18,337 research outputs found
Sensitivity of Astrophysical Observations to Gravity-Induced Wave Dispersion in Vacuo
We discuss possible signatures of quantum gravity for the propagation of
light, including an energy-dependent velocity (refractive index), dispersion in
velocity at a given energy, and birefringence. We also compare the
sensitivities of different astrophysical observations, including BATSE data on
GRB 920229, BeppoSAX data on GRB 980425, the possible HEGRA observation of GRB
920925c, and Whipple observations of the active galaxy Mrk 421. Finally, we
discuss the prospective sensitivities of AMS and GLAST.Comment: LaTex, 3 page
CMAC Trained Optimum Mid course Guidance for Tactical Flight Vehicle
This paper discusses design and validation of neural network based mid-course guidance law of a surface to air flight vehicle. In present study, initially different optimal trajectories have been generated off-line of different pursuer-evader engagements by ensuring minimum flight time, maximum terminal velocity and favorable handing over conditions for seeker based terminal guidance. These optimal trajectories have been evolved by nonlinear programming based direct method of optimisation. The kinematic information of both pursuer and evader, generated based on these trajectories have been used to train cerebellar model articulate controller (CMAC) neural network. Later for a given engagement scenario an on-line near optimal mid-course guidance law has been evolved based on output of trained network. Training has been carried out by CMAC type supervisory neural network. The tested engagement condition is within input/output training space of neural network. Seeker based homing guidance has been used for terminal phase. Complete methodology has been validated along pitch plane of pursuer-evader engagement. During mid-course phase, the guidance demand has been tracked by attitude hold autopilot and during terminal phase, the guidance demanded lateral acceleration has been tracked by acceleration autopilot. System robustness has been studied in presence of plant parameter variations and sensor noise under Monte Carlo Platform
Cornwall-Jackiw-Tomboulis effective potential for canonical noncommutative field theories
We apply the Cornwall-Jackiw-Tomboulis (CJT) formalism to the scalar theory in canonical-noncommutative spacetime. We construct the CJT
effective potential and the gap equation for general values of the
noncommutative parameter . We observe that under the
hypothesis of translational invariance, which is assumed in the effective
potential construction, differently from the commutative case
(), the renormalizability of the gap equation is
incompatible with the renormalizability of the effective potential. We argue
that our result, is consistent with previous studies suggesting that a uniform
ordered phase would be inconsistent with the infrared structure of canonical
noncommutative theories.Comment: 15 pages, LaTe
Implications of observed neutrinoless double beta decay
Recently a positive indication of the neutrinoless double beta decay has been
announced. We study the implications of this result taking into consideration
earlier results on atmospheric neutrinos and solar neutrinos. We also include
in our discussions the recent results from SNO and K2K. We point out that on
the confidence level given for the double beta signal, the neutrino mass
matrices are now highly constrained. All models predicting Dirac masses are
ruled out and leptogenesis becomes a natural choice. Only the degenerate and
the inverted hierarchical solutions are allowed for the three generation
Majorana neutrinos. In both these cases we find that the radiative corrections
destabilize the solutions and the LOW, VO and Just So solutions of the solar
neutrinos are ruled out. For the four generation case only the inverted
hierarchical scenario is allowed.Comment: 16 pages, 2 postscript figure
The Complex Time WKB Approximation And Particle Production
The complex time WKB (CWKB) approximation has been an effective technique to
understand particle production in curved as well as in flat spacetime. Earlier
we obtained the standard results on particle production in time dependent gauge
in various curved spacetime. In the present work we generalize the technique of
CWKB to the equivalent problems in space dependent gauge. Using CWKB, we first
obtain the gauge invariant result for particle production in Minkowski
spacetime in strong electric field. We then carry out particle production in
de-Sitter spacetime in space dependent gauge and obtain the same result that we
obtained earlier in time dependent gauge. The results obtained for de-Sitter
spacetime has a obvious extension to particle production in black hole
spacetime. It is found that the origin of Planckian spectrum is due to repeated
reflections between the turning points. As mentioned earlier, it is now
explicitly shown that particle production is accompanied by rotation of
currents.Comment: 12 pages, Revte
An experimental study of Primary Cosmic Rays at the knee energy region by observation of Extensive Air Showers (EAS)
Simultaneous measurements have been made of the radial (lateral) electron density distribution and the radial muon density distribution at various measured muon energies in the range 2.5–100 GeV in vertically incident EAS in the
size range 3.153104 –1.793106 (primary energy range 2.431014 –8.331015 eV) particles detected near sea level. The characteristics of these radial distributions in terms of the measured shower parameters have been determined and used to draw conclusions about the average nuclear mass of the primaries of these EAS
Predictions for the Cosmogenic Neutrino Flux in Light of New Data from the Pierre Auger Observatory
The Pierre Auger Observatory (PAO) has measured the spectrum and composition
of the ultrahigh energy cosmic rays with unprecedented precision. We use these
measurements to constrain their spectrum and composition as injected from their
sources and, in turn, use these results to estimate the spectrum of cosmogenic
neutrinos generated in their propagation through intergalactic space. We find
that the PAO measurements can be well fit if the injected cosmic rays consist
entirely of nuclei with masses in the intermediate (C, N, O) to heavy (Fe, Si)
range. A mixture of protons and heavier species is also acceptable but (on the
basis of existing hadronic interaction models) injection of pure light nuclei
(p, He) results in unacceptable fits to the new elongation rate data. The
expected spectrum of cosmogenic neutrinos can vary considerably, depending on
the precise spectrum and chemical composition injected from the cosmic ray
sources. In the models where heavy nuclei dominate the cosmic ray spectrum and
few dissociated protons exceed GZK energies, the cosmogenic neutrino flux can
be suppressed by up to two orders of magnitude relative to the all-proton
prediction, making its detection beyond the reach of current and planned
neutrino telescopes. Other models consistent with the data, however, are
proton-dominated with only a small (1-10%) admixture of heavy nuclei and
predict an associated cosmogenic flux within the reach of upcoming experiments.
Thus a detection or non-detection of cosmogenic neutrinos can assist in
discriminating between these possibilities.Comment: 10 pages, 7 figure
Frequency-dependent spin susceptibility in the two-dimensional Hubbard model
A Quantum Monte Carlo calculation of dynamical spin susceptibility in the
half-filled 2D Hubbard model is presented for temperature and an
intermediate on-site repulsion . Using the singular value decomposition
technique we succeed in analytically continuing the Matsubara Green's function
into the real frequency domain and in deriving the spectral representation for
the longitudinal and transverse spin susceptibility. The simulation results,
while contradicting the random-phase approximation prediction of
antiferromagnetic long-range order at this temperature, are in agreement with
an extension of a self-consistent renormalization approach of Moriya. The
static susceptibility calculated using this technique is qualitatively
consistent with the simulation results.Comment: 4 pages, Revtex, encoded figs.uu file with 3 figures enclose
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