3,607 research outputs found
Quantum System under Periodic Perturbation: Effect of Environment
In many physical situations the behavior of a quantum system is affected by
interaction with a larger environment. We develop, using the method of
influence functional, how to deduce the density matrix of the quantum system
incorporating the effect of environment. After introducing characterization of
the environment by spectral weight, we first devise schemes to approximate the
spectral weight, and then a perturbation method in field theory models, in
order to approximately describe the environment. All of these approximate
models may be classified as extended Ohmic models of dissipation whose
differences are in the high frequency part.
The quantum system we deal with in the present work is a general class of
harmonic oscillators with arbitrary time dependent frequency. The late time
behavior of the system is well described by an approximation that employs a
localized friction in the dissipative part of the correlation function
appearing in the influence functional. The density matrix of the quantum system
is then determined in terms of a single classical solution obtained with the
time dependent frequency. With this one can compute the entropy, the energy
distribution function, and other physical quantities of the system in a closed
form.
Specific application is made to the case of periodically varying frequency.
This dynamical system has a remarkable property when the environmental
interaction is switched off: Effect of the parametric resonance gives rise to
an exponential growth of the populated number in higher excitation levels, or
particle production in field theory models. The effect of the environment is
investigated for this dynamical system and it is demonstrated that there existsComment: 55 pages, LATEX file plus 13 PS figures. A few calculational
mistatkes and corresponding figure 1 in field theory model corrected and some
changes made for publication in Phys. Rev.D (in press
Large-Area Scintillator Hodoscope with 50 ps Timing Resolution Onboard BESS
We describe the design and performance of a large-area scintillator hodoscope
onboard the BESS rigidity spectrometer; an instrument with an acceptance of 0.3
m^{2}sr.
The hodoscope is configured such that 10 and 12 counters are respectively
situated in upper and lower layers.
Each counter is viewed from its ends by 2.5 inch fine-mesh photomultiplier
tubes placed in a stray magnetic field of 0.2 Tesla.
Various beam-test data are presented.
Use of cosmic-ray muons at ground-level confirmed 50 ps timing resolution for
each layer, giving an overall time-of-flight resolution of 70 ps rms using a
pure Gaussian resolution function.
Comparison with previous measurements on a similar scintillator hodoscope
indicates good agreement with the scaling law that timing resolution is
proportional to 1/, where is the effective
number of photoelectrons.Comment: 16 pages, 14 figure
Disorder Induced Ferromagnetism in CaRuO3
The magnetic ground state of perovskite structure CaRuO3 has been enigmatic
for decades. Here we show that paramagnetic CaRuO3 can be made ferromagnetic by
very small amounts of partial substitution of Ru by Ti. Magnetic hysteresis
loops are observed at 5 K for as little as 2% Ti substitution. Ti is
non-magnetic and isovalent with Ru, indicating that the primary effect of the
substitution is the disruption of the magnetic ground state of CaRuO3 through
disorder. The data suggest that CaRuO3 is poised at a critical point between
ferromagnetic and paramagnetic ground states
Prolonged Decay and CP-asymmetry
Time evolution of unstable particles that occur in the expanding universe is
investigated. The off-shell effect not included in the Boltzmann-like equation
is important for the decay process when the temperature becomes much below the
mass of unstable particle. When the off-shell effect is taken into account, the
thermal abundance of unstable particles at low temperatures has a power law
behavior of temperature ,
unlike the Boltzmann suppressed , with the power related to
the spectral rise near the threshold of the decay and with the decay
rate. Moreover, the relaxation time towards the thermal value is not governed
by the exponential law; instead, it is the power law of time. The evolution
equation for the occupation number and the number density of the unstable
particle is derived, when both of these effects, along with the cosmic
expansion, are included. We also critically examine how the scattering off
thermal particles may affect the off-shell effect to the unstable particle. As
an application showing the importance of the off-shell effect we compute the
time evolution of the baryon asymmetry generated by the heavy boson decay.
It is shown that the out-of equilibrium kinematics previously discussed is
considerably changed.Comment: 33 pages, LATEX file with 9 PS figure
Remarks on Cosmic String Formation during Preheating on Lattice Simulations
We reconsider the formation of (global) cosmic strings during and after
preheating by calculating the dynamics of a scalar field on both two- and
three- dimensional lattices. We have found that there is little differences
between the results in two and three dimensions about the dynamics of
fluctuations, at least, during preheating. Practically, it is difficult to
determine whether long cosmic strings which may affect the later evolution of
the universe could ever be produced from the results of simulations on
three-dimensional lattices with smaller box sizes than the horizon. Therefore,
using two-dimensional lattices with large box size, we have found that cosmic
strings with the breaking scale 0\eta \sim 10^{16} GeV are produced for broad
range of parameter space in \eta, while for higher breaking scales (\eta \sim
3\times 10^{16} GeV), their production depends crucially on the value of the
breaking scale \eta in our simulations.Comment: 7 pages, RevTex, 14 postscript figures included, to appear in Phys.
Rev.
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