33,686 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
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
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
Collective Dynamics of Random Polyampholytes
We consider the Langevin dynamics of a semi-dilute system of chains which are
random polyampholytes of average monomer charge and with a fluctuations in
this charge of the size and with freely floating counter-ions in the
surrounding. We cast the dynamics into the functional integral formalism and
average over the quenched charge distribution in order to compute the dynamic
structure factor and the effective collective potential matrix. The results are
given for small charge fluctuations. In the limit of finite we then find
that the scattering approaches the limit of polyelectrolyte solutions.Comment: 13 pages including 6 figures, submitted J. Chem. Phy
Characteristics of cloud-to-ground lightning activity over Seoul, South Korea in relation to an urban effect
Cloud-to-ground (CG) lightning flash data collected by the lightning detection network installed at the Korean Meteorological Administration (KMA) have been used to study the urban effect on lightning activity over and around Seoul, the largest metropolitan city of South Korea, for the period of 1989&ndash;1999. Negative and positive flash density and the percentage of positive flashes have been calculated. Calculation reveals that an enhancement of approximately 60% and 42% are observed, respectively, for negative and positive flash density over and downwind of the city. The percentage decrease of positive flashes occurs over and downwind of Seoul and the amount of decrease is nearly 20% compared to upwind values. The results are in good agreement with those obtained by Steiger et al. (2002) and Westcott (1995). CG lightning activities have also been considered in relation to annual averages of PM<sub>10</sub> (particulate matter with an aerodynamic diameter smaller than 10 μm) and sulphur dioxide (SO<sub>2</sub>) concentrations. Interesting results are found, indicating that the higher concentration of SO<sub>2</sub> contributes to the enhancement of CG lightning flashes. On the other hand, the contribution from PM<sub>10</sub> concentration has not appeared in this study to be as significant as SO<sub>2</sub> in the enhancement of CG lightning flashes. Correlation coefficients of 0.33 and 0.64 are found between the change in CG lightning flashes and the PM<sub>10</sub> and SO<sub>2</sub>, respectively, for upwind to downwind areas, suggesting a significant influence of the increased concentration of SO<sub>2</sub> on the enhancement of CG flashes
Size dependent line broadening in the emission spectra of single GaAs quantum dots: Impact of surface charges on spectral diffusion
Making use of droplet epitaxy, we systematically controlled the height of
self-assembled GaAs quantum dots by more than one order of magnitude. The
photoluminescence spectra of single quantum dots revealed the strong dependence
of the spectral linewidth on the dot height. Tall dots with a height of ~30 nm
showed broad spectral peaks with an average width as large as ~5 meV, but
shallow dots with a height of ~2 nm showed resolution-limited spectral lines
(<120 micro eV). The measured height dependence of the linewidths is in good
agreement with Stark coefficients calculated for the experimental shape
variation. We attribute the microscopic source of fluctuating electric fields
to the random motion of surface charges at the vacuum-semiconductor interface.
Our results offer guidelines for creating frequency-locked photon sources,
which will serve as key devices for long-distance quantum key distribution.Comment: 6 pages, 6 figures; updated figs and their description
Combinatorial interpretation of Haldane-Wu fractional exclusion statistics
Assuming that the maximal allowed number of identical particles in state is
an integer parameter, q, we derive the statistical weight and analyze the
associated equation which defines the statistical distribution. The derived
distribution covers Fermi-Dirac and Bose-Einstein ones in the particular cases
q = 1 and q -> infinity (n_i/q -> 1), respectively. We show that the derived
statistical weight provides a natural combinatorial interpretation of
Haldane-Wu fractional exclusion statistics, and present exact solutions of the
distribution equation.Comment: 8 pages, 2 eps-figure
Performance of a prototype active veto system using liquid scintillator for a dark matter search experiment
We report the performance of an active veto system using a liquid
scintillator with NaI(Tl) crystals for use in a dark matter search experiment.
When a NaI(Tl) crystal is immersed in the prototype detector, the detector tags
48% of the internal K-40 background in the 0-10 keV energy region. We also
determined the tagging efficiency for events at 6-20 keV as 26.5 +/- 1.7% of
the total events, which corresponds to 0.76 +/- 0.04 events/keV/kg/day.
According to a simulation, approximately 60% of the background events from U,
Th, and K radioisotopes in photomultiplier tubes are tagged at energies of 0-10
keV. Full shielding with a 40-cm-thick liquid scintillator can increase the
tagging efficiency for both the internal K-40 and external background to
approximately 80%.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section
- …