3,762 research outputs found
Renormalization of One-Pion Exchange and Power Counting
The renormalization of the chiral nuclear interactions is studied. In leading
order, the cutoff dependence is related to the singular tensor interaction of
the one-pion exchange potential. In S waves and in higher partial waves where
the tensor force is repulsive this cutoff dependence can be absorbed by
counterterms expected at that order. In the other partial waves additional
contact interactions are necessary. The implications of this finding for the
effective-field-theory program in nuclear physics are discussed.Comment: 19 pages, 18 figure
Degenerate fermion gas heating by hole creation
Loss processes that remove particles from an atom trap leave holes behind in
the single particle distribution if the trapped gas is a degenerate fermion
system. The appearance of holes increases the temperature and we show that the
heating is (i) significant if the initial temperature is well below the Fermi
temperature , and (ii) increases the temperature to
after half of the system's lifetime, regardless of the initial temperature. The
hole heating has important consequences for the prospect of observing
Cooper-pairing in atom traps.Comment: to be published in PR
Phase separation of Bose-Einstein condensates
The zero-temperature system of two dilute overlapping Bose-Einstein
condensates is unstable against long wavelength excitations if the interaction
strength between the distinguishable bosons exceeds the geometric mean of the
like-boson interaction strengths. If the condensates attract each other, the
instability is similar to the instability of the negative scattering length
condensates. If the condensates repel, they separate spatially into condensates
of equal pressure. We estimate the boundary size, surface tension and energy of
the phase separated condensate system and we discuss the implications for
double condensates in atomic traps.Comment: 11 pages, 1 figur
Boson-Fermion coherence in a spherically symmetric harmonic trap
We consider the photoassociation of a low-density gas of quantum-degenerate
trapped fermionic atoms into bosonic molecules in a spherically symmetric
harmonic potential. For a dilute system and the photoassociation coupling
energy small compared to the level separation of the trap, only those fermions
in the single shell with Fermi energy are coupled to the bosonic molecular
field. Introducing a collective pseudo-spin operator formalism we show that
this system can then be mapped onto the Tavis-Cummings Hamiltonian of quantum
optics, with an additional pairing interaction. By exact diagonalization of the
Hamiltonian, we examine the ground state and low excitations of the Bose-Fermi
system, and study the dynamics of the coherent coupling between atoms and
molecules. In a semiclassical description of the system, the pairing
interaction between fermions is shown to result in a self-trapping transition
in the photoassociation, with a sudden suppression of the coherent oscillations
between atoms and molecules. We also show that the full quantum dynamics of the
system is dominated by quantum fluctuations in the vicinity of the
self-trapping solution.Comment: 16 pages, 14 figure
Two-species mixture of quantum degenerate Bose and Fermi gases
We have produced a macroscopic quantum system in which a Li-6 Fermi sea
coexists with a large and stable Na-23 Bose-Einstein condensate. This was
accomplished using inter-species sympathetic cooling of fermionic Li-6 in a
thermal bath of bosonic Na-23
Variational Thomas-Fermi Theory of a Nonuniform Bose Condensate at Zero Temperature
We derive a description of the spatially inhomogeneous Bose-Einstein
condensate which treats the system locally as a homogeneous system. This
approach, similar to the Thomas-Fermi model for the inhomogeneous many-particle
fermion system, is well-suited to describe the atomic Bose-Einstein condensates
that have recently been obtained experimentally through atomic trapping and
cooling. In this paper, we confine our attention to the zero temperature case,
although the treatment can be generalized to finite temperatures, as we shall
discuss elsewhere.Comment: 24 pages, latex, 6 ps figures, BoxedEPS include
Hyperon-nucleon scattering and hyperon masses in the nuclear medium
We analyze low-energy hyperon-nucleon scattering using an effective field
theory in next-to-leading order. By fitting experimental cross sections for
laboratory hyperon momenta below 200 MeV/c and using information from the
hypertriton we determine twelve contact-interaction coefficients. Based on
these we discuss the low-density expansion of hyperon mass shifts in the
nuclear medium.Comment: 10 pages, 2 figure
Microscopic Dynamics in a Strongly Interacting Bose-Einstein Condensate
An initially stable 85Rb Bose-Einstein condensate (BEC) was subjected to a
carefully controlled magnetic field pulse in the vicinity of a Feshbach
resonance. This pulse probed the strongly interacting regime for the
condensate, with calculated values for the diluteness parameter (na^3) ranging
from 0.01 to 0.5. The field pulse was observed to cause loss of atoms from the
condensate on remarkably short time scales (>=10 microsec). The dependence of
this loss on magnetic field pulse shape and amplitude was measured. For
triangular pulses shorter than 1 ms, decreasing the pulse length actually
increased the loss, until extremely short time scales (a few tens of
microseconds) were reached. Such time scales and dependencies are very
different from those expected in traditional condensate inelastic loss
processes, suggesting the presence of new microscopic BEC physics.Comment: 4 pages in latex2E, 4 eps figures; revised Fig.1, revised
scatt.lengths, added discussion, new refs., resubmitted to PR
Chiral perturbation theory for electroweak reactions on deuterium
I summarize two recent applications of chiral perturbation theory to
electromagnetic reactions on deuterium: elastic electron-deuteron scattering,
and Compton scattering on deuterium. Both calculations have now been carried
out to three orders in the chiral expansion. The expansion shows good
convergence and is able to reproduce data for q < 600 MeV in e-d and for
omega=55-95 MeV in gamma-d. These results demonstrate that ChiPT can be used to
reliably compute operators and wave functions for low-momentum-transfer
reactions in light nuclear systems.Comment: 10 pages, 6 figures. Write-up of invited talk at INT Workshop on
"Nuclear Forces and the Quantum Many-Body Problem", October 4-8, 200
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