132 research outputs found
The use of incorrectly posed inverse problems and catastrophe theory in acoustoplasmic studies
If the discharge current into a plasma contains direct and variable components, the plasma develops wavelike acoustic instabilities and eventually becomes an acoustoplasmа. Such instabilities lead to bistability, multistability, and hysteresis phenomena of the current-voltage characteristics, causing abrupt changes in the state of the plasma medium. These changes can be imagined as phase transitions and described using catastrophe theory. In the present study, the experimental plasma data are approximated by the equations of catastrophes. After reducing the catastrophe equation to canonical form, the points of possible phase transitions are determined. The phase transition coordinates are then converted to coordinates in the experimental system by inverse trans-formations. In this way, we determine the points of possible phase transitions in a real experiment. Finally, the parameter changes in an acoustoplasma discharge are obtained by solving incorrectly posed inverse problems. The inverse problem of the experi-mental data is solved at each current time. Within the neighborhoods of singular points, the incorrectly posed inverse problems are solved by the theory of catastrophes. The proposed methods are applicable to various fields of science and technology.Если ток разряда в плазме содержит прямые и переменные компоненты, плазма развивает волнообразную акустическую нестабильность и в результате становится акустоплазмой. Такие неустойчивости приводят к явлениям бистабильности, мультистабильности и гистерезиса вольт-амперных характеристик, вызывая резкие изменения состояния плазменной среды. Эти изменения могут быть представлены как фазовые переходы. В настоящей работе экспериментальные данные плазмы аппроксимируются уравнениями катастроф. После приведения в соответствие уравнения катастрофы к канонической форме соответствуют точке фазовых переходов. Координаты фазового перехода преобразуются в координаты экспериментальной системы с помощью преобразований.В этом случае мы определяем точки Наконец, изменения параметров в акустической плазме получены с помощью решений. Обратная задача экспериментальных данных решается в каждом текущем моменте времени. В окрестностях особых точек неверно поставлены обратные задачи решаются теорией катастрофы. Предлагаемые методы применимы к различным областям науки и техники
Physics Opportunities with the 12 GeV Upgrade at Jefferson Lab
This white paper summarizes the scientific opportunities for utilization of
the upgraded 12 GeV Continuous Electron Beam Accelerator Facility (CEBAF) and
associated experimental equipment at Jefferson Lab. It is based on the 52
proposals recommended for approval by the Jefferson Lab Program Advisory
Committee.The upgraded facility will enable a new experimental program with
substantial discovery potential to address important topics in nuclear,
hadronic, and electroweak physics.Comment: 64 page
Measurements of the Electric Form Factor of the Neutron up to Q2=3.4 GeV2 using the Reaction He3(e,e'n)pp
The electric form factor of the neutron was determined from studies of the
reaction He3(e,e'n)pp in quasi-elastic kinematics in Hall A at Jefferson Lab.
Longitudinally polarized electrons were scattered off a polarized target in
which the nuclear polarization was oriented perpendicular to the momentum
transfer. The scattered electrons were detected in a magnetic spectrometer in
coincidence with neutrons that were registered in a large-solid-angle detector.
More than doubling the Q2-range over which it is known, we find GEn = 0.0225
+/- 0.0017 (stat) +/- 0.0024 (syst), 0.0200 +/- 0.0023 +/- 0.0018, and 0.0142
+/- 0.0019 +/- 0.0013 for Q2 = 1.72, 2.48, and 3.41 GeV2, respectively.Comment: submitted to PR
Measurement of the Neutron Radius of 208Pb Through Parity-Violation in Electron Scattering
We report the first measurement of the parity-violating asymmetry A_PV in the
elastic scattering of polarized electrons from 208Pb. A_PV is sensitive to the
radius of the neutron distribution (Rn). The result A_PV = 0.656 \pm 0.060
(stat) \pm 0.014 (syst) ppm corresponds to a difference between the radii of
the neutron and proton distributions Rn - Rp = 0.33 +0.16 -0.18 fm and provides
the first electroweak observation of the neutron skin which is expected in a
heavy, neutron-rich nucleus.Comment: 6 pages, 1 figur
Secluded Dark Matter Coupled to a Hidden CFT
Models of secluded dark matter offer a variant on the standard WIMP picture
and can modify our expectations for hidden sector phenomenology and detection.
In this work we extend a minimal model of secluded dark matter, comprised of a
U(1)'-charged dark matter candidate, to include a confining hidden-sector CFT.
This provides a technically natural explanation for the hierarchically small
mediator-scale, with hidden-sector confinement generating m_{gamma'}>0.
Furthermore, the thermal history of the universe can differ markedly from the
WIMP picture due to (i) new annihilation channels, (ii) a (potentially) large
number of hidden-sector degrees of freedom, and (iii) a hidden-sector phase
transition at temperatures T << M_{dm} after freeze out. The mediator allows
both the dark matter and the Standard Model to communicate with the CFT, thus
modifying the low-energy phenomenology and cosmic-ray signals from the secluded
sector.Comment: ~50p, 8 figs; v2 JHEP versio
Accurate and Rapid Estimation of Phosphene Thresholds (REPT)
To calibrate the intensity of transcranial magnetic stimulation (TMS) at the occipital pole, the phosphene threshold is used as a measure of cortical excitability. The phosphene threshold (PT) refers to the intensity of magnetic stimulation that induces illusory flashes of light (phosphenes) on a proportion of trials. The existing PT estimation procedures lack the accuracy and mathematical rigour of modern threshold estimation methods. We present an improved and automatic procedure for estimating the PT which is based on the well-established Ψ Bayesian adaptive staircase approach. To validate the new procedure, we compared it with another commonly used procedure for estimating the PT. We found that our procedure is more accurate, reliable, and rapid when compared with an existing PT measurement procedure. The new procedure is implemented in Matlab and works automatically with the Magstim Rapid2 stimulator using a convenient graphical user interface. The Matlab program is freely available for download
LHC Signatures of a Minimal Supersymmetric Hidden Valley
We investigate the LHC signals of a minimal supersymmetric hidden valley. Our
theory consists of the supersymmetric Standard Model along with a light hidden
U(1)_x gauge multiplet and a pair of hidden chiral superfields that
spontaneously break the new Abelian gauge symmetry near a GeV. The visible and
hidden sectors interact exclusively through supersymmetric gauge kinetic
mixing. We perform a thorough examination of the hidden decay cascades
initiated by the lightest Standard Model superpartner and we study the range of
LHC signals they can produce. In particular, we find parameter regions that
give rise to missing energy, single and multiple lepton jets, and displaced
vertices. Given the simplicity of the underlying theory and the broad range of
collider signals it can produce, we propose that this model is a useful
benchmark for LHC studies of (supersymmetric) hidden valleys.Comment: 45 pages, 15 figures; typos corrected but conclusions unchange
Atoms in Flight and the Remarkable Connections between Atomic and Hadronic Physics
Atomic physics and hadron physics are both based on Yang Mills gauge theory;
in fact, quantum electrodynamics can be regarded as the zero-color limit of
quantum chromodynamics. I review a number of areas where the techniques of
atomic physics provide important insight into the theory of hadrons in QCD. For
example, the Dirac-Coulomb equation, which predicts the spectroscopy and
structure of hydrogenic atoms, has an analog in hadron physics in the form of
light-front relativistic equations of motion which give a remarkable first
approximation to the spectroscopy, dynamics, and structure of light hadrons.
The renormalization scale for the running coupling, which is unambiguously set
in QED, leads to a method for setting the renormalization scale in QCD. The
production of atoms in flight provides a method for computing the formation of
hadrons at the amplitude level. Conversely, many techniques which have been
developed for hadron physics, such as scaling laws, evolution equations, and
light-front quantization have equal utility for atomic physics, especially in
the relativistic domain. I also present a new perspective for understanding the
contributions to the cosmological constant from QED and QCD.Comment: Presented at EXA2011, the International Conference on Exotic Atoms
and Related Topics, Vienna, September 5-9, 201
Search for a vector gauge boson in phi meson decays with the KLOE detector
The existence of a light dark force mediator has been tested with the KLOE
detector at DAPHNE. This particle, called U, is searched for using the decay
chain phi-->etaU, eta-->pi+pi-pi0, U-->e+e-. No evidence is found in 1.5 fb-1
of data. The resulting exclusion plot covers the mass range 5<M_U<470 MeV,
setting an upper limit on the ratio between the U boson coupling constant and
the fine structure constant, alpha'/alpha, of <= 2x10^-5 at 90% C.L. for
50<M_U<420 MeV.Comment: 13 pages, 8 figure
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