1,099 research outputs found
Searching for Quantum Solitons in a 3+1 Dimensional Chiral Yukawa Model
We search for static solitons stabilized by heavy fermions in a 3+1
dimensional Yukawa model. We compute the renormalized energy functional,
including the exact one-loop quantum corrections, and perform a variational
search for configurations that minimize the energy for a fixed fermion number.
We compute the quantum corrections using a phase shift parameterization, in
which we renormalize by identifying orders of the Born series with
corresponding Feynman diagrams. For higher-order terms in the Born series, we
develop a simplified calculational method. When applicable, we use the
derivative expansion to check our results. We observe marginally bound
configurations at large Yukawa coupling, and discuss their interpretation as
soliton solutions subject to general limitations of the model.Comment: 27 pp., 7 EPS files; email correspondence to [email protected]
Experimental study of ultracold neutron production in pressurized superfluid helium
We have investigated experimentally the pressure dependence of the production
of ultracold neutrons (UCN) in superfluid helium in the range from saturated
vapor pressure to 20bar. A neutron velocity selector allowed the separation of
underlying single-phonon and multiphonon pro- cesses by varying the incident
cold neutron (CN) wavelength in the range from 3.5 to 10{\AA}. The predicted
pressure dependence of UCN production derived from inelastic neutron scattering
data was confirmed for the single-phonon excitation. For multiphonon based UCN
production we found no significant dependence on pressure whereas calculations
from inelastic neutron scattering data predict an increase of 43(6)% at 20bar
relative to saturated vapor pressure. From our data we conclude that applying
pressure to superfluid helium does not increase the overall UCN production rate
at a typical CN guide.Comment: 18 pages, 8 figures Version accepted for publication in PR
A Heavy Fermion Can Create a Soliton: A 1+1 Dimensional Example
We show that quantum effects can stabilize a soliton in a model with no
soliton at the classical level. The model has a scalar field chirally coupled
to a fermion in 1+1 dimensions. We use a formalism that allows us to calculate
the exact one loop fermion contribution to the effective energy for a spatially
varying scalar background. This energy includes the contribution from
counterterms fixed in the perturbative sector of the theory. The resulting
energy is therefore finite and unambiguous. A variational search then yields a
fermion number one configuration whose energy is below that of a single free
fermion.Comment: 10 pages, RevTeX, 2 figures composed from 4 .eps files; v2: fixed
minor errors, added reference; v3: corrected reference added in v
A new perturbative approach to the adiabatic approximation
A new and intuitive perturbative approach to time-dependent quantum mechanics
problems is presented, which is useful in situations where the evolution of the
Hamiltonian is slow. The state of a system which starts in an instantaneous
eigenstate of the initial Hamiltonian is written as a power series which has a
straightforward diagrammatic representation. Each term of the series
corresponds to a sequence of "adiabatic" evolutions, during which the system
remains in an instantaneous eigenstate of the Hamiltonian, punctuated by
transitions from one state to another. The first term of this series is the
standard adiabatic evolution, the next is the well-known first correction to
it, and subsequent terms can be written down essentially by inspection.
Although the final result is perhaps not terribly surprising, it seems to be
not widely known, and the interpretation is new, as far as we know. Application
of the method to the adiabatic approximation is given, and some discussion of
the validity of this approximation is presented.Comment: 9 pages. Added references, discussion of previous results, expanded
upon discussion of main result and application of i
Quantum Energies of Interfaces
We present a method for computing the one-loop, renormalized quantum energies
of symmetrical interfaces of arbitrary dimension and codimension using
elementary scattering data. Internal consistency requires finite-energy sum
rules relating phase shifts to bound state energies.Comment: 8 pages, 1 figure, minor changes, Phys. Rev. Lett., in prin
Heavy Fermion Quantum Effects in SU(2)_L Gauge Theory
We explore the effects of a heavy fermion doublet in a simplified version of
the standard electroweak theory. We integrate out the doublet and compute the
exact effective energy functional of spatially varying gauge and Higgs fields.
We perform a variational search for a local minimum of the effective energy and
do not find evidence for a soliton carrying the quantum numbers of the
decoupled fermion doublet. The fermion vacuum polarization energy offsets the
gain in binding energy previously argued to be sufficient to stabilize a
fermionic soliton. The existence of such a soliton would have been a natural
way to maintain anomaly cancellation at the level of the states. We also see
that the sphaleron energy is significantly increased due to the quantum
corrections of the heavy doublet. We find that when the doublet is slightly
heavier than the quantum--corrected sphaleron, its decay is exponentially
suppressed owing to a new barrier. This barrier exists only for an intermediate
range of fermion masses, and a heavy enough doublet is indeed unstable.Comment: 30 pages LaTeX, 3 eps-figure
Heavy Fermion Stabilization of Solitons in 1+1 Dimensions
We find static solitons stabilized by quantum corrections in a
(1+1)-dimensional model with a scalar field chirally coupled to fermions. This
model does not support classical solitons. We compute the renormalized energy
functional including one-loop quantum corrections. We carry out a variational
search for a configuration that minimizes the energy functional. We find a
nontrivial configuration with fermion number whose energy is lower than the
same number of free fermions quantized about the translationally invariant
vacuum. In order to compute the quantum corrections for a given background
field we use a phase-shift parameterization of the Casimir energy. We identify
orders of the Born series for the phase shift with perturbative Feynman
diagrams in order to renormalize the Casimir energy using perturbatively
determined counterterms. Generalizing dimensional regularization, we
demonstrate that this procedure yields a finite and unambiguous energy
functional.Comment: 27 papes Latex, equation labels corrected, version to be published in
Nucl. Phys.
Fractional and Integer Charges from Levinson's Theorem
We compute fractional and integer fermion quantum numbers of static
background field configurations using phase shifts and Levinson's theorem. By
extending fermionic scattering theory to arbitrary dimensions, we implement
dimensional regularization in a 1+1 dimensional gauge theory. We demonstrate
that this regularization procedure automatically eliminates the anomaly in the
vector current that a naive regulator would produce. We also apply these
techniques to bag models in one and three dimensions.Comment: 16 pages, uses RevTex, 1 figure; v2: minor correction
Quasienergy anholonomy and its application to adiabatic quantum state manipulation
The parametric dependence of a quantum map under the influence of a rank-1
perturbation is investigated. While the Floquet operator of the map and its
spectrum have a common period with respect to the perturbation strength
, we show an example in which none of the quasienergies nor the
eigenvectors obey the same period: After a periodic increment of , the
quasienergy arrives at the nearest higher one, instead of the initial one,
exhibiting an anholonomy, which governs another anholonomy of the eigenvectors.
An application to quantum state manipulations is outlined.Comment: 10pages, 1figure. To be published in Phys. Rev. Lett
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