296 research outputs found
On V_ud determination from kaon decays
The pion beta decay pi^+ -> pi^0 e^+ nu proceeds through pure weak vector
hadronic currents and, therefore, the theoretical prediction for it is more
reliable than for the processes with axial-vector current contribution. For
example, recently the pion beta decay has been used for V_ud determination. The
main aim of this letter is to point that kaon beta decay K^0 -> K^+(pi^+ pi^0)
e^- nu-bar analogously can be used for this purpose.Comment: 3 pages, no figures, one reference adde
Hard X-ray Bursts from Collapse of the Super Massive Stars
The very first stars in the Universe can be very massive, up to
. They would leave behind massive black holes that could act as
seeds for growing super massive black holes of active galactic nuclei. Given
the anticipated fast rotation such stars would end their live as super massive
collapsars and drive powerful magnetically-dominated jets. In this paper we
investigate the possibility of observing the bursts of high-energy emission
similar to the Long Gamma Ray Bursts associated with normal collapsars. We show
that during the collapse of supercollapsars, the Blandford-Znajek mechanism can
produce jets as powerful as fewerg/s and release up to
erg of the black hole rotational energy. Due to the higher intrinsic
time scale and higher redshift the initial bright phase of the burst can last
for about seconds whereas the central engine would remain active for
about 10 days. Due to the high redshift the burst spectrum is expected to be
soft, with the spectral energy distribution peaking at around 60keV. The peak
total flux density is relatively low, few,
but not prohibitive. The such events should be rear 0.03 year, the
observations needs long term program and could be done in future.Comment: 4 pages, 2 figures. proceedings of workshop "Many faces of GRB
phenomena - optics vs high energy", SAO Russia, October 12-16, 2009
Hexagons become second if symmetry is broken
Pattern formation on the free surface of a magnetic fluid subjected to a
magnetic field is investigated experimentally. By tilting the magnetic field
the symmetry can be broken in a controllable manner. When increasing the
amplitude of the tilted field, the flat surface gives way to liquid ridges. A
further increase results in a hysteretic transition to a pattern of stretched
hexagons. The instabilities are detected by means of a linear array of magnetic
hall sensors and compared with theoretical predictions.Comment: accepted for publication by Physical Review E/Rapid Communicatio
Gravitational red-shift and deflection of slow light
We explore the nature of the classical propagation of light through media
with strong frequency-dependent dispersion in the presence of a gravitational
field. In the weak field limit, gravity causes a redshift of the optical
frequency, which the slow-light medium converts into a spatially-varying index
of refraction. This results in the bending of a light ray in the medium. We
further propose experimental techniques to amplify and detect the phenomenon
using weak value measurements. Independent heuristic and rigorous derivations
of this effect are given.Comment: 9 pages, 1 figur
Swift J164449.3+573451 event: generation in the collapsing star cluster?
We discuss the multiband energy release in a model of a collapsing galactic
nucleus, and we try to interpret the unique super-long cosmic gamma-ray event
Swift J164449.3+573451 (GRB 110328A by early classification) in this scenario.
Neutron stars and stellar-mass black holes can form evolutionary a compact
self-gravitating subsystem in the galactic center. Collisions and merges of
these stellar remnants during an avalanche contraction and collapse of the
cluster core can produce powerful events in different bands due to several
mechanisms. Collisions of neutron stars and stellar-mass black holes can
generate gamma-ray bursts (GRBs) similar to the ordinary models of short GRB
origin. The bright peaks during the first two days may also be a consequence of
multiple matter supply (due to matter release in the collisions) and accretion
onto the forming supermassive black hole. Numerous smaller peaks and later
quasi-steady radiation can arise from gravitational lensing, late accretion of
gas onto the supermassive black hole, and from particle acceleration by shock
waves. Even if this model will not reproduce exactly all the Swift
J164449.3+573451 properties in future observations, such collapses of galactic
nuclei can be available for detection in other events.Comment: 7 pages, replaced by the final versio
Radiative corrections and parity nonconservation in heavy atoms
The self-energy and the vertex radiative corrections to the effect of parity
nonconservation in heavy atoms are calculated analytically in orders Z alpha^2
and Z^2 alpha^3 ln(lambda_C/r_0), where lambda_C and r_0 being the Compton
wavelength and the nuclear radius, respectively. The value of the radiative
correction is -0.85% for Cs and -1.41% for Tl. Using these results we have
performed analysis of the experimental data on atomic parity nonconservation.
The obtained values of the nuclear weak charge,
Q_W=-72.90(28)_{exp}(35)_{theor} for Cs, and Q_W=-116.7(1.2)_{exp}(3.4)_{theor}
for Tl, agree with predictions of the standard model. As an application of our
approach we have also calculated analytically dependence of the Lamb shift on
the finite nuclear size.Comment: 4 pages, 4 figure
Modeling of the Interaction of GRB Prompt Emission with the Circumburst Medium
We present methodology and results of numerical modeling of the interaction
of GRB prompt emission with the circumburst medium using a modified version of
the multi-group radiation hydrocode STELLA. The modification includes the
nonstationary photoionization, the photoionization heating and the Compton
heating along with the hydrodynamics and radiation transfer. The lightcurves
and spectra of the outcoming gamma-ray, X-ray and optical emission are
calculated for a set of models (shells) of the circumburst environment, which
differ in dimensions, density, density profile, composition, temperature. In
some cases total bolometric and optical luminosities can reach 10^47 and 10^43
erg/s respectively. These effects can be responsible for irregularities which
are seen on lightcurves of some GRB's X-ray and optical afterglows.Comment: 27 pages, 16 colour figures, this version is translated by authors,
so it differs from that, which is published in Astronomy Letter
New Constraints on Dispersive Form Factor Parameterizations from the Timelike Region
We generalize a recent model-independent form factor parameterization derived
from rigorous dispersion relations to include constraints from data in the
timelike region. These constraints dictate the convergence properties of the
parameterization and appear as sum rules on the parameters. We further develop
a new parameterization that takes into account finiteness and asymptotic
conditions on the form factor, and use it to fit to the elastic \pi
electromagnetic form factor. We find that the existing world sample of timelike
data gives only loose bounds on the form factor in the spacelike region, but
explain how the acquisition of additional timelike data or fits to other form
factors are expected to give much better results. The same parameterization is
seen to fit spacelike data extremely well.Comment: 24 pages, latex (revtex), 3 eps figure
Time-reversal violating rotation of polarization plane of light in gas placed in electric field
Rotation of polarization plane of light in gas placed in electric field is
considered. Different factors causing this phenomenon are investigated. Angle
of polarization plane rotation for transition 6S_{1/2} - 7S_{1/2} in cesium
(lambda=539 nm) is estimated. The possibility to observe this effect
experimentally is discussed.Comment: 10 pages, Late
The vector form factor of the pion from unitarity and analyticity: a model-independent approach
We study a model-independent parameterization of the vector pion form factor
that arises from the constraints of analyticity and unitarity. Our description
should be suitable up to sqrt(s) ~ 1.2 GeV and allows a model-independent
determination of the mass of the rho(770) resonance, M(rho) = (775.1 +- 0.5)
MeV. We analyse the experimental data on tau(-) -> pion(-) pion(0)
neutrino(tau), in this framework, and its consequences on the low-energy
observables worked out by chiral perturbation theory. An evaluation of the
two-pion contribution to the anomalous magnetic moment of the muon, a_muon, and
to the fine structure constant, alpha(M(Z)^2), is also performed.Comment: 1+15 pages, 2 figure
- …