255 research outputs found
Is there a "most perfect fluid" consistent with quantum field theory?
It was recently conjectured that the ratio of the shear viscosity to entropy
density, , for any fluid always exceeds . This
conjecture was motivated by quantum field theoretic results obtained via the
AdS/CFT correspondence and from empirical data with real fluids. A theoretical
counterexample to this bound can be constructed from a nonrelativistic gas by
increasing the number of species in the fluid while keeping the dynamics
essentially independent of the species type. The question of whether the
underlying structure of relativistic quantum field theory generically inhibits
the realization of such a system and thereby preserves the possibility of a
universal bound is considered here. Using rather conservative assumptions, it
is shown here that a metastable gas of heavy mesons in a particular controlled
regime of QCD provides a realization of the counterexample and is consistent
with a well-defined underlying relativistic quantum field theory. Thus, quantum
field theory appears to impose no lower bound on , at least for
metastable fluids.Comment: 4 pages; typos corrected and references added in new versio
Minimum of and the phase transition of the Linear Sigma Model in the large-N limit
We reexamine the possibility of employing the viscosity over entropy density
ratio as a diagnostic tool to identify a phase transition in hadron physics to
the strongly coupled quark-gluon plasma and other circumstances where direct
measurement of the order parameter or the free energy may be difficult.
It has been conjectured that the minimum of eta/s does indeed occur at the
phase transition. We now make a careful assessment in a controled theoretical
framework, the Linear Sigma Model at large-N, and indeed find that the minimum
of eta/s occurs near the second order phase transition of the model due to the
rapid variation of the order parameter (here the sigma vacuum expectation
value) at a temperature slightly smaller than the critical one.Comment: 22 pages, 19 figures, v2, some references and several figures added,
typos corrected and certain arguments clarified, revised for PR
Structure-Compressive Stress Relationships in Mixed Dairy Gels
Mixed dairy gels (including a control without fat) of skim milk powder (SMP) and whey protein isolate (WPI) containing fat globules were formed by heating protein emulsions to 90°( and by acid release from glucono- 0-lactone to provide a pH of 4.3-4.4 . Fat globules with artificial protein membranes (FGAPM) were prepared by homogenization of a butter oil /water mixture in the presence of WPI while fat globules without membranes were stabili zed with polyoxyethylene sorbitan monolaurate (Tween 20). Both emulsions were added at a 4% (w/w) leve l to solutions having 3% SMP and 8.3% WPI. The gel contai ning FGAPM had significantly higher compressive streng th than the control without fat (2.4 versus 1.8 kPa , respectively) and microst ru ctural ly it was a mixed gel in which the FGAPM, casein and whey protein aggregates formed a copolymer network. Addition of fat globules without membranes led to a filled gel weaker th an the control without fat (1.4 versus 1.8 kPa, respectivel y). Bonding of the protein membrane in FGAPM to the gel network and presence of in dividually di spersed fat globules without membranes was demonstrated by trans mission electron microscopy. The difference in microstructure is proposed to be responsible for the mechanical properties of each gel
Bose-Einstein Condensate Driven by a Kicked Rotor in a Finite Box
We study the effect of different heating rates of a dilute Bose gas confined
in a quasi-1D finite, leaky box. An optical kicked-rotor is used to transfer
energy to the atoms while two repulsive optical beams are used to confine the
atoms. The average energy of the atoms is localized after a large number of
kicks and the system reaches a nonequilibrium steady state. A numerical
simulation of the experimental data suggests that the localization is due to
energetic atoms leaking over the barrier. Our data also indicates a correlation
between collisions and the destruction of the Bose-Einstein condensate
fraction.Comment: 7 pages, 8 figure
On freeze-out problem in hydro-kinetic approach to A+A collisions
A new method for evaluating spectra and correlations in the hydrodynamic
approach is proposed. It is based on an analysis of Boltzmann equations (BE) in
terms of probabilities for constituent particles to escape from the interacting
system. The conditions of applicability of Cooper-Frye freeze-out prescription
are considered within the method. The results are illustrated with a
non-relativistic exact solution of BE for expanding spherical fireball as well
as with approximate solutions for ellipsoidally expanding ones.Comment: 4 pages including 2 figures, RevTex, stylistic and clarifying
corrections are made, submitted to Phys. Rev. Let
Time parameterization and stationary distributions in a relativistic gas
In this paper we consider the effect of different time parameterizations on
the stationary velocity distribution function for a relativistic gas. We
clarify the distinction between two such distributions, namely the J\"{u}ttner
and the modified J\"{u}ttner distributions. Using a recently proposed model of
a relativistic gas, we show that the obtained results for the proper-time
averaging does not lead to modified J\"{u}ttner distribution (as recently
conjectured), but introduces only a Lorentz factor to the well-known
J\"{u}ttner function which results from observer-time averaging. We obtain
results for rest frame as well as moving frame in order to support our claim.Comment: 5 pages, 2 figure
Decoherence and dephasing errors caused by D.C. Stark effect in rapid ion transport
We investigate the error due to D.C. Stark effect for quantum information
processing for trapped ion quantum computers using the scalable architecture
proposed in J. Res. Natl. Inst. Stan. 103, 259 (1998) and Nature 417, 709
(2002). As the operation speed increases, dephasing and decoherence due to the
D.C. Stark effect becomes prominent as a large electric field is applied for
transporting ions rapidly. We estimate the relative significance of the
decoherence and dephasing effects and find that the latter is dominant. We find
that the minimum possible of dephasing is quadratic in the time of flight, and
an inverse cubic in the operational time scale. From these relations, we obtain
the operational speed-range at which the shifts caused by D.C. Stark effect, no
matter follow which trajectory the ion is transported, are no longer
negligible. Without phase correction, the maximum speed a qubit can be
transferred across a 100 micron-long trap, without excessive error, in about 10
ns for Calcium ion and 50 ps for Beryllium ion. In practice, the accumulated
error is difficult to be tracked and calculated, our work gives an estimation
to the range of speed limit imposed by D.C. Stark effect.Comment: 7 pages, 1 figure. v2: Title is changed in this version to make our
argument more focused. Introduction is rewritten. A new section IV is added
to make our point more prominent. v3: Title is changed to make our argument
more specific. Abstract, introduction, and summary are revise
The non-self-adjointness of the radial momentum operator in n dimensions
The non self-adjointness of the radial momentum operator has been noted
before by several authors, but the various proofs are incorrect. We give a
rigorous proof that the -dimensional radial momentum operator is not self-
adjoint and has no self-adjoint extensions. The main idea of the proof is to
show that this operator is unitarily equivalent to the momentum operator on
which is not self-adjoint and has no self-adjoint
extensions.Comment: Some text and a reference adde
A Model for the Diffuse γ-Ray Spectrum
A model is proposed to describe the observed shelf in the cosmic diffuse radiation spectrum just above 1 MeV. This model is based on induced positronium annihilation, which at incident photon energy 2 mc^2 gives rise to enhancement of the radiative field. It is proposed that this amplification may occur in double radio sources or other accreting objects. Recent observation of γ-ray line emission from the double radio sources SS 433 at 1.2 and 1.5 MeV is in good agreement with the proposed model
Nonlinear Relativistic and Quantum Equations with a Common Type of Solution
Generalizations of the three main equations of quantum physics, namely, the
Schr\"odinger, Klein-Gordon, and Dirac equations, are proposed. Nonlinear
terms, characterized by exponents depending on an index , are considered in
such a way that the standard linear equations are recovered in the limit . Interestingly, these equations present a common, soliton-like,
travelling solution, which is written in terms of the -exponential function
that naturally emerges within nonextensive statistical mechanics. In all cases,
the well-known Einstein energy-momentum relation is preserved for arbitrary
values of
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