734,261 research outputs found
Intra-day variability observations and the VLBI structure analysis of quasar S4 0917+624
The IDV observations of S4 0917+624 were carried out monthly, from August
2005 to January 2010, with the Urumqi 25m radio telescope at 4.8 GHz. The
quasar S4 0917+624 exhibits only very weak or no IDV during our 4.5 year
observing interval. Prior to the year 2000, the source S4 0917+624 was one of
the most prominent IDV sources. Our new data indicate that the previous strong
IDV has ceased. We analyzed the long-term VLBI structural variability using
Gaussian model-fitting. From this we obtained the flux densities and the
deconvolved sizes of core and inner-jet components of the source. We studied
the properties such as core fraction, angular size, spectral index, and
brightness temperature of VLBI core for S4 0917+624, as well as the time delay
between 5 and 15 GHz variations, and compared them with the IDV properties of
S4 0917+624. The source shows ejection of several jet components that are
suspected to have partially reduced the IDV amplitude of S4 0917+624. However,
during 2005-2006, the VLBI core size was comparable to the size before the year
2000, but no strong IDV was detected in the period, suggesting that the
quenching effect due to source size changes may not be responsible for the lack
of strong IDV after the year 2000. The refractive scattering properties for the
strong IDV phase of S4 0917+624 before the year 2000 are discussed. The
disappearance of strong IDV in S4 0917+624 after the year 2000 is a mystery and
cannot be explained via the quenching effect by changes in the observable VLBI
structure. However, it may be caused by changes in the interstellar medium,
i.e. by interstellar weather, which induces changes in the scintillation
pattern on timescales of several years. Further coordinated multi-frequency
observations will be required to distinguish between the effect of
source-intrinsic variability and changing properties of the interstellar
medium.Comment: 8 pages, 6 figures, accepted for publication in A&
Trimaximal neutrino mixing from vacuum alignment in A4 and S4 models
Recent T2K results indicate a sizeable reactor angle theta_13 which would
rule out exact tri-bimaximal lepton mixing. We study the vacuum alignment of
the Altarelli-Feruglio A4 family symmetry model including additional flavons in
the 1' and 1" representations and show that it leads to trimaximal mixing in
which the second column of the lepton mixing matrix consists of the column
vector (1,1,1)^T/sqrt{3}, with a potentially large reactor angle. In order to
limit the reactor angle and control the higher order corrections, we propose a
renormalisable S4 model in which the 1' and 1" flavons of A4 are unified into a
doublet of S4 which is spontaneously broken to A4 by a flavon which enters the
neutrino sector at higher order. We study the vacuum alignment in the S4 model
and show that it predicts accurate trimaximal mixing with approximate
tri-bimaximal mixing, leading to a new mixing sum rule testable in future
neutrino experiments. Both A4 and S4 models preserve form dominance and hence
predict zero leptogenesis, up to renormalisation group corrections.Comment: 24 pages, 2 figures, version to be published in JHE
Neutrino Mixings and the S4 Discrete Flavour Symmetry
Discrete non-Abelian Symmetries have been extensively used to reproduce the
lepton mixings. In particular, the S4 group turned out to be suitable to
describe predictive mixing patterns, such as the well-known Tri-Bimaximal and
the Bimaximal schemes, which all represent possible first approximations of the
experimental lepton mixing matrix. We review the main application of the S4
discrete group as a flavour symmetry, first dealing with the formalism and
later with the phenomenological implications. In particular, we summarize the
main features of flavour models based on S4, commenting on their ability in
reproducing a reactor angle in agreement with the recent data and on their
predictions for lepton flavour violating transitions.Comment: 33 pages, 7 figures, 4 tables. Review written for the special issue
on "Flavor Symmetries and Neutrino Oscillations", published in Fortschritte
der Physik - Progress of Physic
S_4-symmetry of 6j-symbols and Frobenius-Schur indicators in rigid monoidal C^*-categories
We show that a left-rigid monoidal C^*-category with irreducible monoidal
unit is also a sovereign and spherical category. Defining a Frobenius-Schur
type indicator we obtain selection rules for the fusion coefficients of
irreducible objects. As a main result we prove S_4-invariance of 6j-symbols in
such a category.Comment: 21 pages + 16 pages with figures; LaTeX2e plus macro package XYpic;
file with included pictures available as
http://www.desy.de/~jfuchs/s4/s4.ps.g
Towards Minimal S4 Lepton Flavor Model
We study lepton flavor models with the flavor symmetry. We construct
simple models with smaller numbers of flavon fields and free parameters, such
that we have predictions among lepton masses and mixing angles. The model with
a triplet flavon is not realistic, but we can construct realistic models
with two triplet flavons, or one triplet and one doublet flavons.Comment: 18 pages, 4 figures, references are adde
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