374 research outputs found
Search for Small Trans-Neptunian Objects by the TAOS Project
The Taiwan-America Occultation Survey (TAOS) aims to determine the number of
small icy bodies in the outer reach of the Solar System by means of stellar
occultation. An array of 4 robotic small (D=0.5 m), wide-field (f/1.9)
telescopes have been installed at Lulin Observatory in Taiwan to simultaneously
monitor some thousand of stars for such rare occultation events. Because a
typical occultation event by a TNO a few km across will last for only a
fraction of a second, fast photometry is necessary. A special CCD readout
scheme has been devised to allow for stellar photometry taken a few times per
second. Effective analysis pipelines have been developed to process stellar
light curves and to correlate any possible flux changes among all telescopes. A
few billion photometric measurements have been collected since the routine
survey began in early 2005. Our preliminary result of a very low detection rate
suggests a deficit of small TNOs down to a few km size, consistent with the
extrapolation of some recent studies of larger (30--100 km) TNOs.Comment: 4 pages, 3 figures, IAU Symposium 23
Ferromagnetism and giant magnetoresistance in the rare earth fullerides Eu6-xSrxC60
We have studied crystal structure, magnetism and electric transport
properties of a europium fulleride Eu6C60 and its Sr-substituted compounds,
Eu6-xSrxC60. They have a bcc structure, which is an isostructure of other M6C60
(M represents an alkali atom or an alkaline earth atom). Magnetic measurements
revealed that magnetic moment is ascribed to the divalent europium atom with S
= 7/2 spin, and a ferromagnetic transition was observed at TC = 10 - 14 K. In
Eu6C60, we also confirm the ferromagnetic transition by heat capacity
measurement. The striking feature in Eu6-xSrxC60} is very large negative
magnetoresistance at low temperature; the resistivity ratio \rho(H = 9
T)/\rho(H = 0 T) reaches almost 10^{-3} at 1 K in Eu6C60. Such large
magnetoresistance is the manifestation of a strong pi-f interaction between
conduction carriers on C60 and 4f electrons of Eu.Comment: 5 pages, 4 figure
Multiwavelength Study on Solar and Interplanetary Origins of the Strongest Geomagnetic Storm of Solar Cycle 23
We study the solar sources of an intense geomagnetic storm of solar cycle 23
that occurred on 20 November 2003, based on ground- and space-based
multiwavelength observations. The coronal mass ejections (CMEs) responsible for
the above geomagnetic storm originated from the super-active region NOAA 10501.
We investigate the H-alpha observations of the flare events made with a 15 cm
solar tower telescope at ARIES, Nainital, India. The propagation
characteristics of the CMEs have been derived from the three-dimensional images
of the solar wind (i.e., density and speed) obtained from the interplanetary
scintillation data, supplemented with other ground- and space-based
measurements. The TRACE, SXI and H-alpha observations revealed two successive
ejections (of speeds ~350 and ~100 km/s), originating from the same filament
channel, which were associated with two high speed CMEs (~1223 and ~1660 km/s,
respectively). These two ejections generated propagating fast shock waves
(i.e., fast drifting type II radio bursts) in the corona. The interaction of
these CMEs along the Sun-Earth line has led to the severity of the storm.
According to our investigation, the interplanetary medium consisted of two
merging magnetic clouds (MCs) that preserved their identity during their
propagation. These magnetic clouds made the interplanetary magnetic field (IMF)
southward for a long time, which reconnected with the geomagnetic field,
resulting the super-storm (Dst_peak=-472 nT) on the Earth.Comment: 24 pages, 16 figures, Accepted for publication in Solar Physic
Primordial He4 Abundance Constrains the Possible Time Variation of the Higgs Vacuum Expectation Value
We constrain the possible time variation of the Higgs vacuum expectation
value () by recent results on the primordial abundance (). For
that, we use an analytic approach which enables us to take important issues
into consideration, that have been ignored by previous works, like the
-dependence of the relevant cross sections of deuterium production and
photodisintegration, including the full Klein- Nishina cross section.
Furthermore, we take a non-equilibrium Ansatz for the freeze-out concentration
of neutrons and protons and incorporate the latest results on the neutron
decay. Finally, we approximate the key-parameters of the primordial
production (the mean lifetime of the free neutron and the binding energy of the
deuteron) by terms of (where denotes the present theoretical
estimate). Eventually, we derive the relation and the most stringent
limit on a possible time variation of is given by: .Comment: Accepted for publication in IJT
On Vanishing Theorems For Vector Bundle Valued p-Forms And Their Applications
Let be a strictly increasing function
with . We unify the concepts of -harmonic maps, minimal
hypersurfaces, maximal spacelike hypersurfaces, and Yang-Mills Fields, and
introduce -Yang-Mills fields, -degree, -lower degree, and generalized
Yang-Mills-Born-Infeld fields (with the plus sign or with the minus sign) on
manifolds. When and
the -Yang-Mills field becomes an ordinary Yang-Mills field,
-Yang-Mills field, a generalized Yang-Mills-Born-Infeld field with the plus
sign, and a generalized Yang-Mills-Born-Infeld field with the minus sign on a
manifold respectively. We also introduce the energy functional (resp.
-Yang-Mills functional) and derive the first variational formula of the
energy functional (resp. -Yang-Mills functional) with
applications. In a more general frame, we use a unified method to study the
stress-energy tensors that arise from calculating the rate of change of various
functionals when the metric of the domain or base manifold is changed. These
stress-energy tensors, linked to -conservation laws yield monotonicity
formulae. A "macroscopic" version of these monotonicity inequalities enables us
to derive some Liouville type results and vanishing theorems for forms with
values in vector bundles, and to investigate constant Dirichlet boundary value
problems for 1-forms. In particular, we obtain Liouville theorems for
harmonic maps (e.g. -harmonic maps), and Yang-Mills fields (e.g.
generalized Yang-Mills-Born-Infeld fields on manifolds). We also obtain
generalized Chern type results for constant mean curvature type equations for
forms on and on manifolds with the global doubling property
by a different approach. The case and is due to Chern.Comment: 1. This is a revised version with several new sections and an
appendix that will appear in Communications in Mathematical Physics. 2. A
"microscopic" approach to some of these monotonicity formulae leads to
celebrated blow-up techniques and regularity theory in geometric measure
theory. 3. Our unique solution of the Dirichlet problems generalizes the work
of Karcher and Wood on harmonic map
D-Brane Propagation in Two-Dimensional Black Hole Geometries
We study propagation of D0-brane in two-dimensional Lorentzian black hole
backgrounds by the method of boundary conformal field theory of SL(2,R)/U(1)
supercoset at level k. Typically, such backgrounds arise as near-horizon
geometries of k coincident non-extremal NS5-branes, where 1/k measures
curvature of the backgrounds in string unit and hence size of string worldsheet
effects. At classical level, string worldsheet effects are suppressed and
D0-brane propagation in the Lorentzian black hole geometry is simply given by
the Wick rotation of D1-brane contour in the Euclidean black hole geometry.
Taking account of string worldsheet effects, boundary state of the Lorentzian
D0-brane is formally constructible via Wick rotation from that of the Euclidean
D1-brane. However, the construction is subject to ambiguities in boundary
conditions. We propose exact boundary states describing the D0-brane, and
clarify physical interpretations of various boundary states constructed from
different boundary conditions. As it falls into the black hole, the D0-brane
radiates off to the horizon and to the infinity. From the boundary states
constructed, we compute physical observables of such radiative process. We find
that part of the radiation to infinity is in effective thermal distribution at
the Hawking temperature. We also find that part of the radiation to horizon is
in the Hagedorn distribution, dominated by massive, highly non-relativistic
closed string states, much like the tachyon matter. Remarkably, such
distribution emerges only after string worldsheet effects are taken exactly
into account. From these results, we observe that nature of the radiation
distribution changes dramatically across the conifold geometry k=1 (k=3 for the
bosonic case), exposing the `string - black hole transition' therein.Comment: 51 pages, 5 figures, v2: referece added, note added replying the
comment made in hep-th/060206
Inflation and late time acceleration in braneworld cosmological models with varying brane tension
Braneworld models with variable brane tension introduce a new
degree of freedom that allows for evolving gravitational and cosmological
constants, the latter being a natural candidate for dark energy. We consider a
thermodynamic interpretation of the varying brane tension models, by showing
that the field equations with variable can be interpreted as
describing matter creation in a cosmological framework. The particle creation
rate is determined by the variation rate of the brane tension, as well as by
the brane-bulk energy-matter transfer rate. We investigate the effect of a
variable brane tension on the cosmological evolution of the Universe, in the
framework of a particular model in which the brane tension is an exponentially
dependent function of the scale factor. The resulting cosmology shows the
presence of an initial inflationary expansion, followed by a decelerating
phase, and by a smooth transition towards a late accelerated de Sitter type
expansion. The varying brane tension is also responsible for the generation of
the matter in the Universe (reheating period). The physical constraints on the
model parameters, resulted from the observational cosmological data, are also
investigated.Comment: 25 pages, 8 figures, accepted for publication in European Physical
Journal
Unitarity Meets Channel-Duality for Rolling / Decaying D-Branes
Investigations for decay of unstable D-brane and rolling of accelerated
D-brane dynamics have revealed that various proposed prescriptions give
different result for spectral amplitudes and observables. Here, we study them
with particular attention to unitarity and open-closed channel duality. From
"ab initio" derivation in the open string channel, both in Euclidean and
Lorentzian worldsheet approaches, we find heretofore overlooked contribution to
the spectral amplitudes and obervables. The contribution is fortuitously absent
for decay of unstable D-brane, but is present for rolling of accelerated
D-brane. We finally show that the contribution is imperative for ensuring
unitarity and optical theorem at each order in string loop expansion.Comment: Latex, 28 pages, 2 figures (colored
Thermal Partition Functions for S-branes
We calculate the thermal partition functions of open strings on the S-brane
backgrounds (the bouncing or rolling tachyon backgrounds) both in the bosonic
and superstring cases. According to hep-th/0302146, we consider the discretized
temperatures compatible with the pure imaginary periodicity of tachyon
profiles. The ``effective Hagedorn divergence'' is shown to appear no matter
how low temperature is chosen (including zero-temperature). This feature is
likely to be consistent with the large rate of open string pair production
discussed in hep-th/0209090 and also emission of closed string massive modes
hep-th/0303139. We also discuss the possibility to remove the divergence by
considering the space-like linear dilaton backgrounds as in hep-th/0306132.Comment: 33 pages, no figure; v2 typos corrected, a reference adde
Rescattering and chiral dynamics in B\to \rho\pi decay
We examine the role of B^0(\bar B^0) \to \sigma \pi^0 \to \pi^+\pi^- \pi^0
decay in the Dalitz plot analysis of B^0 (\bar B^0) \to \rho\pi \to
\pi^+\pi^-\pi^0 decays, employed to extract the CKM parameter \alpha. The
\sigma \pi channel is significant because it can break the relationship between
the penguin contributions in B\to\rho^0\pi^0, B\to\rho^+\pi^-, and
B\to\rho^-\pi^+ decays consequent to an assumption of isospin symmetry. Its
presence thus mimics the effect of isospin violation. The \sigma\pi^0 state is
of definite CP, however; we demonstrate that the B\to\rho\pi analysis can be
generalized to include this channel without difficulty. The \sigma or
f_0(400-1200) ``meson'' is a broad I=J=0 enhancement driven by strong \pi\pi
rescattering; a suitable scalar form factor is constrained by the chiral
dynamics of low-energy hadron-hadron interactions - it is rather different from
the relativistic Breit-Wigner form adopted in earlier B\to\sigma\pi and
D\to\sigma\pi analyses. We show that the use of this scalar form factor leads
to an improved theoretical understanding of the measured ratio Br(\bar B^0 \to
\rho^\mp \pi^\pm) / Br(B^-\to \rho^0 \pi^-).Comment: 26 pages, 8 figs, published version. typos fixed, minor change
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