1,325 research outputs found
Neutron matter on the lattice with pionless effective field theory
We study neutron matter by combining pionless effective field theory with
non-perturbative lattice methods. The neutron contact interaction is determined
by zero temperature scattering data. We simulate neutron matter on the lattice
at temperatures 4 and 8 MeV and densities below one-fifth normal nuclear matter
density. Our results at different lattice spacings agree with one another and
match bubble chain calculations at low densities. The equation of state of pure
neutron matter obtained from our simulations agrees quantitatively with
variational calculations based on realistic potentials.Comment: 28 pages, 13 figure
On the modification of the Efimov spectrum in a finite cubic box
Three particles with large scattering length display a universal spectrum of
three-body bound states called "Efimov trimers''. We calculate the modification
of the Efimov trimers of three identical bosons in a finite cubic box and
compute the dependence of their energies on the box size using effective field
theory. Previous calculations for positive scattering length that were
perturbative in the finite volume energy shift are extended to arbitrarily
large shifts and negative scattering lengths. The renormalization of the
effective field theory in the finite volume is explicitly verified. Moreover,
we investigate the effects of partial wave mixing and study the behavior of
shallow trimers near the dimer energy. Finally, we provide numerical evidence
for universal scaling of the finite volume corrections.Comment: 21 pages, 8 figures, published versio
High Energy Theorems at Large-N
Sum rules for products of two, three and four QCD currents are derived using
chiral symmetry at infinite momentum in the large-N limit. These exact
relations among meson decay constants, axialvector couplings and masses
determine the asymptotic behavior of an infinite number of QCD correlators. The
familiar spectral function sum rules for products of two QCD currents are among
the relations derived. With this precise knowledge of asymptotic behavior, an
infinite number of large-N QCD correlators can be constructed using dispersion
relations. A detailed derivation is given of the exact large-N pion vector form
factor and forward pion-pion scattering amplitudes.Comment: 34 pages TeX and mtexsis.tex, 10 figures (uses epsf
Deconstructing triplet nucleon-nucleon scattering
Nucleon-nucleon scattering in spin-triplet channels is analysed within an
effective field theory where one-pion exchange is treated nonperturbatively.
Justifying this requires the identification of an additional low-energy scale
in the strength of that potential. Short-range interactions are organised
according to the resulting power counting, in which the leading term is
promoted to significantly lower order than in the usual perturbative counting.
In each channel there is a critical momentum above which the waves probe the
singular core of the tensor potential and the new counting is necessary. When
the effects of one- and two-pion exchange have been removed using a
distorted-wave Born approximation, the residual scattering in waves with L<=2
is well described by the first three terms in the new counting. In contrast,
the scattering in waves with L>=3 is consistent with the perturbative counting,
at least for energies up to 300 MeV. This pattern is in agreement with
estimates of the critical momenta in these channels.Comment: 13 pages, RevTeX, 8 figures, minor clarifications adde
Bridging over p-wave pi-production and weak processes in few-nucleon systems with chiral perturbation theory
I study an aspect of chiral perturbation theory (\chi PT) which enables one
to ``bridge'' different reactions. That is, an operator fixed in one of the
reactions can then be used to predict the other. For this purpose, I calculate
the partial wave amplitude for the p-wave pion production (pp\to pn\pi^+) using
the pion production operator from the lowest and the next nonvanishing orders.
The operator includes a contact operator whose coupling has been fixed using a
matrix element of a low-energy weak process (pp\to de^+\nu_e). I find that this
operator does not reproduce the partial wave amplitude extracted from
experimental data, showing that the bridging over the reactions with
significantly different kinematics is not necessarily successful. I study the
dependence of the amplitude on the various inputs such as the NN potential, the
\pi N\Delta coupling, and the cutoff. I argue the importance of a higher order
calculation. In order to gain an insight into a higher order calculation, I add
a higher order counter term to the operator used above, and fit the couplings
to both the low-energy weak process and the pion production. The energy
dependence of the partial wave amplitude for the pion production is described
by the operator consistently with the data. However, I find a result which
tells us to be careful about the convergence of the chiral expansion for the
pp\to pn\pi^+ reaction.Comment: 30 pages, 13 figures, figures changed, compacted tex
Baryon Axial Charge in a Finite Volume
We compute finite-volume corrections to nucleon matrix elements of the
axial-vector current. We show that knowledge of this finite-volume dependence
--as well as that of the nucleon mass-- obtained using lattice QCD will allow a
clean determination of the chiral-limit values of the nucleon and
Delta-resonance axial-vector couplings.Comment: 11 pages, 8 figure
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