464 research outputs found
Higgs Boson Bounds in Three and Four Generation Scenarios
In light of recent experimental results, we present updated bounds on the
lightest Higgs boson mass in the Standard Model (SM) and in the Minimal
Supersymmetric extension of the Standard Model (MSSM). The vacuum stability
lower bound on the pure SM Higgs boson mass when the SM is taken to be valid up
to the Planck scale lies above the MSSM lightest Higgs boson mass upper bound
for a large amount of SUSY parameter space. If the lightest Higgs boson is
detected with a mass M_{H} < 134 GeV (150 GeV) for a top quark mass M_{top} =
172 GeV (179 GeV), it may indicate the existence of a fourth generation of
fermions. The region of inconsistency is removed and the MSSM is salvagable for
such values of M_{H} if one postulates the existence of a fourth generation of
leptons and quarks with isodoublet degenerate masses M_{L} and M_{Q} such that
60 GeV 170 GeV.Comment: 7 pages, 4 figures. To be published in Physical Review
Tunneling conductance in strained graphene-based superconductor: Effect of asymmetric Weyl-Dirac fermions
Based on the BTK theory, we investigate the tunneling conductance in a
uniaxially strained graphene-based normal metal (NG)/ barrier
(I)/superconductor (SG) junctions. In the present model, we assume that
depositing the conventional superconductor on the top of the uniaxially
strained graphene, normal graphene may turn to superconducting graphene with
the Cooper pairs formed by the asymmetric Weyl-Dirac electrons, the massless
fermions with direction-dependent velocity. The highly asymmetrical velocity,
vy/vx>>1, may be created by strain in the zigzag direction near the transition
point between gapless and gapped graphene. In the case of the highly
asymmetrical velocity, we find that the Andreev reflection strongly depends on
the direction and the current perpendicular to the direction of strain can flow
in the junction as if there was no barrier. Also, the current parallel to the
direction of strain anomalously oscillates as a function of the gate voltage
with very high frequency. Our predicted result is found as quite different from
the feature of the quasiparticle tunneling in the unstrained graphene-based
NG/I/SG conventional junction. This is because of the presence of the
direction-dependent-velocity quasiparticles in the highly strained graphene
system.Comment: 18 pages, 7 Figures; Eq.13 and 14 are correcte
Form factors of the XXZ Heisenberg spin-1/2 finite chain
Form factors for local spin operators of the XXZ Heisenberg spin-1/2 finite
chain are computed. Representation theory of Drinfel'd twists for the sl2
quantum affine algebra in finite dimensional modules is used to calculate
scalar products of Bethe states (leading to Gaudin formula) and to solve the
quantum inverse problem for local spin operators in the finite XXZ chain.
Hence, we obtain the representation of the n-spin correlation functions in
terms of expectation values(in ferromagnetic reference state) of the operator
entries of the quantum monodromy matrix satisfying Yang-Baxter algebra. This
leads to the direct calculation of the form factors of the XXZ Heisenberg
spin-1/2 finite chain as determinants of usual functions of the parameters of
the model. A two-point correlation function for adjacent sites is also derived
using similar techniques.Comment: 30 pages, LaTeX2
Holographic principle in the BDL brane cosmology
We study the holographic principle in the brane cosmology. Especially we
describe how to accommodate the 5D anti de Sitter Schwarzschild (AdSS)
black hole in the Binetruy-Deffayet-Langlois (BDL) approach of brane cosmology.
It is easy to make a connection between a mass of the AdSS black hole
and a conformal field theory (CFT)-radiation dominated universe on the brane in
the moving domain wall approach. But this is not established in the BDL
approach. In this case we use two parameters in the Friedmann
equation. These arise from integration and are really related to the choice of
initial bulk matter. If one chooses a bulk energy density to account
for a mass of the AdSS black hole and the static fifth dimension, a
CFT-radiation term with comes out from the bulk
matter without introducing a localized matter distribution on the brane. This
means that the holographic principle can be established in the BDL brane
cosmology.Comment: 9 pages, a version to appear in PR
A texture of neutrino mass matrix in view of recent neutrino experimental results
In view of recent neutrino experimental results such as SNO, Super-Kamiokande
(SK), CHOOZ and neutrinoless double beta decay , we
consider a texture of neutrino mass matrix which contains three parameters in
order to explain those neutrino experimental results. We have first fitted
parameters in a model independent way with solar and atmospheric neutrino mass
squared differences and solar neutrino mixing angle which satisfy LMA solution.
The maximal value of atmospheric neutrino mixing angle comes out naturally in
the present texture. Most interestingly, fitted parameters of the neutrino mass
matrix considered here also marginally satisfy recent limit on effective
Majorana neutrino mass obtained from neutrinoless double beta decay experiment.
We further demonstrate an explicit model which gives rise to the texture
investigated by considering an gauge group with two
extra real scalar singlets and discrete symmetry. Majorana
neutrino masses are generated through higher dimensional operators at the scale
. We have estimated the scales at which singlets get VEV's and M by
comparing with the best fitted results obtained in the present work.Comment: Journal Ref.: Phys. Rev. D66, 053004 (2002
U(2)-like Flavor Symmetries and Approximate Bimaximal Neutrino Mixing
Models involving a U(2) flavor symmetry, or any of a number of its
non-Abelian discrete subgroups, can explain the observed hierarchy of charged
fermion masses and CKM angles. It is known that a large neutrino mixing angle
connecting second and third generation fields may arise via the seesaw
mechanism in these models, without a fine tuning of parameters. Here we show
that it is possible to obtain approximate bimaximal mixing in a class of models
with U(2)-like Yukawa textures. We find a minimal form for Dirac and Majorana
neutrino mass matrices that leads to two large mixing angles, and show that our
result can quantitatively explain atmospheric neutrino oscillations while
accommodating the favored, large angle MSW solution to the solar neutrino
problem. We demonstrate that these textures can arise in models by presenting a
number of explicit examples.Comment: 20 pages RevTex4, 2 figure
Leptogenesis and low energy observables in left-right symmetric models
In the context of left-right symmetric models we study the connection of
leptogenesis and low energy parameters such as neutrinoless double beta decay
and leptonic CP violation. Upon imposition of a unitarity constraint, the
neutrino parameters are significantly restricted and the Majorana phases are
determined within a narrow range, depending on the kind of solar solution. One
of the Majorana phases gets determined to a good accuracy and thereby the
second phase can be probed from the results of neutrinoless double beta decay
experiments. We examine the contributions of the solar and atmospheric mass
squared differences to the asymmetry and find that in general the solar scale
dominates. In order to let the atmospheric scale dominate, some finetuning
between one of the Majorana phases and the Dirac CP phase is required. In this
case, one of the Majorana phases is determined by the amount of CP violation in
oscillation experiments.Comment: 18 pages, 6 figures. Matches version to appear in PR
Leptogenesis and Neutrino Oscillations Within A Predictive G(224)/SO(10)-Framework
A framework based on an effective symmetry that is either G(224)= SU(2)_L x
SU(2)_R xSU(4)^c or SO(10) has been proposed (a few years ago) that
successfully describes the masses and mixings of all fermions including
neutrinos, with seven predictions, in good accord with the data. Baryogenesis
via leptogenesis is considered within this framework by allowing for natural
phases (~ 1/20-1/2) in the entries of the Dirac and Majorana mass-matrices. It
is shown that the framework leads quite naturally, for both thermal as well as
non-thermal leptogenesis, to the desired magnitude for the baryon asymmetry.
This result is obtained in full accord with the observed features of the
atmospheric and solar neutrino oscillations, as well as with those of the quark
and charged lepton masses and mixings, and the gravitino-constraint. Hereby one
obtains a unified description of fermion masses, neutrino oscillations and
baryogenesis (via leptogenesis) within a single predictive framework.Comment: Efficiency factor updated, some clarifications and new references
added. 19 page
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