2,025 research outputs found
Do the Profile Function singularities explain the high energy reflection of fermions in a phase transition?
We investigate the scope of a previous result concerning the behaviour of
fermions hitting a general wall caused by a first-order phase transition. The
wall profile function was considered to be analytic in the real axis. The
previous result is valid for analytic functions in the whole complex plane
except in certain isolated singularities located out of the real axis. A
non-analytic profile function in the real axis is studied in order to show the
validity of the result for any profile which can be put as a certain limit of a
function which verifies the latter. A new understanding of the high energy
behaviour of the quantum reflection caused by a sharp profile, as the step,
arises from that study.Comment: 6 pages, 1 figur
-Flavour Violation at the LHC
We study the conditions required for
decays to yield observable tau flavour violation at the LHC, for cosmologically
interesting values of the neutralino relic density. These condition can be
achieved in the framework of a SU(5) model with a see-saw mechanism that allows
a possible coexistence of a LHC signal a low prediction for radiative LFV
decays.Comment: 7 pages, 5 figures, Prepared for the proceedings of the workshop:
"LC09: Physics at the TeV Scale and the Dark Matter Connection",
21-24 September 2009, Perugia, Ital
On the zero crossing of the three-gluon vertex
We report on new results on the infrared behaviour of the three-gluon vertex
in quenched Quantum Chormodynamics, obtained from large-volume lattice
simulations. The main focus of our study is the appearance of the
characteristic infrared feature known as 'zero crossing', the origin of which
is intimately connected with the nonperturbative masslessness of the
Faddeev-Popov ghost. The appearance of this effect is clearly visible in one of
the two kinematic configurations analyzed, and its theoretical origin is
discussed in the framework of Schwinger-Dyson equations. The effective coupling
in the momentum subtraction scheme that corresponds to the three-gluon vertex
is constructed, revealing the vanishing of the effective interaction at the
exact location of the zero crossing.Comment: 6 pages, 4 figure
Multibody neutrino exchange in a neutron star: neutrino sea and border effects
The interaction due to the exchange of massless neutrinos between neutrons is
a long-range force. Border effects on this multibody exchange inside a dense
core are studied and computed analytically in 1 + 1 dimensions. We demonstrate
in this work that a proper treatment of the star's border effect automatically
incorporates the condensate contribution as a consequence of the appropriate
boundary conditions for the neutrino Feynman propagator inside the star.
The total multibody exchange contribution is infrared-safe and vanishes
exactly in 1 + 1 dimensions. The general conclusion of this work is that the
border effect does not modify the result that neutrino exchange is
infrared-safe. This toy model prepares the ground and gives the tools for the
study of the realistic 3 + 1 star.Comment: Latex file (Revtex), 11 pages, 2 figure
Instanton traces in lattice gluon correlation functions
Strong coupling constant computed in Landau gauge and MOM renormalization
scheme from lattice two and three gluon Green Functions exhibits an unexpected
behavior in the deep IR, showing a maximum value around . We
analise this coupling below this maximum within a semiclassical approach, were
gluon degrees of freedom at very low energies are described in terms of the
classical solutions of the lagrangian, namely instantons. We provide some new
results concerning the relationship between instantons and the low energy
dynamics of QCD, by analising gluon two- and three-point Green functions
separately and with the help of a cooling procedure to eliminate short range
correlations.Comment: 4 pages, talk given at XXXX Rencontres de Moriond on QCD and Hadronic
Interactions, La Thuile (Italy
The Infrared Behaviour of the Pure Yang-Mills Green Functions
We study the infrared behaviour of the pure Yang-Mills correlators using
relations that are well defined in the non-perturbative domain. These are the
Slavnov-Taylor identity for three-gluon vertex and the Schwinger-Dyson equation
for ghost propagator in the Landau gauge. We also use several inputs from
lattice simulations. We show that lattice data are in serious conflict with a
widely spread analytical relation between the gluon and ghost infrared critical
exponents. We conjecture that this is explained by a singular behaviour of the
ghost-ghost-gluon vertex function in the infrared. We show that, anyhow, this
discrepancy is not due to some lattice artefact since lattice Green functions
satisfy the ghost propagator Schwinger-Dyson equation. We also report on a
puzzle concerning the infrared gluon propagator: lattice data seem to favor a
constant non vanishing zero momentum gluon propagator, while the Slavnov-Taylor
identity (complemented with some regularity hypothesis of scalar functions)
implies that it should diverge.Comment: 25 pages, 7 figures; replaced version with some references adde and
an enlarged discussion of the non-renormalization theorem; second replacement
with improved figures and added reference
Asymptotic behavior of the ghost propagator in SU3 lattice gauge theory
We study the asymptotic behavior of the ghost propagator in the quenched
SU(3) lattice gauge theory with Wilson action. The study is performed on
lattices with a physical volume fixed around 1.6 fm and different lattice
spacings: 0.100 fm, 0.070 fm and 0.055 fm. We implement an efficient algorithm
for computing the Faddeev-Popov operator on the lattice. We are able to
extrapolate the lattice data for the ghost propagator towards the continuum and
to show that the extrapolated data on each lattice can be described up to
four-loop perturbation theory from 2.0 GeV to 6.0 GeV. The three-loop values
are consistent with those extracted from previous perturbative studies of the
gluon propagator. However the effective \Lambda_{\ms} scale which reproduces
the data does depend strongly upon the order of perturbation theory and on the
renormalization scheme used in the parametrization. We show how the truncation
of the perturbative series can account for the magnitude of the dependency in
this energy range. The contribution of non-perturbative corrections will be
discussed elsewhere.Comment: 26 pages, 7 figure
Refining the detection of the zero crossing for the three-gluon vertex in symmetric and asymmetric momentum subtraction schemes
This article reports on the detailed study of the three-gluon vertex in four-dimensional SU(3) Yang-Mills theory employing lattice simulations with large physical volumes and high statistics. A meticulous scrutiny of the so-called symmetric and asymmetric kinematical configurations is performed, and it is shown that the associated form factor changes sign at a given range of momenta. The lattice results are compared to the model-independent predictions of Schwinger-Dyson equations, and a very good agreement between the two is found
Search for Tau Flavour Violation at the LHC
We explore the prospects for searches at the LHC for sparticle decays that
violate lepton number, in the light of neutrino oscillation data and the
seesaw model for neutrino masses and mixing. We analyse the theoretical and
phenomenological conditions required for tau flavour violation to be observable
in \chi_2 \to \chi + \tau^\pm \mu^\mp decays, for cosmologically interesting
values of the relic neutralino LSP density. We study the relevant
supersymmetric parameter space in the context of the Constrained Minimal
Supersymmetric Extension of the Standard Model (CMSSM) and in SU(5) extensions
of the theory. We pay particular attention to the possible signals from
hadronic tau decays, that we analyse using PYTHIA event simulation. We find
that a signal for \tau flavour-violating \chi_2 decays may be observable if the
branching ratio exceeds about 10%. This may be compatible with the existing
upper limit on \tau \to \mu \gamma decays if there is mixing between
right-handed sleptons, as could be induced in non-minimal SU(5) GUTs.Comment: 24 pages, 10 fig
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