6 research outputs found
One-loop Higgs mass finiteness in supersymmetric Kaluza-Klein theories
We analyze the one-loop ultraviolet sensitivity of the Higgs mass in a
five-dimensional supersymmetric theory compactified on the orbifold S^1/Z_2,
with superpotential localized on a fixed-point brane. Four-dimensional
supersymmetry is broken by Scherk-Schwarz boundary conditions. Kaluza-Klein
interactions are regularized by means of a brane Gaussian distribution along
the extra dimension with length l_s\simeq\Lambda^{-1}_s, where \Lambda_s is the
cutoff of the five-dimensional theory. The coupling of the n-mode, with mass
M^{(n)}, acquires the n-dependent factor exp{-(M^{(n)}/\Lambda_s)^2/2}, which
makes it to decouple for M^{(n)}\gg \Lambda_s. The sensitivity of the Higgs
mass on \Lambda_s is strongly suppressed and quadratic divergences cancel by
supersymmetry. The one-loop correction to the Higgs mass is finite and equals,
for large values of \Lambda_s, the value obtained by the so-called
KK-regularization.Comment: 8 pages, 1 figure. The discussion on the distribution giving rise to
couplings suppressed by exp(-M/Lambda) is revised and the result is finite
and equals that of the Gaussian cas
Radiative Scherk-Schwarz supersymmetry breaking
We analyze the Scherk-Schwarz (SS) supersymmetry breaking in brane-world five
dimensional theories compactified on the orbifold . The SS
breaking parameter is undetermined at the tree-level (no-scale supergravity)
and can be interpreted as the Hosotani vacuum expectation value corresponding
to the group in five dimensional N=2 (ungauged) supergravity. We show
that the SS breaking parameter is fixed at the loop level to either 0 or 1/2
depending on the matter content propagating in the bulk but in a rather
model-independent way. Supersymmetry breaking is therefore fixed through a
radiative Scherk-Schwarz mechanism. We also show that the two discrete values
of the SS parameter, as well as the supersymmetry breaking shift in the
spectrum of the bulk fields, are altered in the presence of a brane-localized
supersymmetry breaking arising from some hidden sector dynamics. The interplay
between the SS and the brane localized breaking is studied in detail.Comment: 16 pages, 2 figures, uses axodraw. References adde
Towards a renormalizable standard model without fundamental Higgs scalar
We investigate the possibility of constructing a renormalizable standard
model with purely fermionic matter content. The Higgs scalar is replaced by
point-like fermionic self-interactions with couplings growing large at the
Fermi scale. An analysis of the UV behavior in the point-like approximation
reveals a variety of non-Gaussian fixed points for the fermion couplings. If
real, such fixed points would imply nonperturbative renormalizability and evade
triviality of the Higgs sector. For point-like fermionic self-interactions and
weak gauge couplings, one encounters a hierarchy problem similar to the one for
a fundamental Higgs scalar.Comment: 18 pages, 4 figure
A Complete Theory of Grand Unification in Five Dimensions
A fully realistic unified theory is constructed, with SU(5) gauge symmetry
and supersymmetry both broken by boundary conditions in a fifth dimension.
Despite the local explicit breaking of SU(5) at a boundary of the dimension,
the large size of the extra dimension allows precise predictions for gauge
coupling unification, alpha_s(M_Z) = 0.118 \pm 0.003, and for Yukawa coupling
unification, m_b(M_Z) = 3.3 \pm 0.2 GeV. A complete understanding of the MSSM
Higgs sector is given; with explanations for why the Higgs triplets are heavy,
why the Higgs doublets are protected from a large tree-level mass, and why the
mu and B parameters are naturally generated to be of order the SUSY breaking
scale. All sources of d=4,5 proton decay are forbidden, while a new origin for
d=6 proton decay is found to be important. Several aspects of flavor follow
from an essentially unique choice of matter location in the fifth dimension:
only the third generation has an SU(5) mass relation, and the lighter two
generations have small mixings with the heaviest generation. The entire
superpartner spectrum is predicted in terms of only two free parameters. The
squark and slepton masses are determined by their location in the fifth
dimension, allowing a significant experimental test of the detailed structure
of the extra dimension. Lepton flavor violation is found to be generically
large in higher dimensional unified theories with high mediation scales of SUSY
breaking. In our theory this forces a common location for all three neutrinos,
predicting large neutrino mixing angles. Rates for mu -> e gamma, mu -> e e e,
mu -> e conversion and tau -> mu gamma are larger in our theory than in
conventional 4D supersymmetric GUTs. Proposed experiments probing mu -> e
transitions will probe the entire interesting parameter space of our theory.Comment: 51 pages, late
Critical exponents and equation of state of the three-dimensional Heisenberg universality class
We improve the theoretical estimates of the critical exponents for the
three-dimensional Heisenberg universality class. We find gamma=1.3960(9),
nu=0.7112(5), eta=0.0375(5), alpha=-0.1336(15), beta=0.3689(3), and
delta=4.783(3). We consider an improved lattice phi^4 Hamiltonian with
suppressed leading scaling corrections. Our results are obtained by combining
Monte Carlo simulations based on finite-size scaling methods and
high-temperature expansions. The critical exponents are computed from
high-temperature expansions specialized to the phi^4 improved model. By the
same technique we determine the coefficients of the small-magnetization
expansion of the equation of state. This expansion is extended analytically by
means of approximate parametric representations, obtaining the equation of
state in the whole critical region. We also determine a number of universal
amplitude ratios.Comment: 40 pages, final version. In publication in Phys. Rev.
Nonperturbative renormalization group approach to frustrated magnets
This article is devoted to the study of the critical properties of classical
XY and Heisenberg frustrated magnets in three dimensions. We first analyze the
experimental and numerical situations. We show that the unusual behaviors
encountered in these systems, typically nonuniversal scaling, are hardly
compatible with the hypothesis of a second order phase transition. We then
review the various perturbative and early nonperturbative approaches used to
investigate these systems. We argue that none of them provides a completely
satisfactory description of the three-dimensional critical behavior. We then
recall the principles of the nonperturbative approach - the effective average
action method - that we have used to investigate the physics of frustrated
magnets. First, we recall the treatment of the unfrustrated - O(N) - case with
this method. This allows to introduce its technical aspects. Then, we show how
this method unables to clarify most of the problems encountered in the previous
theoretical descriptions of frustrated magnets. Firstly, we get an explanation
of the long-standing mismatch between different perturbative approaches which
consists in a nonperturbative mechanism of annihilation of fixed points between
two and three dimensions. Secondly, we get a coherent picture of the physics of
frustrated magnets in qualitative and (semi-) quantitative agreement with the
numerical and experimental results. The central feature that emerges from our
approach is the existence of scaling behaviors without fixed or pseudo-fixed
point and that relies on a slowing-down of the renormalization group flow in a
whole region in the coupling constants space. This phenomenon allows to explain
the occurence of generic weak first order behaviors and to understand the
absence of universality in the critical behavior of frustrated magnets.Comment: 58 pages, 15 PS figure