80 research outputs found
Four Generations: SUSY and SUSY Breaking
We revisit four generations within the context of supersymmetry. We compute
the perturbativity limits for the fourth generation Yukawa couplings and show
that if the masses of the fourth generation lie within reasonable limits of
their present experimental lower bounds, it is possible to have perturbativity
only up to scales around 1000 TeV. Such low scales are ideally suited to
incorporate gauge mediated supersymmetry breaking, where the mediation scale
can be as low as 10-20 TeV. The minimal messenger model, however, is highly
constrained. While lack of electroweak symmetry breaking rules out a large part
of the parameter space, a small region exists, where the fourth generation stau
is tachyonic. General gauge mediation with its broader set of boundary
conditions is better suited to accommodate the fourth generation.Comment: 27 pages, 5 figure
Long Lived Fourth Generation and the Higgs
A chiral fourth generation is a simple and well motivated extension of the
standard model, and has important consequences for Higgs phenomenology. Here we
consider a scenario where the fourth generation neutrinos are long lived and
have both a Dirac and Majorana mass term. Such neutrinos can be as light as 40
GeV and can be the dominant decay mode of the Higgs boson for Higgs masses
below the W-boson threshold. We study the effect of the Majorana mass term on
the Higgs branching fractions and reevaluate the Tevatron constraints on the
Higgs mass. We discuss the prospects for the LHC to detect the semi-invisible
Higgs decays into fourth generation neutrino pairs. Under the assumption that
the lightest fourth generation neutrino is stable, it's thermal relic density
can be up to 20% of the observed dark matter density in the universe. This is
in agreement with current constraints on the spin dependent neutrino-neutron
cross section, but can be probed by the next generation of dark matter direct
detection experiments.Comment: v1: 19 pages, 5 figures; v2: References added; v3: version to appear
in JHE
Composite Dirac Neutrinos
We present a mechanism that naturally produces light Dirac neutrinos. The
basic idea is that the right-handed neutrinos are composite. Any realistic
composite model must involve `hidden flavor' chiral symmetries. In general some
of these symmetries may survive confinement, and in particular, one of them
manifests itself at low energy as an exact symmetry. Dirac neutrinos are
therefore produced. The neutrinos are naturally light due to compositeness. In
general, sterile states are present in the model, some of them can naturally be
warm dark matter candidates.Comment: 12 pages; Sec. IIC updated; minor corrections; published versio
Fermion Masses in Emergent Electroweak Symmetry Breaking
We consider the generation of fermion masses in an emergent model of
electroweak symmetry breaking with composite gauge bosons. A universal
bulk fermion profile in a warped extra dimension is used for all fermion
flavors. Electroweak symmetry is broken at the UV (or Planck) scale where
boundary mass terms are added to generate the fermion flavor structure. This
leads to flavor-dependent nonuniversality in the gauge couplings. The effects
are suppressed for the light fermion generations but are enhanced for the top
quark where the and couplings can deviate at the
level in the minimal setup. By the AdS/CFT correspondence our model
implies that electroweak symmetry is not a fundamental gauge symmetry. Instead
the Standard Model with massive fermions and gauge bosons is an effective
chiral Lagrangian for some underlying confining strong dynamics at the TeV
scale, where mass is generated without a Higgs mechanism.Comment: modified discussion in Sec 3.1, version published in JHE
Higgs and Dark Matter Hints of an Oasis in the Desert
Recent LHC results suggest a standard model (SM)-like Higgs boson in the
vicinity of 125 GeV with no clear indications yet of physics beyond the SM. At
the same time, the SM is incomplete, since additional dynamics are required to
accommodate cosmological dark matter (DM). In this paper we show that
interactions between weak scale DM and the Higgs which are strong enough to
yield a thermal relic abundance consistent with observation can easily
destabilize the electroweak vacuum or drive the theory into a non-perturbative
regime at a low scale. As a consequence, new physics--beyond the DM
itself--must enter at a cutoff well below the Planck scale and in some cases as
low as O(10 - 1000 TeV), a range relevant to indirect probes of flavor and CP
violation. In addition, this cutoff is correlated with the DM mass and
scattering cross-section in a parameter space which will be probed
experimentally in the near term. Specifically, we consider the SM plus
additional spin 0 or 1/2 states with singlet, triplet, or doublet electroweak
quantum numbers and quartic or Yukawa couplings to the Higgs boson. We derive
explicit expressions for the full two-loop RGEs and one-loop threshold
corrections for these theories.Comment: 29 pages, 13 figure
Three-loop \beta-functions for top-Yukawa and the Higgs self-interaction in the Standard Model
We analytically compute the dominant contributions to the \beta-functions for
the top-Yukawa coupling, the strong coupling and the Higgs self-coupling as
well as the anomalous dimensions of the scalar, gluon and quark fields in the
unbroken phase of the Standard Model at three-loop level. These are mainly the
QCD and top-Yukawa corrections. The contributions from the Higgs
self-interaction which are negligible for the running of the top-Yukawa and the
strong coupling but important for the running of the Higgs self-coupling are
also evaluated.Comment: 22 pages, 7 figures. Few extra citations are added; the plots are
improved. Results in computer readable form can be retrieved from
http://www-ttp.particle.uni-karlsruhe.de/Progdata/ttp12/ttp12-012
Impact of massive neutrinos on the Higgs self-coupling and electroweak vacuum stability
The presence of right-handed neutrinos in the type I seesaw mechanism may
lead to significant corrections to the RG evolution of the Higgs self-coupling.
Compared to the Standard Model case, the Higgs mass window can become narrower,
and the cutoff scale become lower. Naively, these effects decrease with
decreasing right-handed neutrino mass. However, we point out that the unknown
Dirac Yukawa matrix may impact the vacuum stability constraints even in the low
scale seesaw case not far away from the electroweak scale, hence much below the
canonical seesaw scale of 10^15 GeV. This includes situations in which
production of right-handed neutrinos at colliders is possible. We illustrate
this within a particular parametrization of the Dirac Yukawas and with explicit
low scale seesaw models. We also note the effect of massive neutrinos on the
top quark Yukawa coupling, whose high energy value can be increased with
respect to the Standard Model case.Comment: 17 pages, 7 figures, minor revisions, version to appear in JHE
Higgs mass and vacuum stability in the Standard Model at NNLO
We present the first complete next-to-next-to-leading order analysis of the
Standard Model Higgs potential. We computed the two-loop QCD and Yukawa
corrections to the relation between the Higgs quartic coupling (lambda) and the
Higgs mass (Mh), reducing the theoretical uncertainty in the determination of
the critical value of Mh for vacuum stability to 1 GeV. While lambda at the
Planck scale is remarkably close to zero, absolute stability of the Higgs
potential is excluded at 98% C.L. for Mh < 126 GeV. Possible consequences of
the near vanishing of lambda at the Planck scale, including speculations about
the role of the Higgs field during inflation, are discussed.Comment: 35 pages, 8 figures. Final published version, misprints fixed,
figures update
Universality of Strength of Yukawa Couplings, Quark Singlets and Strength of CP Violation
We analyse the strength of CP violation in an extension of the standard model
with an extra vector-like singlet quark, in the framework of the
hypothesis of universality of strength of Yukawa couplings connecting standard
quarks. We show that the correct pattern of quark masses and mixing can be
obtained, including the observed strength of CP violation.Comment: 9 pages, no figure
Sensitivity analysis for generalized quasi-variational relation problems in locally G-convex spaces
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