160 research outputs found
Development of a new diagnostic device for extracorporeal shock-wave lithotripsy
Extracorporeal Shock-Wave Lithotripsy (ESWL) is the leading technique for the non-invasive treatment of urinary stones. Thousands of ultrasound shocks are focused on the stones in order to break them into fragments small enough to be passed naturally by the body. The procedure is well established, though the re-treatment rate is around
50%. One of the limits of the procedure is that there is no capability for on-line monitoring of the degree of fragmentation of the stone. The output of the treatments could probably be improved if this facility was made available. The underlying physical mechanisms responsible for the break-up of the stone are still subject to investigation. However both direct stress damage and indirect cavitation erosion seem to be necessary to obtain eliminable fragments. In previous studies, Coleman et al. monitored cavitation in-vivo through the associated acoustic emissions. The objective of this research was to design a new diagnostic device for lithotripsy, exploiting the information carried by these acoustic emissions. After preliminary laboratory experiments
some clinical prototypes were developed in collaboration with Precision Acoustic Ltd., UK. The prototypes are currently been tested in the clinic
The spatial distribution of cavitation induced acoustic emission, sonoluminescence and cell lysis in the field of a shock wave lithotripter
This study examines the spatial distribution of various properties attributed to the cavitation field generated by a shock wave lithotripter. These properties include acoustic emission and sonoluminescence, which result from violent bubble collapse, and the degree of cell lysis in vitro, which appears to be related to cavitation. The acoustic emission detected with a 1 MHz, 12 cm diameter focused hydrophone occurs in two distinct bursts. The immediate signal is emitted from a small region contained within the 4 MPa peak negative pressure contour. A second, delayed, burst is emitted from a region extending further along the beam axis. The delay between these two bursts has also been mapped, and the longest delay occurs at positions close to the regions of maximum peak negative pressure. Sonoluminescence from both single and multiple shocks occurs in a broader region than the acoustic emission but the measurement technique does not allow time resolution of the signal. Cell lysis occurs in a relatively small region that correlates closely with the immediate acoustic emission for a shock propagating in a gelatine solution
Searching for Lee-Wick Gauge Bosons at the LHC
In an extension of the Standard Model(SM) based on the ideas of Lee and Wick,
Grinstein, O'Connell and Wise have found an interesting way to remove the usual
quadratically divergent contributions to the Higgs mass induced by radiative
corrections. Phenomenologically, the model predicts the existence of Terascale,
negative-norm copies of the usual SM fields with rather unique properties:
ghost-like propagators and negative decay widths, but with otherwise SM-like
couplings. The model is both unitary and causal on macroscopic scales. In this
paper we examine whether or not such states with these unusual properties can
be uniquely identified as such at the LHC. We find that in the extended strong
and electroweak gauge boson sector of the model, which is the simplest one to
analyze, such an identification can be rather difficult. Observation of heavy
gluon-like resonances in the dijet channel offers the best hope for this
identification.Comment: 17 pages, 4 figs; discussion adde
Stability of Subsequent-to-Leading-Logarithm Corrections to the Effective Potential for Radiative Electroweak Symmetry Breaking
We demonstrate the stability under subsequent-to-leading logarithm
corrections of the quartic scalar-field coupling constant and the
running Higgs boson mass obtained from the (initially massless) effective
potential for radiatively broken electroweak symmetry in the
single-Higgs-Doublet Standard Model. Such subsequent-to-leading logarithm
contributions are systematically extracted from the renormalization group
equation considered beyond one-loop order. We show to be the dominant
coupling constant of the effective potential for the radiatively broken case of
electroweak symmetry. We demonstrate the stability of and the running
Higgs boson mass through five orders of successively subleading logarithmic
corrections to the scalar-field-theory projection of the effective potential
for which all coupling constants except the dominant coupling constant
are disregarded. We present a full next-to-leading logarithm
potential in the three dominant Standard Model coupling constants
(-quark-Yukawa, , and ) from these coupling constants'
contribution to two loop - and -functions. Finally, we
demonstrate the manifest order-by-order stability of the physical Higgs boson
mass in the 220-231 GeV range. In particular, we obtain a 231 GeV physical
Higgs boson mass inclusive of the -quark-Yukawa and coupling
constants to next-to-leading logarithm order, and inclusive of the smaller
gauge coupling constants to leading logarithm order.Comment: 21 pages, latex2e, 2 eps figures embedded in latex file. Updated
version contains expanded analysis in Section
Compound effect of EHD and surface roughness in pool boiling and CHF with R-123
This article is a post-print version of the fianl published article which may be accessed at the link below.Saturated pool boiling of R-123 at 1 bar, including the critical heat flux (CHF), was enhanced by modifying the surface characteristics and applying a high intensity electrostatic field, the latter termed electrohydrodynamic (and abbreviated EHD) enhancement. The heat flux was varied from very low values in the natural convection regime up to CHF. Experiments were performed with increasing and decreasing heat flux to study boiling hysteresis without and with EHD. Boiling occurred on the sand blasted surface of a
cylindrical copper block with embedded electrical heating elements, with standardized surface parameter Pa = 3.5 ÎŒm. The electric field was generated by a potential of 5 kV to 25 kV, applied through a 40 mm diameter circular electrode of ss-304 wire mesh, aperture size 5.1 mm, located at distances of 5 - 60 mm from the surface, with most of the data obtained for 20 mm. The data for the rough surface were compared with earlier data for a smooth surface and indicated a significant increase in the heat transfer rates. EHD produced a further increase in the heat transfer rates, particularly at low heat flux values and near the CHF. Boiling hysteresis was reduced progressively by EHD and eliminated at high field strength.This work was supported by Government of Pakistan under a scholarship programme
Optimal Renormalization-Group Improvement of Two Radiatively-Broken Gauge Theories
In the absence of a tree-level scalar-field mass, renormalization-group (RG)
methods permit the explicit summation of leading-logarithm contributions to all
orders of the perturbative series for the effective-potential functions
utilized in radiative symmetry breaking. For scalar-field electrodynamics, such
a summation of leading logarithm contributions leads to upper bounds on the
magnitudes of both gauge and scalar-field coupling constants, and suggests the
possibility of an additional phase of spontaneous symmetry breaking
characterized by a scalar-field mass comparable to that of the theory's gauge
boson. For radiatively-broken electroweak symmetry, the all-orders summation of
leading logarithm terms involving the dominant three couplings (quartic
scalar-field, t-quark Yukawa, and QCD) contributing to standard-model radiative
corrections leads to an RG-improved potential characterized by a 216 GeV Higgs
boson mass. Upon incorporation of electroweak gauge couplants we find that the
predicted Higgs mass increases to 218 GeV. The potential is also characterized
by a quartic scalar-field coupling over five times larger than that anticipated
for an equivalent Higgs mass obtained via conventional spontaneous symmetry
breaking, leading to a concomitant enhancement of processes (such as ) sensitive to this coupling. Moreover, if the QCD coupling constant is
taken to be sufficiently strong, the tree potential's local minimum at is shown to be restored for the summation of leading logarithm corrections.
Thus if QCD exhibits a two-phase structure similar to that of
supersymmetric Yang-Mills theory, the weaker asymptotically-free phase of QCD
may be selected by the large logarithm behaviour of the RG-improved effective
potential for radiatively broken electroweak symmetry.Comment: latex2e using amsmath, 36 pages, 7 eps figures embedded in latex.
Section 8.3 errors asociated with electroweak coupling effects are correcte
X-boson cumulant approach to the periodic Anderson model
The Periodic Anderson Model (PAM) can be studied in the infinite U limit by
employing the Hubbard X operators to project out the unwanted states. We have
already studied this problem employing the cumulant expansion with the
hybridization as perturbation, but the probability conservation of the local
states (completeness) is not usually satisfied when partial expansions like the
Chain Approximation (CHA) are employed. Here we treat the problem by a
technique inspired in the mean field approximation of Coleman's slave-bosons
method, and we obtain a description that avoids the unwanted phase transition
that appears in the mean-field slave-boson method both when the chemical
potential is greater than the localized level Ef at low temperatures (T) and
for all parameters at intermediate T.Comment: Submited to Physical Review B 14 pages, 17 eps figures inserted in
the tex
75th Anniversary of âExistence of Electromagnetic-Hydrodynamic Wavesâ
We have recently passed the 75th anniversary of one of the most important
results in solar and space physics: Hannes Alfv\'en's discovery of Alfv\'en
waves and the Alfv\'en speed. To celebrate the anniversary, this article
recounts some major episodes in the history of MHD waves. Following an
initially cool reception, Alfv\'en's ideas were propelled into the spotlight by
Fermi's work on cosmic rays, the new mystery of coronal heating and, as
scientific perception of interplanetary space shifted dramatically and the
space race started, detection of Alfv\'en waves in the solar wind. From then
on, interest in MHD waves boomed, laying the foundations for modern remote
observations of MHD waves in the Sun, coronal seismology and some of today's
leading theories of coronal heating and solar wind acceleration. In 1970,
Alfv\'en received the Nobel Prize for his work in MHD, including these
discoveries. The article concludes with some reflection about what the history
implies about the way we do science, especially the advantages and pitfalls of
idealised mathematical models.Comment: 10 pages, accepted by Solar Physic
Lorentz breaking Effective Field Theory and observational tests
Analogue models of gravity have provided an experimentally realizable test
field for our ideas on quantum field theory in curved spacetimes but they have
also inspired the investigation of possible departures from exact Lorentz
invariance at microscopic scales. In this role they have joined, and sometime
anticipated, several quantum gravity models characterized by Lorentz breaking
phenomenology. A crucial difference between these speculations and other ones
associated to quantum gravity scenarios, is the possibility to carry out
observational and experimental tests which have nowadays led to a broad range
of constraints on departures from Lorentz invariance. We shall review here the
effective field theory approach to Lorentz breaking in the matter sector,
present the constraints provided by the available observations and finally
discuss the implications of the persisting uncertainty on the composition of
the ultra high energy cosmic rays for the constraints on the higher order,
analogue gravity inspired, Lorentz violations.Comment: 47 pages, 4 figures. Lecture Notes for the IX SIGRAV School on
"Analogue Gravity", Como (Italy), May 2011. V.3. Typo corrected, references
adde
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
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