3,514 research outputs found
Hypervelocity particle capture: Some considerations regarding suitable target media
Hypervelocity particles colliding with passive capture media will be traversed by shock waves; depending on the stress amplitude, the particle may remain solid or it may melt or vaporize. Any capture mechanism considered for cosmic dust collection in low Earth-orbit must be designed such that sample alteration and hence loss of scientific information is minimized. Capture of pristine particles is fundamentally difficult, because the specific heat of melting and even vaporization is exceeded upon impact at typical, geocentric encounter velocities. From the results of calculated and observed melting behaviors it is concluded that shock stresses in excess of 50 GPA should be avoided during hypervelocity particle capture on board Space Station and that stresses 20 GPa, even at 15 km/s collision velocities, should constitute desirable instrument design goals. Some principal characteristics of the capture medium that may satisfy these requirements are identified
Impact cratering in reduced-gravity environments: Early experiments on the NASA KC-135 aircraft
Impact experimentation on the NASA KC-135 Reduced-Gravity Aircraft was shown to be possible, practical, and of considerable potential use in examining the role of gravity on various impact phenomena. With a minimal facility, crater dimensional and growth-times were measured, and have demonstrated both agreement and disagreement with predictions. A larger facility with vacuum capability and a high-velocity gun would permit a much wider range of experimentation
The Johnson Space Center Experimental Impact Lab: Contributions Toward Understanding the Evolution of the Solar System
Impact is the most common and only weathering phenomenon affecting all the planetary bodies (e.g., planets, satellites, asteroids, comets, etc.) in the solar system. NASA Johnson Space Center s Experimental Impact Laboratory (EIL) includes three accelerators that are used in support of research into the effects of impact on the formation and evolution of the solar system. They permit researchers to study a wide variety of phenomena associated with high-velocity impacts into a wide range of geologic targets and materials relevant to astrobiological studies. By studying these processes, researchers can investigate the histories and evolution of planetary bodies and the solar system as a whole. While the majority of research conducted in the EIL addresses questions involving planetary impacts, work involving spacecraft components has been performed on occasion. An example of this is the aerogel collector material flown on the Stardust spacecraft that traveled to Comet Wild-2. This capture medium was tested and flight qualified using the 5 mm Light-Gas Gun located in the EIL
Neutrino masses in the Lepton Number Violating MSSM
We consider the most general supersymmetric model with minimal particle
content and an additional discrete Z_3 symmetry (instead of R-parity), which
allows lepton number violating terms and results in non-zero Majorana neutrino
masses. We investigate whether the currently measured values for lepton masses
and mixing can be reproduced. We set up a framework in which Lagrangian
parameters can be initialised without recourse to assumptions concerning
trilinear or bilinear superpotential terms, CP-conservation or
intergenerational mixing and analyse in detail the one loop corrections to the
neutrino masses. We present scenarios in which the experimental data are
reproduced and show the effect varying lepton number violating couplings has on
the predicted atmospheric and solar mass^2 differences. We find that with
bilinear lepton number violating couplings in the superpotential of the order 1
MeV the atmospheric mass scale can be reproduced. Certain trilinear
superpotential couplings, usually, of the order of the electron Yukawa coupling
can give rise to either atmospheric or solar mass scales and bilinear
supersymmetry breaking terms of the order 0.1 GeV^2 can set the solar mass
scale. Further details of our calculation, Lagrangian, Feynman rules and
relevant generic loop diagrams, are presented in three Appendices.Comment: 48 pages, 7 figures, v2 references added, typos corrected, published
versio
Magnetic permeability of near-critical 3d abelian Higgs model and duality
The three-dimensional abelian Higgs model has been argued to be dual to a
scalar field theory with a global U(1) symmetry. We show that this duality,
together with the scaling and universality hypotheses, implies a scaling law
for the magnetic permeablity chi_m near the line of second order phase
transition: chi_m ~ t^nu, where t is the deviation from the critical line and
nu ~ 0.67 is a critical exponent of the O(2) universality class. We also show
that exactly on the critical lines, the dependence of magnetic induction on
external magnetic field is quadratic, with a proportionality coefficient
depending only on the gauge coupling. These predictions provide a way for
testing the duality conjecture on the lattice in the Coulomb phase and at the
phase transion.Comment: 11 pages; updated references and small changes, published versio
Higgs mediated Double Flavor Violating top decays in Effective Theories
The possibility of detecting double flavor violating top quark transitions at
future colliders is explored in a model-independent manner using the effective
Lagrangian approach through the () decays. A
Yukawa sector that contemplates invariants of up to
dimension six is proposed and used to derive the most general flavor violating
and CP violating and vertices of renormalizable type.
Low-energy data, on high precision measurements, and experimental limits are
used to constraint the and vertices and then used to
predict the branching ratios for the decays. It is found
that this branching ratios may be of the order of , for a
relative light Higgs boson with mass lower than , which could be more
important than those typical values found in theories beyond the standard model
for the rare top quark decays () or . %% LHC experiments, by using a total integrated luminosity of of data, will be able to rule out, at 95% C.L., DFV top quark
decays up to a Higgs mass of 155 GeV/ or discover such a process up to a
Higgs mass of 147 GeV/.Comment: 24 pages, 11 figure
Quantum group symmetry of the Quantum Hall effect on the non-flat surfaces
After showing that the magnetic translation operators are not the symmetries
of the QHE on non-flat surfaces , we show that there exist another set of
operators which leads to the quantum group symmetries for some of these
surfaces . As a first example we show that the symmetry of the QHE on
sphere leads to algebra in the equator . We explain this result by a
contraction of . Secondly , with the help of the symmetry operators of
QHE on the Pioncare upper half plane , we will show that the ground state wave
functions form a representation of the algebra .Comment: 8 pages,latex,no figur
Stripe formation in high-Tc superconductors
The non-uniform ground state of the two-dimensional three-band Hubbard model
for the oxide high-Tc superconductors is investigated using a variational Monte
Carlo method. We examine the effect produced by holes doped into the
antiferromagnetic (AF) background in the underdoped region. It is shown that
the AF state with spin modulations and stripes is stabilized du to holes
travelling in the CuO plane. The structures of the modulated AF spins are
dependent upon the parameters used in the model. The effect of the boundary
conditions is reduced for larger systems. We show that there is a region where
incommensurability is proportional to the hole density. Our results give a
consistent description of stripes observed by the neutron- scattering
experiments based on the three-band model for CuO plane.Comment: 8 pages, 9 figure
Dynamical Symmetry Breaking in Planar QED
We investigate (2+1)-dimensional QED coupled with Dirac fermions both at zero
and finite temperature. We discuss in details two-components (P-odd) and
four-components (P-even) fermion fields. We focus on P-odd and P-even Dirac
fermions in presence of an external constant magnetic field. In the spontaneous
generation of the magnetic condensate survives even at infinite temperature. We
also discuss the spontaneous generation of fermion mass in presence of an
external magnetic field.Comment: 34 pages, 8 postscript figures, final version to appear on J. Phys.
Critical Exponents of the Three Dimensional Random Field Ising Model
The phase transition of the three--dimensional random field Ising model with
a discrete () field distribution is investigated by extensive Monte
Carlo simulations. Values of the critical exponents for the correlation length,
specific heat, susceptibility, disconnected susceptibility and magnetization
are determined simultaneously via finite size scaling. While the exponents for
the magnetization and disconnected susceptibility are consistent with a first
order transition, the specific heat appears to saturate indicating no latent
heat. Sample to sample fluctuations of the susceptibilty are consistent with
the droplet picture for the transition.Comment: Revtex, 10 pages + 4 figures included as Latex files and 1 in
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