtain the correct
analytic structure of the singularities in the low energy region, we have
performed the calculation in a relativistic framework (infrared
regularization). The result can be written in terms of functions of a single
variable. We study the corresponding dispersion relations and discuss the
problems encountered in the straightforward nonrelativistic expansion of the
infrared singularities. As an application, we evaluate the corrections to the
Goldberger-Treiman relation and to the low energy theorem that relates the
value of the amplitude at the Cheng-Dashen point to the \sigma-term. While
chiral symmetry does govern the behaviour of the amplitude in the vicinity of
this point, the representation for the scattering amplitude is not accurate
enough to use it for an extrapolation of the experimental data to the
subthreshold region. We propose to perform this extrapolation on the basis of a
set of integral equations that interrelate the lowest partial waves and are
analogous to the Roy equations for \pi\pi scattering.Comment: 97 pages (LaTeX), 16 figures. Two references added, correction in
table one. Published versio
Fabrication and characterization of hot- pressed tantalum carbide
Microstructure and chemistry of hot pressed powder compacts of tantalum carbid
Baryon Chiral Perturbation Theory in Manifestly Lorentz Invariant Form
We show that in the presence of massive particles such as nucleons, the
standard low energy expansion in powers of meson momenta and light quark masses
in general only converges in part of the low energy region. The expansion of
the scalar form factor , for instance, breaks down in the vicinity
of . In the language of heavy baryon chiral perturbation theory,
the proper behaviour in the threshold region only results if the multiple
internal line insertions generated by relativistic kinematics are summed up to
all orders. We propose a method that yields a coherent representation
throughout the low energy region while keeping Lorentz and chiral invariance
explicit at all stages. The method is illustrated with a calculation of the
nucleon mass and of the scalar form factor to order .Comment: 66 pages, 12 postscript figure
Systematic Low-Energy Effective Field Theory for Magnons and Holes in an Antiferromagnet on the Honeycomb Lattice
Based on a symmetry analysis of the microscopic Hubbard and t-J models, a
systematic low-energy effective field theory is constructed for hole-doped
antiferromagnets on the honeycomb lattice. In the antiferromagnetic phase,
doped holes are massive due to the spontaneous breakdown of the
symmetry, just as nucleons in QCD pick up their mass from spontaneous chiral
symmetry breaking. In the broken phase the effective action contains a
single-derivative term, similar to the Shraiman-Siggia term in the square
lattice case. Interestingly, an accidental continuous spatial rotation symmetry
arises at leading order. As an application of the effective field theory we
consider one-magnon exchange between two holes and the formation of two-hole
bound states. As an unambiguous prediction of the effective theory, the wave
function for the ground state of two holes bound by magnon exchange exhibits
-wave symmetry.Comment: 33 pages, 6 figure
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