35,011 research outputs found
Influence of Light and Heavy Thresholds on SUSY Unification
In this paper we study and compare susy unification using two different
approaches in order to take into account the effect of light particle
thresholds on the evolution of gauge couplings: the step--function
approximation, on the one hand, and a mass dependent procedure, which gives a
more accurate description of the dependence of the results on the masses, on
the other. We also include the effect of heavy thresholds, when is
chosen as the unifying group. We find that the mass--dependent procedure
excludes scenarios where all susy masses are below , and favors a value
of near its upper experimental bound, contrary to the results
obtained with the step--function approximation. We underline the dependence of
the results on the procedure chosen to deal with light thresholds.Comment: 18 pages,LAEFF-93/014,REVTEX-2.1, 5 figures not included, available
upon request (include FAX number)
Semi-perturbative unification with extra vector-like families
We make a comprehensive analysis of an extended supersymmetric model(ESSM)
obtained by adding a pair of vector-like families to the minimal supersymmetric
standard model and having specific forms of 5 x 5 fermion mass matrices. The
singlet Higgs couplings which link the ordinary to vector-like generations do
not have the renormalization effects of the gauge interactions and hence the
"quasi-infrared fixed point" near the scale of the top quark mass. The two-loop
Yukawa effects on gauge couplings lead to an unified coupling around
0.2 with an unification scale M_X of 10^{16.9} GeV. Large Yukawa effects in the
high energy region arrest the growth of the QCD coupling near M_X making the
evolution flat. The renormalization effects of the vector-like generations on
soft mass parameters has important effects on the charge and color
breaking(CCB)minima. We will show that there exists parameter space where there
is no charge and color breaking. We will also demonstrate that there exists
minima of the Higgs potential which satisfies the mass of the Z boson but avoid
CCB. Upper limits on the mass of the lightest Higgs boson from the one-loop
effective scalar potential is obtained for sets of universal soft supersymmetry
breaking mass parameters.Comment: LaTeX, 38 pages and 7 eps figures. Many revisions include
Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models
We study the renormalization group flow for the Higgs self coupling in the
presence of gravitational correction terms. We show that the resulting equation
is equivalent to a singular linear ODE, which has explicit solutions in terms
of hypergeometric functions. We discuss the implications of this model with
gravitational corrections on the Higgs mass estimates in particle physics
models based on the spectral action functional.Comment: 25 pages, LaTeX, 8 PDF figure
Supersymmetry Breakdown at a Hidden Wall
We consider hidden sector supersymmetry breakdown in the strongly coupled
heterotic theory of Ho\v{r}ava and Witten. Using effective
field theory methods in four dimensions, we can show that gravitational
interactions induce soft breaking terms in the observable sector that are of
order of the gravitiono mass. We apply these methods to the mechanism of
gaugino condensation at the hidden wall. Although the situation is very similar
to the weakly coupled case, there is a decisive difference concerning the
observable sector gaugino mass; with desirable phenomenological as well as
cosmological consequences.Comment: 29 pages, latex, no figures, To appear in Nuclear Physics B. Minor
change
Strong Unification
We investigate the possibility that unification occurs at strong coupling. We
show that, despite the fact the couplings pass through a strong coupling
regime, accurate predictions for their low energy values are possible because
the couplings of the theory flow to infrared fixed points. We determine the
low-energy QCD coupling in a favoured class of strong coupling models and find
it is reduced from the weak coupling predictions, lying close to the
experimentally measured value. We extend the analysis to the determination of
quark and lepton masses and show that (even without Grand Unification) the
infra-red fixed point structure may lead to good predictions for the top mass,
the bottom to tau mass ratio and . Finally we discuss the
implications for the unification scale finding it to be increased from the MSSM
value and closer to the heterotic string prediction.Comment: 12 pages, LateX, no figure
String-Unification, Universal One-Loop Corrections and Strongly Coupled Heterotic String Theory
We derive the universal threshold corrections in heterotic string theory
including a continuous Wilson line. Unification of gauge and gravitational
couplings is shown to be possible even within perturbative string theory. The
relative importance of gauge group dependent and independent thresholds on
unification is clarified. Equipped with these results we can then attempt an
extrapolation to the strongly coupled heterotic string -- M-theory. We argue
that such an extrapolation might be meaningful because of the holomorphic
structure of the gauge coupling function and the close connection of the
threshold corrections to the anomaly cancelation mechanism.Comment: 28 LaTex pages with 2 fig
Time variation of fundamental couplings and dynamical dark energy
Scalar field dynamics may give rise to a nonzero cosmological variation of
fundamental constants. Within different scenarios based on the unification of
gauge couplings, the various claimed observations and bounds may be combined in
order to trace or restrict the time history of the couplings and masses. If the
scalar field is responsible for a dynamical dark energy or quintessence,
cosmological information becomes available for its time evolution. Combining
this information with the time variation of couplings, one can determine the
interaction strength between the scalar and atoms, which may be observed by
tests of the Weak Equivalence Principle. We compute bounds on the present rate
of coupling variation from experiments testing the differential accelerations
for bodies with equal mass and different composition and compare the
sensitivity of various methods. In particular, we discuss two specific models
of scalar evolution: crossover quintessence and growing neutrino models.Comment: 26 pages, 2 figures; minor typos & added references, to be published
in JCA
Grand Unification at Intermediate Mass Scales through Extra Dimensions
One of the drawbacks of conventional grand unification scenarios has been
that the unification scale is too high to permit direct exploration. In this
paper, we show that the unification scale can be significantly lowered (perhaps
even to the TeV scale) through the appearance of extra spacetime dimensions.
Such extra dimensions are a natural consequence of string theories with
large-radius compactifications. We show that extra spacetime dimensions
naturally lead to gauge coupling unification at intermediate mass scales, and
moreover may provide a natural mechanism for explaining the fermion mass
hierarchy by permitting the fermion masses to evolve with a power-law
dependence on the mass scale. We also show that proton-decay constraints may be
satisfied in our scenario due to the higher-dimensional cancellation of
proton-decay amplitudes to all orders in perturbation theory. Finally, we
extend these results by considering theories without supersymmetry;
experimental collider signatures; and embeddings into string theory. The latter
also enables us to develop several novel methods of explaining the fermion mass
hierarchy via -branes. Our results therefore suggest a new approach towards
understanding the physics of grand unification as well as the phenomenology of
large-radius string compactifications.Comment: 65 pages, LaTeX, 20 figure
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