963 research outputs found
Final state interaction in the production of heavy unstable particles
We make an attempt to discuss in detail the effects originating from the
final state interaction in the processes involving production of unstable
elementary particles and their subsequent decay. Two complementary scenarios
are considered: the single resonance production and the production of two
resonances. We argue that part of the corrections due to the final state
interaction can be connected with the Coulomb phases of the involved charge
particles; the presence of the unstable particle in the problem makes the
Coulomb phase ``visible''. It is shown how corrections due to the final state
interaction disappear when one proceeds to the total cross-sections. We derive
one-loop non-factorizable radiative corrections to the lowest order matrix
element of both single and double resonance production. We discuss how the
infrared limit of the theories with the unstable particles is modified. In
conclusion we briefly discuss our results in the context of the forthcoming
experiments on the and the production at LEP and NLC.Comment: 33 pages, latex, 6 figures (added), version accepted for publication
in Nuc. Phys. B, substantial revisio
QCD Radiative Correction to Zero Recoil Sum Rules for Heavy Flavor Transitions in the Small Velocity Limit.
We consider the small velocity sum rules for heavy flavour semileptonic
transitions that are used to estimate the zero recoil values of semileptonic
heavy flavour form factors. We analyze the complete O() radiative
correction to these sum rules. The corrections are universal and influence all
"model-independent" bounds previously derived for semileptonic form factors at
zero recoil.Comment: 13 pages, LaTeX, 3 figures
Two -loop radiative corrections to the Higgs decay width for large Higgs boson masses
This note is devoted to the calculation of the two-loop
radiative corrections to the Higgs decay width for large
values of the Higgs boson mass within the Minimal Standard Model.
The use of the Equivalence Theorem makes it possible to reduce the problem to
the consideration of the physical Higgs boson field and the Goldstone bosons
. We present analytical results for the various two- and
three-particle absorptive parts of two-loop contributions, using dispersive
techniques, analytic results for all but one of the dispersive contributions.
The typical size of the correction is percent for a Higgs boson
mass of order .Comment: 21 pages, 7 uuencoded figure
Black Holes in AdS/BCFT and Fluid/Gravity Correspondence
A proposal to describe gravity duals of conformal theories with boundaries
(AdS/BCFT correspondence) was put forward by Takayanagi few years ago. However
interesting solutions describing field theories at finite temperature and
charge density are still lacking. In this paper we describe a class of theories
with boundary, which admit black hole type gravity solutions. The theories are
specified by stress-energy tensors that reside on the extensions of the
boundary to the bulk. From this perspective AdS/BCFT appears analogous to the
fluid/gravity correspondence. Among the class of the boundary extensions there
is a special (integrable) one, for which the stress-energy tensor is
fluid-like. We discuss features of that special solution as well as its
thermodynamic properties.Comment: 18 pages, 4 figures (7 pdf-files). Save and view with Adobe Reader if
images appear corrupted in the browse
High-resolution radio imaging of two luminous quasars beyond redshift 4.5
Context. Radio-loud active galactic nuclei in the early Universe are rare.
The quasars J0906+6930 at redshift z=5.47 and J2102+6015 at z=4.57 stand out
from the known sample with their compact emission on milliarcsecond (mas)
angular scale with high (0.1-Jy level) flux densities measured at GHz radio
frequencies. This makes them ideal targets for very long baseline
interferometry (VLBI) observations. Aims. By means of VLBI imaging we can
reveal the inner radio structure of quasars and model their brightness
distribution to better understand the geometry of the jet and the physics of
the sources. Methods. We present sensitive high-resolution VLBI images of
J0906+6930 and J2102+6015 at two observing frequencies, 2.3 and 8.6 GHz. The
data were taken in an astrometric observing programme involving a global
five-element radio telescope array. We combined the data from five different
epochs from 2017 February to August. Results. For one of the highest redshift
blazars known, J0906+6930, we present the first-ever VLBI image obtained at a
frequency below 8 GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that
this source has a sharply bent helical inner jet structure within ~3 mas from
the core. The quasar J2102+6015 shows an elongated radio structure in the
east-west direction within the innermost ~2 mas that can be described with a
symmetric three-component brightness distribution model at 8.6 GHz. Because of
their non-pointlike mas-scale structure, these sources are not ideal as
astrometric reference objects. Our results demonstrate that VLBI observing
programmes conducted primarily with astrometric or geodetic goals can be
utilized for astrophysical purposes as well.Comment: 8 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Models of G time variations in diverse dimensions
A review of different cosmological models in diverse dimensions leading to a
relatively small time variation of the effective gravitational constant G is
presented. Among them: 4-dimensional general scalar-tensor model,
multidimensional vacuum model with two curved Einstein spaces, multidimensional
model with multicomponent anisotropic "perfect fluid", S-brane model with
scalar fields and two form field etc. It is shown that there exist different
possible ways of explanation of relatively small time variation of the
effective gravitational constant G compatible with present cosmological data
(e.g. acceleration): 4-dimensional scalar-tensor theories or multidimensional
cosmological models with different matter sources. The experimental bounds on
G-dot may be satisfied ether in some restricted interval or for all allowed
values of the synchronous time variable.Comment: 27 pages, Late
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