45,961 research outputs found
An Underlying Theory for Gravity
A new direction to understand gravity has recently been explored by
considering classical gravity to be a derived interaction from an underlying
theory. This underlying theory would involve new degrees of freedom at a deeper
level and it would be structurally different from classical gravitation. It may
conceivably be a quantum theory or a non-quantum theory. The relation between
this underlying theory and Einstein's gravity is similar to the connection
between statistical mechanics and thermodynamics. We discuss the apparent lack
of evidence of any quantum nature of gravity in this context.Comment: Contributed paper to VIIth International Conference on Gravitation
and Cosmology, 14 - 19 December, 2011 GOA, INDIA. 4 page
Diffusion in a continuum model of self-propelled particles with alignment interaction
In this paper, we provide the corrections to the hydrodynamic
model derived by Degond and Motsch from a kinetic version of the model by
Vicsek & coauthors describing flocking biological agents. The parameter
stands for the ratio of the microscopic to the macroscopic scales.
The corrected model involves diffusion terms in both the mass and
velocity equations as well as terms which are quadratic functions of the first
order derivatives of the density and velocity. The derivation method is based
on the standard Chapman-Enskog theory, but is significantly more complex than
usual due to both the non-isotropy of the fluid and the lack of momentum
conservation
Semiflexible Chains under Tension
A functional integral formalism is used to derive the extension of a stiff
chain subject to an external force. The force versus extension curves are
calculated using a meanfield approach in which the hard constraint
is replaced by a global constraint where is the
tangent vector describing the chain and is the arc length. The theory
``quantitatively'' reproduces the experimental results for DNA that is subject
to a constant force.
We also treat the problems of a semiflexible chain in a nematic field. In the
limit of weak nematic field strength our treatment reproduces the exact results
for chain expansion parallel to the director. When the strength of nematic
field is large, a situation in which there are two equivalent minima in the
free energy, the intrinsically meanfield approach yields incorrect results for
the dependence of the persistence length on the nematic field.Comment: 14 pages, 1 figure available upon request, submitted to J. Chem. Phy
boosting in kernel regression
In this paper, we investigate the theoretical and empirical properties of
boosting with kernel regression estimates as weak learners. We show that
each step of boosting reduces the bias of the estimate by two orders of
magnitude, while it does not deteriorate the order of the variance. We
illustrate the theoretical findings by some simulated examples. Also, we
demonstrate that boosting is superior to the use of higher-order kernels,
which is a well-known method of reducing the bias of the kernel estimate.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ160 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Neutrino reactions on La and Ta via charged and neutral currents by the Quasi-particle Random Phase Approximation (QRPA)
Cosmological origins of the two heaviest odd-odd nuclei, La and
Ta, are believed to be closely related to the neutrino-process. We
investigate in detail neutrino-induced reactions on the nuclei. Charged current
(CC) reactions, BaLa and HfTa, are calculated by the standard Quasi-particle Random Phase
Approximation (QRPA) with neutron-proton pairing as well as neutron-neutron,
proton-proton pairing correlations. For neutral current (NC) reactions,
La{La} and TaTa, we generate ground and excited states of odd-even target nuclei,
La and Ta, by operating one quasi-particle to even-even nuclei,
Ba and Hf, which are assumed as the BCS ground state. Numerical
results for CC reactions are shown to be consistent with recent semi-empirical
data deduced from the Gamow-Teller strength distributions measured in the
(He, t) reaction. Results for NC reactions are estimated to be smaller by
a factor about 4 5 rather than those by CC reactions. Finally, cross
sections weighted by the incident neutrino flux in the core collapsing
supernova are presented for further applications to the network calculations
for relevant nuclear abundances
Electron Addition Spectrum in the Supersymmetric t-J Model with Inverse-Square Interaction
The electron addition spectrum A^+(k,omega) is obtained analytically for the
one-dimensional (1D) supersymmetric t-J model with 1/r^2 interaction. The
result is obtained first for a small-sized system and its validity is checked
against the numerical calculation. Then the general expression is found which
is valid for arbitrary size of the system. The thermodynamic limit of
A^+(k,omega) has a simple analytic form with contributions from one spinon, one
holon and one antiholon all of which obey fractional statistics. The upper edge
of A^+(k,omega) in the (k,omega) plane includes a delta-function peak which
reduces to that of the single-electron band in the low-density limit.Comment: 5 pages, 1 figure, accepted for publication in Phys. Rev. Let
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