459 research outputs found
Fluctuations of the Retarded Van der Waals Force
The retarded Van der Waals force between a polarizable particle and a
perfectly conducting plate is re-examined. The expression for this force given
by Casimir and Polder represents a mean force, but there are large fluctuations
around this mean value on short time scales which are of the same order of
magnitude as the mean force itself. However, these fluctuations occur on time
scales which are typically of the order of the light travel time between the
atom and the plate. As a consequence, they will not be observed in an
experiment which measures the force averaged over a much longer time. In the
large time limit, the magnitude of the mean squared velocity of a test particle
due to this fluctuating Van der Waals force approaches a constant, and is
similar to a Brownian motion of a test particle in an thermal bath with an
effective temperature. However the fluctuations are not isotropic in this case,
and the shift in the mean square velocity components can even be negative. We
interpret this negative shift to correspond to a reduction in the velocity
spread of a wavepacket. The force fluctuations discussed in this paper are
special case of the more general problem of stress tensor fluctuations. These
are of interest in a variety of areas fo physics, including gravity theory.
Thus the effects of Van der Waals force fluctuations serve as a useful model
for better understanding quantum effects in gravity theory.Comment: 14 pages, no figure
Three-generation flavor transitions and decays of supernova relic neutrinos
If neutrinos have mass, they can also decay. Decay lifetimes of cosmological
interest can be probed, in principle, through the detection of the redshifted,
diffuse neutrino flux produced by all past supernovae--the so-called supernova
relic neutrino (SRN) flux. In this work, we solve the SRN kinetic equations in
the general case of three-generation flavor transitions followed by invisible
(nonradiative) two-body decays. We then use the general solution to calculate
observable SRN spectra in some representative decay scenarios. It is shown
that, in the presence of decay, the SRN event rate can basically span the whole
range below the current experimental upper bound--a range accessible to future
experimental projects. Radiative SRN decays are also briefly discussed.Comment: 25 pages, including 7 figure
k=0Magnetic Structure and Absence of Ferroelectricity in SmFeO3
SmFeO3 has attracted considerable attention very recently due to the reported
multiferroic properties above room-temperature. We have performed powder and
single crystal neutron diffraction as well as complementary polarization
dependent soft X-ray absorption spectroscopy measurements on floating-zone
grown SmFeO3 single crystals in order to determine its magnetic structure. We
found a k=0 G-type collinear antiferromagnetic structure that is not compatible
with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While
the structural data reveals a clear sign for magneto-elastic coupling at the
N\'eel-temperature of ~675 K, the dielectric measurements remain silent as far
as ferroelectricity is concerned
Corrosion behaviour of micro-plasma arc welded stainless steels in H3PO4 under flowing conditions at different temperatures
[EN] This paper studies the general corrosion behaviour of the micro-plasma arc welded AISI 316L stainless steel in phosphoric acid at different temperatures (25-60°C) and at a Reynolds number of 1456. Galvanic corrosion has been studied using zero-resistance ammeter (ZRA) measurements and polarization curves (by the mixed potential theory). Results show that the microstructure of the stainless steel is modified due to the micro-plasma arc welding procedure. Coupled current density values obtained from polarization curves increase with temperature. ZRA tests present the highest iG values at 60°C; however, the values are very close to zero for all the temperatures studied. This is in agreement with the low value of the compatibility limit and of the parameter which evaluates the importance of the galvanic phenomenon. Both techniques present the most positive potentials at the highest temperature. This study reveals that micro-plasma arc welded AISI 316L stainless steels are appropriated working in the studied H3PO4 media from a corrosion point of view for all the temperatures analysedThe authors would like to express their gratitude to the Spanish MAEC (PCI Mediterráneo C/8196/07, C/018046/08, D/023608/09) and to Asuncion Jaime for her translation assistance.Sánchez Tovar, R.; Montañés Sanjuan, MT.; García Antón, J.; Guenbour, A.; Ben Bachir, A. (2011). Corrosion behaviour of micro-plasma arc welded stainless steels in H3PO4 under flowing conditions at different temperatures. Corrosion Science. 53(4):1237-1246. https://doi.org/10.1016/j.corsci.2010.12.017S1237124653
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
SN1987A and the properties of neutrino burst
We reanalyze the neutrino events from SN1987A in IMB and Kamiokande-II (KII)
detectors, and compare them with the expectations from simple theoretical
models of the neutrino emission. In both detectors the angular distributions
are peaked in the forward direction, and the average cosines are 2 sigma above
the expected values. Furthermore, the average energy in KII is low if compared
with the expectations; but, as we show, the assumption that a few (probably
one) events at KII have been caused by elastic scattering is not in contrast
with the 'standard' picture of the collapse and yields a more satisfactory
distributions in angle and (marginally) in energy. The observations give useful
information on the astrophysical parameters of the collapse: in our
evaluations, the mean energy of electron antineutrinos is =12-16 MeV, the
total energy radiated around (2-3)*1.E53 erg, and there is a hint for a
relatively large radiation of non-electronic neutrino species. These properties
of the neutrino burst are not in disagreement with those suggested by the
current theoretical paradigm, but the data leave wide space to non-standard
pictures, especially when neutrino oscillations are included.Comment: 14 pages, 5 figure
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