41 research outputs found
Low Energy Continuum and Lattice Effective Field Theories
In the first part of the thesis we consider the constraints of causality and
unitarity for particles interacting via strictly finite-range interactions. We
generalize Wigner's causality bound to the case of non-vanishing partial-wave
mixing. Specifically we analyze the system of the low-energy interactions
between protons and neutrons. We also analyze low-energy scattering for systems
with arbitrary short-range interactions plus an attractive tail
for . In particular, we focus on the case of and we
derive the constraints of causality and unitarity also for these systems and
find that the van der Waals length scale dominates over parameters
characterizing the short-distance physics of the interaction. This separation
of scales suggests a separate universality class for physics characterizing
interactions with an attractive tail. We argue that a similar
universality class exists for any attractive potential for
.
In the second part of the thesis we present lattice Monte Carlo calculations
of fermion-dimer scattering in the limit of zero-range interactions using the
adiabatic projection method. The adiabatic projection method uses a set of
initial cluster states and Euclidean time projection to give a systematically
improvable description of the low-lying scattering cluster states in a finite
volume. We use L\"uscher's finite-volume relations to determine the -wave,
-wave, and -wave phase shifts. For comparison, we also compute exact
lattice results using Lanczos iteration and continuum results using the
Skorniakov-Ter-Martirosian equation. For our Monte Carlo calculations we use a
new lattice algorithm called impurity lattice Monte Carlo. This algorithm can
be viewed as a hybrid technique which incorporates elements of both worldline
and auxiliary-field Monte Carlo simulations.Comment: Ph.D. thesis, 201 pages, 19 figures, 17 table
Ab initio calculation of the alpha-particle monopole transition form factor: No puzzle for nuclear forces
We present a parameter-free ab initio calculation of the -particle
monopole transition form factor in the framework of nuclear lattice effective
field theory. We use a minimal nuclear interaction that was previously used to
reproduce the ground state properties of light nuclei, medium-mass nuclei, and
neutron matter simultaneously with no more than a few percent error in the
energies and charge radii. The results for the monopole transition form factor
are in good agreement with recent precision data from Mainz.Comment: 5 pages, 3 figure
Ab initio study of nuclear clustering in hot dilute nuclear matter
We present a systematic ab initio study of clustering in hot dilute nuclear
matter using nuclear lattice effective field theory with an SU(4)-symmetric
interaction. We introduce a method called light-cluster distillation to
determine the abundances of dimers, trimers, and alpha clusters as a function
of density and temperature. Our lattice results are compared with an ideal gas
model composed of free nucleons and clusters. Excellent agreement is found at
very low density, while deviations from ideal gas abundances appear at
increasing density due to cluster-nucleon and cluster-cluster interactions. In
addition to determining the composition of hot dilute nuclear matter as a
function of density and temperature, the lattice calculations also serve as
benchmarks for virial expansion calculations, statistical models, and transport
models of fragmentation and clustering in nucleus-nucleus collisions.Comment: 6+8 pages, 4+8 figure
Lattice Monte Carlo Simulations with Two Impurity Worldlines
We develop the impurity lattice Monte Carlo formalism, for the case of two
distinguishable impurities in a bath of polarized fermions. The majority
particles are treated as explicit degrees of freedom, while the impurities are
described by worldlines. The latter serve as localized auxiliary fields, which
affect the majority particles. We apply the method to non-relativistic
three-dimensional systems of two impurities and a number of majority particles
where both the impurity-impurity interaction and the impurity-majority
interaction have zero range. We consider the case of an attractive
impurity-majority interaction, and we study the formation and disintegration of
bound states as a function of the impurity-impurity interaction strength. We
also discuss the potential applications of this formalism to other quantum
many-body systems.Comment: 7 pages, 4 figure
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
Hidden spin-isospin exchange symmetry
The strong interactions among nucleons have an approximate spin-isospin
exchange symmetry that arises from the properties of quantum chromodynamics in
the limit of many colors, . However this large- symmetry is well
hidden and reveals itself only when averaging over intrinsic spin orientations.
Furthermore, the symmetry is obscured unless the momentum resolution scale is
close to an optimal scale that we call . We show
that the large- derivation requires a momentum resolution scale of
MeV. We derive a set of spin-isospin
exchange sum rules and discuss implications for the spectrum of P and
applications to nuclear forces, nuclear structure calculations, and
three-nucleon interactions.Comment: 5 pages (main) + 3 pages (supplemental materials), 1 figure (main) +
4 figures (supplemental materials), final version to appear in Phys. Rev.
Let