535 research outputs found
Transverse Observables and Mass Determination at Hadron Colliders
I consider the two-body decay of a particle at a hadron collider into a
visible and an invisible particle, generalizing , where the masses
of the decaying particle and the invisible decay particle are, {\em a priori},
unknown. I prove that the transverse mass, when maximized over possible
kinematic configurations, can be used to determine both of the unknown masses.
I argue that the proof can be generalized to cover cases such as decays of
pair-produced superpartners to the lightest, stable superpartner at the Large
Hadron Collider.Comment: 4 pages; version published in JHE
Singularities in scalar-tensor gravity
The analysis of certain singularities in scalar-tensor gravity contained in a
recent paper is completed, and situations are pointed out in which these
singularities cannot occur.Comment: 6 pages, LaTe
Statistical anisotropy of magnetohydrodynamic turbulence
Direct numerical simulations of decaying and forced magnetohydrodynamic (MHD)
turbulence without and with mean magnetic field are analyzed by higher-order
two-point statistics. The turbulence exhibits statistical anisotropy with
respect to the direction of the local magnetic field even in the case of global
isotropy. A mean magnetic field reduces the parallel-field dynamics while in
the perpendicular direction a gradual transition towards two-dimensional MHD
turbulence is observed with inertial-range scaling of the
perpendicular energy spectrum. An intermittency model based on the Log-Poisson
approach, , is able to describe the observed
structure function scalings.Comment: 4 pages, 3 figures. To appear in Phys.Rev.
FRW Cosmology From Five Dimensional Vacuum Brans-Dicke Theory
We follow approach of induced matter theory for 5D vacuum BD, introduce
induced matter and potential in 4D hypersurfaces, and employ generalized FRW
type solution. We confine ourselves to scalar field and scale factors be
functions of the time. This makes the induced potential, by its definition,
vanishes. When the scale factor of fifth dimension and scalar field are not
constants, 5D eqs for any geometry admit a power law relation between scalar
field and scale factor of fifth dimension. Hence the procedure exhibits that 5D
vacuum FRW like eqs are equivalent, in general, to corresponding 4D vacuum ones
with the same spatial scale factor but new scalar field and coupling constant.
We show that 5D vacuum FRW like eqs or its equivalent 4D vacuum ones admit
accelerated solutions. For constant scalar field, eqs reduce to usual FRW eqs
with typical radiation dominated universe. For this situation we obtain
dynamics of scale factors for any geometry without any priori assumption. For
nonconstant scalar fields and spatially flat geometries, solutions are found to
be power law and exponential ones. We also employ weak energy condition for
induced matter, that allows negative/positive pressures. All types of solutions
fulfill WEC in different ranges. The power law solutions with negative/positive
pressures admit both decelerating and accelerating ones. Some solutions accept
shrinking extra dimension. By considering nonghost scalar fields and recent
observational measurements, solutions are more restricted. We illustrate that
accelerating power law solutions, which satisfy WEC and have nonghost fields,
are compatible with recent observations in ranges -4/3 < \omega </- -1.3151 and
1.5208 </- n < 1.9583 for dependence of fifth dimension scale factor with usual
scale factor. These ranges also fulfill condition nonghost fields in the
equivalent 4D vacuum BD eqs.Comment: 18 pages, 16 figures, 11 table
Theory and Applications of Non-Relativistic and Relativistic Turbulent Reconnection
Realistic astrophysical environments are turbulent due to the extremely high
Reynolds numbers. Therefore, the theories of reconnection intended for
describing astrophysical reconnection should not ignore the effects of
turbulence on magnetic reconnection. Turbulence is known to change the nature
of many physical processes dramatically and in this review we claim that
magnetic reconnection is not an exception. We stress that not only
astrophysical turbulence is ubiquitous, but also magnetic reconnection itself
induces turbulence. Thus turbulence must be accounted for in any realistic
astrophysical reconnection setup. We argue that due to the similarities of MHD
turbulence in relativistic and non-relativistic cases the theory of magnetic
reconnection developed for the non-relativistic case can be extended to the
relativistic case and we provide numerical simulations that support this
conjecture. We also provide quantitative comparisons of the theoretical
predictions and results of numerical experiments, including the situations when
turbulent reconnection is self-driven, i.e. the turbulence in the system is
generated by the reconnection process itself. We show how turbulent
reconnection entails the violation of magnetic flux freezing, the conclusion
that has really far reaching consequences for many realistically turbulent
astrophysical environments. In addition, we consider observational testing of
turbulent reconnection as well as numerous implications of the theory. The
former includes the Sun and solar wind reconnection, while the latter include
the process of reconnection diffusion induced by turbulent reconnection, the
acceleration of energetic particles, bursts of turbulent reconnection related
to black hole sources as well as gamma ray bursts. Finally, we explain why
turbulent reconnection cannot be explained by turbulent resistivity or derived
through the mean field approach.Comment: 66 pages, 24 figures, a chapter of the book "Magnetic Reconnection -
Concepts and Applications", editors W. Gonzalez, E. N. Parke
Onset of Superfluidity in 4He Films Adsorbed on Disordered Substrates
We have studied 4He films adsorbed in two porous glasses, aerogel and Vycor,
using high precision torsional oscillator and DC calorimetry techniques. Our
investigation focused on the onset of superfluidity at low temperatures as the
4He coverage is increased. Torsional oscillator measurements of the 4He-aerogel
system were used to determine the superfluid density of films with transition
temperatures as low as 20 mK. Heat capacity measurements of the 4He-Vycor
system probed the excitation spectrum of both non-superfluid and superfluid
films for temperatures down to 10 mK. Both sets of measurements suggest that
the critical coverage for the onset of superfluidity corresponds to a mobility
edge in the chemical potential, so that the onset transition is the bosonic
analog of a superconductor-insulator transition. The superfluid density
measurements, however, are not in agreement with the scaling theory of an onset
transition from a gapless, Bose glass phase to a superfluid. The heat capacity
measurements show that the non-superfluid phase is better characterized as an
insulator with a gap.Comment: 15 pages (RevTex), 21 figures (postscript
Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification
Unmodified, Na-modified, and Sr-modified castings of Al-7 pct Si and Al-12.5 pct Si alloys were cast in molds in which it was possible to create different cooling conditions. It is shown how solidification influences the distribution of porosity at the surface and the center of the castings as a function of modification and Si content in sand- and chill-cast samples. Eutectic modification, Si content, and cooling conditions have a great impact on the distribution of porosity. Unmodified and Na-modified castings are more easily fed with porosity tending to congregate near the centerline of the casting, while Sr-modified castings solidify in a mushy manner that creates a more homogeneous distribution of porosity in the casting. The amount of porosity was highest in the Sr-modified alloys, lower in the Na-modified alloys, and lowest in the unmodified alloys. The size of the porosity-free layer and the effectiveness of the feeders were greater in the castings made with the steel chills due to the increased thermal gradients and consequent increase in the directionality of solidification
New hadrons as ultra-high energy cosmic rays
Ultra-high energy cosmic ray (UHECR) protons produced by uniformly
distributed astrophysical sources contradict the energy spectrum measured by
both the AGASA and HiRes experiments, assuming the small scale clustering of
UHECR observed by AGASA is caused by point-like sources. In that case, the
small number of sources leads to a sharp exponential cutoff at the energy
E<10^{20} eV in the UHECR spectrum. New hadrons with mass 1.5-3 GeV can solve
this cutoff problem. For the first time we discuss the production of such
hadrons in proton collisions with infrared/optical photons in astrophysical
sources. This production mechanism, in contrast to proton-proton collisions,
requires the acceleration of protons only to energies E<10^{21} eV. The diffuse
gamma-ray and neutrino fluxes in this model obey all existing experimental
limits. We predict large UHE neutrino fluxes well above the sensitivity of the
next generation of high-energy neutrino experiments. As an example we study
hadrons containing a light bottom squark. These models can be tested by
accelerator experiments, UHECR observatories and neutrino telescopes.Comment: 17 pages, revtex style; v2: shortened, as to appear in PR
On Physical Equivalence between Nonlinear Gravity Theories
We argue that in a nonlinear gravity theory, which according to well-known
results is dynamically equivalent to a self-gravitating scalar field in General
Relativity, the true physical variables are exactly those which describe the
equivalent general-relativistic model (these variables are known as Einstein
frame). Whenever such variables cannot be defined, there are strong indications
that the original theory is unphysical. We explicitly show how to map, in the
presence of matter, the Jordan frame to the Einstein one and backwards. We
study energetics for asymptotically flat solutions. This is based on the
second-order dynamics obtained, without changing the metric, by the use of a
Helmholtz Lagrangian. We prove for a large class of these Lagrangians that the
ADM energy is positive for solutions close to flat space. The proof of this
Positive Energy Theorem relies on the existence of the Einstein frame, since in
the (Helmholtz--)Jordan frame the Dominant Energy Condition does not hold and
the field variables are unrelated to the total energy of the system.Comment: 37 pp., TO-JLL-P 3/93 Dec 199
Solar Wind Turbulence and the Role of Ion Instabilities
International audienc
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