33,320 research outputs found
Contribution of Scalar Loops to the Three-Photon Decay of the Z
I corrected 3 mistakes from the first version: that were an omitted Feynman
integration in the function f^3_{ij}, a factor of 2 in front of log f^3_{ij} in
eq.2 and an overall factor of 2 in Fig.1 c). The final result is changed
drastically. Doing an expansion in the Higgs mass I show that the matrix
element is identically 0 in the order (MZ/MH)^2, which is due to gauge
invariance. Left with an amplitude of the order (MZ/MH)^4 the final result is
that the scalar contribution to this decay rate is several orders of magnitude
smaller than those of the W boson and fermions.Comment: 6 pages, plain Tex, 1 figure available under request via fax or mail,
OCIP/C-93-5, UQAM-PHE-93/0
R-parity as a residual gauge symmetry : probing a theory of cosmological dark matter
We present a non-supersymmetric scenario in which the R-parity symmetry arises as a result of spontaneous gauge symmetry breaking,
leading to a viable Dirac fermion WIMP dark matter candidate. Direct detection
in nuclear recoil experiments probes dark matter masses around TeV for
TeV consistent with searches at the LHC, while lepton
flavor violation rates and flavor changing neutral currents in neutral meson
systems lie within reach of upcoming experiments.Comment: 7 pages, 3 figure
Inelastic cotunneling induced decoherence and relaxation, charge and spin currents in an interacting quantum dot under a magnetic field
We present a theoretical analysis of several aspects of nonequilibirum
cotunneling through a strong Coulomb-blockaded quantum dot (QD) subject to a
finite magnetic field in the weak coupling limit. We carry this out by
developing a generic quantum Heisenberg-Langevin equation approach leading to a
set of Bloch dynamical equations which describe the nonequilibrium cotunneling
in a convenient and compact way. These equations describe the time evolution of
the spin variables of the QD explicitly in terms of the response and
correlation functions of the free reservoir variables. This scheme not only
provides analytical expressions for the relaxation and decoherence of the
localized spin induced by cotunneling, but it also facilitates evaluations of
the nonequilibrium magnetization, the charge current, and the spin current at
arbitrary bias-voltage, magnetic field, and temperature. We find that all
cotunneling events produce decoherence, but relaxation stems only from {\em
inelastic} spin-flip cotunneling processes. Moreover, our specific calculations
show that cotunneling processes involving electron transfer (both spin-flip and
non-spin-flip) contribute to charge current, while spin-flip cotunneling
processes are required to produce a net spin current in the asymmetric coupling
case. We also point out that under the influence of a nonzero magnetic field,
spin-flip cotunneling is an energy-consuming process requiring a sufficiently
strong external bias-voltage for activation, explaining the behavior of
differential conductance at low temperature: in particular, the splitting of
the zero-bias anomaly in the charge current and a broad zero-magnitude "window"
of differential conductance for the spin current near zero-bias-voltage.Comment: 15 pages, 5 figures, published version, to appear in Phys. Rev.
Zero Modes in Electromagnetic Form Factors of the Nucleon in a Light-Cone Diquark Model
We use a diquark model of the nucleon to calculate the electromagnetic form
factors of the nucleon described as a scalar and axialvector diquark bound
state. We provide an analysis of the zero-mode contribution in the diquark
model. We find there are zero-mode contributions to the form factors arising
from the instantaneous part of the quark propagator, which cannot be neglected
compared with the valence contribution but can be removed by the choice of wave
function. We also find that the charge and magnetic radii and magnetic moment
of the proton can be reproduced, while the magnetic moment of the neutron is
too small. The dipole shape of the form factors, and
can be reproduced. The ratio decreases
with but too fast.Comment: 22 pages, 6 pages, accepted by J.Phys.
- …