5 research outputs found
SUSY Quivers, Intersecting Branes and the Modest Hierarchy Problem
We present a class of chiral non-supersymmetric D=4 field theories in which
quadratic divergences appear only at two loops. They may be depicted as ``SUSY
quivers'' in which the nodes represent a gauge group with extended e.g., N=4
SUSY whereas links represent bifundamental matter fields which transform as
chiral multiplets with respect to different N=1 subgroups. One can obtain this
type of field theories from simple D6-brane configurations on Type IIA string
theory compactified on a six-torus. We discuss the conditions under which this
kind of structure is obtained from D6-brane intersections. We also discuss some
aspects of the effective low-energy field theory. In particular we compute
gauge couplings and Fayet-Iliopoulos terms from the Born-Infeld action and show
how they match the field theory results. This class of theories may be of
phenomenological interest in order to understand the modest hierarchy problem
i.e., the stability of the hierarchy between the weak scale and a fundamental
scale of order 10-100 TeV which appears e.g. in low string scale models.
Specific D-brane models with the spectrum of the SUSY Standard Model and three
generations are presented.Comment: 36 pages, using JHEP3.cls, 8 figures. References update
Lepton Flavour Violating Leptonic/Semileptonic Decays of Charged Leptons in the Minimal Supersymmetric Standard Model
We consider the leptonic and semileptonic (SL) lepton flavour violating (LFV)
decays of the charged leptons in the minimal supersymmetric standard model
(MSSM). The formalism for evaluation of branching fractions for the SL LFV
charged-lepton decays with one or two pseudoscalar mesons, or one vector meson
in the final state, is given. Previous amplitudes for the SL LFV charged-lepton
decays in MSSM are improved, for instance the -penguin amplitude is
corrected to assure the gauge invariance. The decays are studied not only in
the model-independent formulation of the theory in the frame of MSSM, but also
within the frame of the minimal supersymmetric SO(10) model within which the
parameters of the MSSM are determined. The latter model gives predictions for
the neutrino-Dirac Yukawa coupling matrix, once free parameters in the model
are appropriately fixed to accommodate the recent neutrino oscillation data.
Using this unambiguous neutrino-Dirac Yukawa couplings, we calculate the LFV
leptonic and SL decay processes assuming the minimal supergravity scenario. A
very detailed numerical analysis is done to constrain the MSSM parameters.
Numerical results for SL LFV processes are given, for instance for tau -> e
(mu) pi0, tau -> e (mu) eta, tau -> e (mu) eta', tau -> e (mu) rho0, tau -> e
(mu) phi, tau -> e (mu) omega, etc.Comment: 36 pages, 3 tables, 5 .eps figure
Intersecting Brane Models of Particle Physics and the Higgs Mechanism
We analyze a recently constructed class of D-brane theories with the fermion
spectrum of the SM at the intersection of D6-branes wrapping a compact toroidal
space. We show how the SM Higgs mechanism appears as a brane recombination
effect in which the branes giving rise to U(2)_L \times U(1) recombine into a
single brane related to U(1)_{em}. We also show how one can construct D6-brane
models which respect some supersymmetry at every intersection. These are
quasi-supersymmetric models of the type introduced in hep-th/0201205 which may
be depicted in terms of SUSY-quivers and may stabilize the hierarchy between
the weak scale and a fundamental scale of order 10-100 TeV present in low
string scale models. Several explicit D6-brane models with three generations of
quarks and leptons and different SUSY-quiver structure are presented. One can
prove on general grounds that if one wants to build a (factorizable) D6-brane
configuration with the SM gauge group and N = 1 SUSY (or quasi-SUSY), also a
massless (B-L) generator must be initially present in any model. If in addition
we insist on lef- and right-handed fermions respecting the same N=1 SUSY, the
brane configurations are forced to have intersections giving rise to Higgs
multiplets, providing for a rationale for the very existence of the SM Higgs
sector.Comment: 58 pages, using JHEP3.cls, 13 figures. Minor corrections and
references update