941 research outputs found
On unitarity of a linearized Yang-Mills formulation for massless and massive gravity with propagating torsion
A perturbative regime based on contortion as a dynamical variable and metric
as a (classical) fixed background, is performed in the context of a pure
Yang-Mills formulation for gravity in a dimensional space-time. In the
massless case we show that the theory contains three degrees of freedom and
only one is a non-unitary mode. Next, we introduce quadratical terms dependent
on torsion, which preserve parity and general covariance. The linearized
version reproduces an analogue Hilbert-Einstein-Fierz-Pauli unitary massive
theory plus three massless modes, two of them represents non-unitary ones.
Finally we confirm the existence of a family of unitary Yang-Mills-extended
theories which are classically consistent with Einstein's solutions coming from
non massive and topologically massive gravity. The unitarity of these
YM-extended theories is shown in a perturbative regime. A possible way to
perform a non-perturbative study is remarked.Comment: To appear in International Journal of Modern Physics
Rare top quark decays in extended models
Flavor changing neutral currents (FCNC) decays t to H + c, t to Z + c, and H
to t + bar{c} are discussed in the context of Alternative Left-Right symmetric
Models (ALRM) with extra isosinglet heavy fermions where FCNC decays may take
place at tree-level and are only suppressed by the mixing between ordinary top
and charm quarks, which is poorly constraint by current experimental values.
The non-manifest case is also briefly discussed.Comment: Contributed talk given at the 10th Mexican Workhop on Particles and
Fields, Morelia, Michoacan, Mexico, 7-12 Nov 200
Quadratic Dynamical Decoupling with Non-Uniform Error Suppression
We analyze numerically the performance of the near-optimal quadratic
dynamical decoupling (QDD) single-qubit decoherence errors suppression method
[J. West et al., Phys. Rev. Lett. 104, 130501 (2010)]. The QDD sequence is
formed by nesting two optimal Uhrig dynamical decoupling sequences for two
orthogonal axes, comprising N1 and N2 pulses, respectively. Varying these
numbers, we study the decoherence suppression properties of QDD directly by
isolating the errors associated with each system basis operator present in the
system-bath interaction Hamiltonian. Each individual error scales with the
lowest order of the Dyson series, therefore immediately yielding the order of
decoherence suppression. We show that the error suppression properties of QDD
are dependent upon the parities of N1 and N2, and near-optimal performance is
achieved for general single-qubit interactions when N1=N2.Comment: 17 pages, 22 figure
Neutrino mixing and masses in a left-right model with mirror fermions
In the framework of a left-right model containing mirror fermions with gauge
group SU(3),
we estimate the neutrino masses, which are found to be consistent with their
experimental bounds and hierarchy. We evaluate the decay rates of the Lepton
Flavor Violation (LFV) processes , and . We obtain upper limits for the
flavor-changing branching ratios in agreement with their present experimental
bounds. We also estimate the decay rates of heavy Majorana neutrinos in the
channels , and , which are roughly equal for large values of the heavy
neutrino mass. Starting from the most general Majorana neutrino mass matrix,
the smallness of active neutrino masses turns out from the interplay of the
hierarchy of the involved scales and the double application of seesaw
mechanism. An appropriate parameterization on the structure of the neutrino
mass matrix imposing a symmetric mixing of electron neutrino with muon and tau
neutrinos leads to Tri-bimaximal mixing matrix for light neutrinos.Comment: Accepted by European Physical Journal
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