239 research outputs found
Bound states of neutral particles in external electric fields
Neutral fermions of spin with magnetic moment can interact with
electromagnetic fields through nonminimal coupling. The Dirac--Pauli equation
for such a fermion coupled to a spherically symmetric or central electric field
can be reduced to two simultaneous ordinary differential equations by
separation of variables in spherical coordinates. For a wide variety of central
electric fields, bound-state solutions of critical energy values can be found
analytically. The degeneracy of these energy levels turns out to be numerably
infinite. This reveals the possibility of condensing infinitely many fermions
into a single energy level. For radially constant and radially linear electric
fields, the system of ordinary differential equations can be completely solved,
and all bound-state solutions are obtained in closed forms. The radially
constant field supports scattering solutions as well. For radially linear
fields, more energy levels (in addition to the critical one) are infinitely
degenerate. The simultaneous presence of central magnetic and electric fields
is discussed.Comment: REVTeX, 14 pages, no figur
Higgs algebraic symmetry of screened system in a spherical geometry
The orbits and the dynamical symmetries for the screened Coulomb potentials
and isotropic harmonic oscillators have been studied by Wu and Zeng [Z. B. Wu
and J. Y. Zeng, Phys. Rev. A 62,032509 (2000)]. We find the similar properties
in the responding systems in a spherical space, whose dynamical symmetries are
described by Higgs Algebra. There exists a conserved aphelion and perihelion
vector, which, together with angular momentum, constitute the generators of the
geometrical symmetry group at the aphelia and perihelia points .Comment: 8 pages, 1 fi
The Similarity Hypothesis in General Relativity
Self-similar models are important in general relativity and other fundamental
theories. In this paper we shall discuss the ``similarity hypothesis'', which
asserts that under a variety of physical circumstances solutions of these
theories will naturally evolve to a self-similar form. We will find there is
good evidence for this in the context of both spatially homogenous and
inhomogeneous cosmological models, although in some cases the self-similar
model is only an intermediate attractor. There are also a wide variety of
situations, including critical pheneomena, in which spherically symmetric
models tend towards self-similarity. However, this does not happen in all cases
and it is it is important to understand the prerequisites for the conjecture.Comment: to be submitted to Gen. Rel. Gra
Measurement of atmospheric elemental carbon: Real-time data for Los Angeles during summer 1987
Two fundamentally different techniques for measuring atmospheric elemental carbon (EC) aerosol were compared to validate the methods. One technique, photoacoustic spectroscopy, was used to measure the optical absorption ([lambda] = 514.5 nm) of in situ atmospheric aerosol in real time. This optical absorption can be converted to EC concentration using the appropriate value of the absorption cross-section for C, so that a comparison could be made with the second technique, thermal-optical analysis of filter-collected samples, which measures the collected EC by combustion. Solvent extraction of the filter samples prior to the thermal analysis procedure was required to minimize errors due to pyrolysis of organic carbon. Excellent 1:1 correlation of atmospheric EC concentrations resulted for measurements by the photoacoustic method vs the thermal method over coincident sampling times. The linear regression gave y = 1.006 (+/-0.056) x+0.27 (+/-0.56) with R = 0.945 (n = 41), where y is the photoacoustic EC concentration and x is the thermal elemental carbon concentration, both in [mu]g m-3. This data set was collected in Los Angeles as part of the Southern California Air Quality Study (SCAQS) during the summer 1987, and supplements the results of an earlier, more limited data set taken in Dearborn, MI. The diurnal variability of EC aerosol in Los Angeles during SCAQS, as determined by photoacoustic spectroscopy, is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28903/1/0000740.pd
Acceleration of Relativistic Protons during the 20 January 2005 Flare and CME
The origin of relativistic solar protons during large flare/CME events has
not been uniquely identified so far.We perform a detailed comparative analysis
of the time profiles of relativistic protons detected by the worldwide network
of neutron monitors at Earth with electromagnetic signatures of particle
acceleration in the solar corona during the large particle event of 20 January
2005. The intensity-time profile of the relativistic protons derived from the
neutron monitor data indicates two successive peaks. We show that microwave,
hard X-ray and gamma-ray emissions display several episodes of particle
acceleration within the impulsive flare phase. The first relativistic protons
detected at Earth are accelerated together with relativistic electrons and with
protons that produce pion decay gamma-rays during the second episode. The
second peak in the relativistic proton profile at Earth is accompanied by new
signatures of particle acceleration in the corona within approximatively 1
solar radius above the photosphere, revealed by hard X-ray and microwave
emissions of low intensity, and by the renewed radio emission of electron beams
and of a coronal shock wave. We discuss the observations in terms of different
scenarios of particle acceleration in the corona.Comment: 22 pages, 5 figure
Updated meta-analysis of randomized controlled trials comparing conventional excisional haemorrhoidectomy with LigaSure for haemorrhoids
Background:\ud
To compare the surgical outcome of haemorrhoidectomy performed using LigaSure bipolar diathermy with conventional haemorrhoidectomy.\ud
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Methods:\ud
Only randomized and alternate allocated studies were included from the major electronic databases using the search terms "ligasure" and "haemorrhoids" Duration of operation, blood loss during operation, postoperative pain score, wound healing, in-hospital stay, time to return to normal activities and complications were assessed.\ud
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Results:\ud
The 11 trials contained a total of 1,046 patients; the largest study was based on 273 patients and two earlier studies were based on 34 patients. No significant gender mismatch between the groups was reported in any of the studies. The patients’ ages were similar between groups in the studies, as was disease severity. All 11 studies reported a shorter duration of the operation when using LigaSure compared to the conventional technique (p<0.001). The postoperative pain score (p=0.001) and blood loss during operation (p=0.001) were significantly reduced. After LigaSure haemorrhoidectomy wound healing (p=0.004) and the return to normal activities (p=0.001) were significantly faster than after conventional haemorrhoidectomy. However, the overall incidence of complications reported was not significantly different (p=0.056).\ud
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Conclusions:\ud
LigaSure is an effective instrument for haemorrhoidectomy which results in less blood loss, quicker wound healing and earlier return to work.\u
Real-time, in situ measurements of atmospheric optical absorption in the visible via photoacoustic spectroscopy--IV. Visibility degradation and aerosol optical properties in Los Angeles
Aerosol light absorption (babs) has been measured in real-time in Los Angeles with a validated photoacoustic technique, and its impact on visibility degradation has been examined. These measurements were collected during ten days in the summer of 1987 for the Southern California Air Quality Study (SCAQS). Aerosol babs ([lambda] = 514.5 nm) varied from an hourly average value of 7 x 10-6 m-1 in the 3-4 and 4-5 a.m. periods of 13 July to 9 x 10-5 m-1 in the 7-8 a.m. period of both 28 August and 3 September. This babs, which is due solely to elemental carbon (EC) showed a distinct diurnal pattern with low values at night, increasing around sunrise to higher values through mid-afternoon. Comparison of these data with aerosol light scattering data clearly illustrates that the contribution of aerosol light absorption to visibility degradation increases in importance under less polluted conditions. Other urban and rural studies show similar results.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28906/1/0000743.pd
Probing exotic phenomena at the interface of nuclear and particle physics with the electric dipole moments of diamagnetic atoms: A unique window to hadronic and semi-leptonic CP violation
The current status of electric dipole moments of diamagnetic atoms which
involves the synergy between atomic experiments and three different theoretical
areas -- particle, nuclear and atomic is reviewed. Various models of particle
physics that predict CP violation, which is necessary for the existence of such
electric dipole moments, are presented. These include the standard model of
particle physics and various extensions of it. Effective hadron level combined
charge conjugation (C) and parity (P) symmetry violating interactions are
derived taking into consideration different ways in which a nucleon interacts
with other nucleons as well as with electrons. Nuclear structure calculations
of the CP-odd nuclear Schiff moment are discussed using the shell model and
other theoretical approaches. Results of the calculations of atomic electric
dipole moments due to the interaction of the nuclear Schiff moment with the
electrons and the P and time-reversal (T) symmetry violating
tensor-pseudotensor electron-nucleus are elucidated using different
relativistic many-body theories. The principles of the measurement of the
electric dipole moments of diamagnetic atoms are outlined. Upper limits for the
nuclear Schiff moment and tensor-pseudotensor coupling constant are obtained
combining the results of atomic experiments and relativistic many-body
theories. The coefficients for the different sources of CP violation have been
estimated at the elementary particle level for all the diamagnetic atoms of
current experimental interest and their implications for physics beyond the
standard model is discussed. Possible improvements of the current results of
the measurements as well as quantum chromodynamics, nuclear and atomic
calculations are suggested.Comment: 46 pages, 19 tables and 16 figures. A review article accepted for
EPJ
Photonic molecules and spectral engineering
This chapter reviews the fundamental optical properties and applications of
pho-tonic molecules (PMs) - photonic structures formed by electromagnetic
coupling of two or more optical microcavities (photonic atoms). Controllable
interaction between light and matter in photonic atoms can be further modified
and en-hanced by the manipulation of their mutual coupling. Mechanical and
optical tunability of PMs not only adds new functionalities to
microcavity-based optical components but also paves the way for their use as
testbeds for the exploration of novel physical regimes in atomic physics and
quantum optics. Theoretical studies carried on for over a decade yielded novel
PM designs that make possible lowering thresholds of semiconductor microlasers,
producing directional light emission, achieving optically-induced transparency,
and enhancing sensitivity of microcavity-based bio-, stress- and
rotation-sensors. Recent advances in material science and nano-fabrication
techniques make possible the realization of optimally-tuned PMs for cavity
quantum electrodynamic experiments, classical and quantum information
processing, and sensing.Comment: A review book chapter: 29 pages, 19 figure
Wnt signalling and cancer stem cells
[Abstract] Intracellular signalling mediated by secreted Wnt proteins is essential for the establishment of cell fates and proper tissue patterning during embryo development and for the regulation of tissue homeostasis and stem cell function in adult tissues. Aberrant activation of Wnt signalling pathways has been directly linked to the genesis of different tumours. Here, the components and molecular mechanisms implicated in the transduction of Wnt signal, along with important results supporting a central role for this signalling pathway in stem cell function regulation and carcinogenesis will be briefly reviewed.Ministerio de Ciencia e Innovación; SAF2008-0060
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