1,766 research outputs found
Stability of an electroweak string with a fermion condensate
A solution of the standard electroweak theory with a single lepton family is
constructed, consisting of a cosmic string and a fermion condensate within its
core. The stability of this system to small perturbations is examined, and it
is found that stability is not enhanced relative to the bare electroweak
string. The presence of quark zero modes is shown to violate the existence
criteria for embedded defects.Comment: 13 pages, preprint DAMTP 94-9, SWAT/2
On the structure and spectrum of classical two-dimensional clusters with a logarithmic interaction potential
We present a numerical study of the effect of the repulsive logarithmic
inter-particle interaction on the ground state configuration and the frequency
spectrum of a confined classical two-dimensional cluster containing a finite
number of particles. In the case of a hard wall confinement all particles form
one ring situated at the boundary of the potential. For a general r^n
confinement potential, also inner rings can form and we find that all
frequencies lie below the frequency of a particular mode, namely the
breathing-like mode. An interesting situation arises for the parabolic confined
system(i.e. n=2). In this case the frequency of the breathing mode is
independent of the number of particles leading to an upper bound for all
frequencies. All results can be understood from Earnshaw's theorem in two
dimensions. In order to check the sensitivity of these results, the spectrum of
vortices in a type II superconductor which, in the limit of large penetration
depths, interact through a logarithmic potential, is investigated.Comment: 11 pages, 6 figure
Constraints on stable equilibria with fluctuation-induced forces
We examine whether fluctuation-induced forces can lead to stable levitation.
First, we analyze a collection of classical objects at finite temperature that
contain fixed and mobile charges, and show that any arrangement in space is
unstable to small perturbations in position. This extends Earnshaw's theorem
for electrostatics by including thermal fluctuations of internal charges.
Quantum fluctuations of the electromagnetic field are responsible for
Casimir/van der Waals interactions. Neglecting permeabilities, we find that any
equilibrium position of items subject to such forces is also unstable if the
permittivities of all objects are higher or lower than that of the enveloping
medium; the former being the generic case for ordinary materials in vacuum.Comment: 4 pages, 1 figur
The Dirac Sea Contribution To The Energy Of An Electroweak String
We present a systematic determination of the order hbar fermionic energy
shift when an electroweak string is perturbed. We show that the combined effect
of zero modes, bound states and continuum states is to lower the total
fermionic ground state energy of the string when the Higgs instability of the
string is excited. The effect of the Dirac sea is thus to destabilise the
string. However, this effect can be offset by populating positive energy
states. Fermions enhance the stability of an electroweak string with
sufficiently populated fermionic bound states.Comment: 57 pages, 11 figure
Performance of the Colorado wind-profiling network, part 1.5A
The Wave Propagation Laboratory (WPL) has operated a network of radar wind Profilers in Colorado for about 1 year. The network consists of four VHF (50-MHz) radars and a UHF (915-MHz) radar. The Platteville VHF radar was developed by the Aeronomy Laboratory (AL) and has been operated jointly by WPL and AL for several years. The other radars were installed between February and May 1983. Experiences with these radars and some general aspects of tropospheric wind measurements with Doppler radar are discussed
Generic Current-Carrying Strings
We investigate the standard model in a cosmic string background and show that
the electroweak symmetry is partially restored. For a range of parameters the
electroweak Higgs field can wind in this region, producing an electroweak
string stabilised by the cosmic string background. In this case there are quark
and lepton zero modes that result in the string becoming superconducting at the
electroweak scale. If the electroweak Higgs field does not wind, there are no
zero modes, but there are fermion bound states. These bound states can also
carry a current.Comment: 8 pages, TeX, 5 postscript figure
Clinicopathological evidence that neovascularisation is a cause of recurrent varicose veins
Objectives:Recurrent varicose veins may result from poor initial surgical technique or progression of varicosities in collateral veins. In some cases new veins may develop at the saphenofemoral junction (neovascularisation) and cause recurrent saphenofemoral incompetence. This was a histological study of recurrent varicose veins.Design:This clinicopathological study included 20 patients (median age 55 years) who had surgery for recurrent saphenofemoral incompetence.Materials and methods:A total of 28 legs had groin re-exploration with repeat flush saphenofemoral ligation. The venous tissue block from the saphenofemoral region (including the proximal thigh varicosity) was excised and orientated for histological analysis. Evidence of neovascularisation was sought using routine histological sections and S100 immunohistochemistry.Results:At operation, thin-walled, serpentine neovascular veins were detected clinically as the principal cause of recurrence in 19 groins. In five groins recurrence was due to a residual missed vein at the saphenofemoral junction, and in four recurrence was caused by cross groin collaterals. On histological sections, evidence of neovascularisation was present in 27 of 28 groins. In eight it co-existed with the veins missed at the original operation but it was the sole identified cause of recurrent saphenofemoral incompetence in 19 (68%) groins.Conclusions:Neovascularisation was the principal cause of recurrent saphenofemoral incompetence in this series
Opioid-Induced Constipation in Advanced Illness: Safety and Efficacy of Methylnaltrexone Bromide
Constipation, one of the major side effects of opiates used in palliative care, can impair patients’ quality of life to a point where it prevents sufficient pain control. Methylnaltrexone is a novel μ-receptor antagonist, which does not pass the blood brain barrier. It is licensed to treat opiate induced constipation for patients with advanced diseases. This review article presents an overview of pharmacology and safety of its application, evidence of its efficacy and economic aspects of its use in clinical practice. Available data are limited but strongly suggest that methylnaltrexone causes laxation in less than 24 hours for at least half of those patients over the first two weeks of usage without impairing pain control or causing serious adverse effects. To avoid danger of gastrointestinal perforation it is contraindicated for patients at risk for that complication. More research is needed to evaluate its long-term efficacy and economic impact
Exploring the climate of Proxima B with the Met Office Unified Model
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.The corrigendum to this article is in ORE at: http://hdl.handle.net/10871/34331We present results of simulations of the climate of the newly discovered planet Proxima Centauri B, performed using the Met Office
Unified Model (UM). We examine the responses of both an ‘Earth-like’ atmosphere and simplified nitrogen and trace carbon dioxide
atmosphere to the radiation likely received by Proxima Centauri B. Additionally, we explore the effects of orbital eccentricity on the
planetary conditions using a range of eccentricities guided by the observational constraints. Overall, our results are in agreement with
previous studies in suggesting Proxima Centauri B may well have surface temperatures conducive to the presence of liquid water.
Moreover, we have expanded the parameter regime over which the planet may support liquid water to higher values of eccentricity
(& 0.1) and lower incident fluxes (881.7 W m−2
) than previous work. This increased parameter space arises because of the low
sensitivity of the planet to changes in stellar flux, a consequence of the stellar spectrum and orbital configuration. However, we also
find interesting differences from previous simulations, such as cooler mean surface temperatures for the tidally-locked case. Finally,
we have produced high resolution planetary emission and reflectance spectra, and highlight signatures of gases vital to the evolution
of complex life on Earth (oxygen, ozone and carbon dioxide).I.B., J.M. and P.E. acknowledge the support of a Met Office Academic Partnership secondment. B.D. thanks the University of Exeter for
support through a Ph.D. studentship. N.J.M. and J.G.’s contributions were in part
funded by a Leverhulme Trust Research Project Grant, and in part by a University
of Exeter College of Engineering, Mathematics and Physical Sciences studentship.
We acknowledge use of the MONSooN system, a collaborative facility
supplied under the Joint Weather and Climate Research Programme, a strategic
partnership between the Met Office and the Natural Environment Research
Council. This work also used the University of Exeter Supercomputer, a DiRAC
Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and
the University of Exeter
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