4,149 research outputs found
Dimensional versus cut-off renormalization and the nucleon-nucleon interaction
The role of dimensional regularization is discussed and compared with that of
cut-off regularization in some quantum mechanical problems with ultraviolet
divergence in two and three dimensions with special emphasis on the
nucleon-nucleon interaction. Both types of renormalizations are performed for
attractive divergent one- and two-term separable potentials, a divergent tensor
potential, and the sum of a delta function and its derivatives. We allow
energy-dependent couplings, and determine the form that these couplings should
take if equivalence between the two regularization schemes is to be enforced.
We also perform renormalization of an attractive separable potential superposed
on an analytic divergent potential.Comment: 19 pages + one postscript figur
The eigenspectra of Indian musical drums
In a family of drums used in the Indian subcontinent, the circular drum head
is made of material of non-uniform density. Remarkably, and in contrast to a
circular membrane of uniform density, the low eigenmodes of the non-uniform
membrane are harmonic. In this work we model the drum head by a non-uniform
membrane whose density varies smoothly between two prescribed values. Using a
Fourier-Chebyshev spectral collocation method we obtain the eigenmodes and
eigenvalues of the drum head. For a suitable choice of parameters, which we
find by optimising a cost function, the eigenspectra obtained from our model
are in excellent agreement with experimental values. Our model and the
numerical method should find application in numerical sound synthesis
Performance of a prototype active veto system using liquid scintillator for a dark matter search experiment
We report the performance of an active veto system using a liquid
scintillator with NaI(Tl) crystals for use in a dark matter search experiment.
When a NaI(Tl) crystal is immersed in the prototype detector, the detector tags
48% of the internal K-40 background in the 0-10 keV energy region. We also
determined the tagging efficiency for events at 6-20 keV as 26.5 +/- 1.7% of
the total events, which corresponds to 0.76 +/- 0.04 events/keV/kg/day.
According to a simulation, approximately 60% of the background events from U,
Th, and K radioisotopes in photomultiplier tubes are tagged at energies of 0-10
keV. Full shielding with a 40-cm-thick liquid scintillator can increase the
tagging efficiency for both the internal K-40 and external background to
approximately 80%.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section
Effective Nonlinear Schr\"odinger Equations for Cigar-Shaped and Disk-Shaped Fermi Superfluids at Unitarity
In the case of tight transverse confinement (cigar-shaped trap) the
three-dimensional (3D) nonlinear Schr\"odinger equation, describing superfluid
Fermi atoms at unitarity (infinite scattering length ), is
reduced to an effective one-dimensional form by averaging over the transverse
coordinates. The resultant effective equation is a 1D nonpolynomial Schrodinger
equation, which produces results in good agreement with the original 3D one. In
the limit of small and large fermion number the nonlinearity is of simple
power-law type. A similar reduction of the 3D theory to a two-dimensional form
is also performed for a tight axial confinement (disk-shaped trap). The
resultant effective 2D nonpolynomial equation also produces results in
agreement with the original 3D equation and has simple power-law nonlinearity
for small and large . For both cigar- and disk-shaped superfluids our
nonpolynomial Schr\"odinger equations are quite attractive for phenomenological
application.Comment: 22 pages, 5 figure
Lattice Discretization in Quantum Scattering
The utility of lattice discretization technique is demonstrated for solving
nonrelativistic quantum scattering problems and specially for the treatment of
ultraviolet divergences in these problems with some potentials singular at the
origin in two and three space dimensions. This shows that lattice
discretization technique could be a useful tool for the numerical solution of
scattering problems in general. The approach is illustrated in the case of the
Dirac delta function potential.Comment: 9 page
Endogenous Quasicycles and Stochastic Coherence in a Closed Endemic Model
We study the role of demographic fluctuations in typical endemics as
exemplified by the stochastic SIRS model. The birth-death master equation of
the model is simulated using exact numerics and analysed within the linear
noise approximation. The endemic fixed point is unstable to internal
demographic noise, and leads to sustained oscillations. This is ensured when
the eigenvalues () of the linearised drift matrix are complex, which
in turn, is possible only if detailed balance is violated. In the oscillatory
state, the phases decorrelate asymptotically, distinguishing such oscillations
from those produced by external periodic forcing. These so-called quasicycles
are of sufficient strength to be detected reliably only when the ratio
is of order unity. The coherence or regularity of
these oscillations show a maximum as a function of population size, an effect
known variously as stochastic coherence or coherence resonance. We find that
stochastic coherence can be simply understood as resulting from a non-monotonic
variation of with population size. Thus, within the
linear noise approximation, stochastic coherence can be predicted from a purely
deterministic analysis. The non-normality of the linearised drift matrix,
associated with the violation of detailed balance, leads to enhanced
fluctuations in the population amplitudes.Comment: 21 pages, 8 figure
Dynamics of collapsing and exploding Bose-Einstein condensed vortex state
Using the time-dependent mean-field Gross-Pitaevskii equation we study the
dynamics of small repulsive Bose-Einstein condensed vortex states of ^{85}Rb
atoms in a cylindrical trap with low angular momentum hbar L per atom (L <= 6),
when the atomic interaction is suddenly turned attractive by manipulating the
external magnetic field near a Feshbach resonance. Consequently, the condensate
collapses and ejects atoms via explosion and a remnant condensate with a
smaller number of atoms emerges that survives for a long time. Detail of this
collapse and explosion is compared critically with a similar experiment
performed with zero angular momentum (L=0). Suggestion for future experiment
with vortex state is made.Comment: 8 REVTEX4 pages, 8 EPS figures, final version accepted in Phys. Rev.
A after minor change
Numerical study of the coupled time-dependent Gross-Pitaevskii equation: Application to Bose-Einstein condensation
We present a numerical study of the coupled time-dependent Gross-Pitaevskii
equation, which describes the Bose-Einstein condensate of several types of
trapped bosons at ultralow temperature with both attractive and repulsive
interatomic interactions. The same approach is used to study both stationary
and time-evolution problems. We consider up to four types of atoms in the study
of stationary problems. We consider the time-evolution problems where the
frequencies of the traps or the atomic scattering lengths are suddenly changed
in a stable preformed condensate. We also study the effect of periodically
varying these frequencies or scattering lengths on a preformed condensate.
These changes introduce oscillations in the condensate which are studied in
detail. Good convergence is obtained in all cases studied.Comment: 9 pages, 10 figures, accepted in Physical Review
Resonance in Bose-Einstein condensate oscillation from a periodic variation in scattering length
Using the explicit numerical solution of the axially-symmetric
Gross-Pitaevskii equation we study the oscillation of the Bose-Einstein
condensate induced by a periodic variation in the atomic scattering length .
When the frequency of oscillation of is an even multiple of the radial or
axial trap frequency, respectively, the radial or axial oscillation of the
condensate exhibits resonance with novel feature. In this nonlinear problem
without damping, at resonance in the steady state the amplitude of oscillation
passes through maximum and minimum. Such growth and decay cycle of the
amplitude may keep on repeating. Similar behavior is also observed in a
rotating Bose-Einstein condensate.Comment: 14 REVTEX4 pages, 18 PS figures, final version Accepted in Journal of
Physics
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