6,030 research outputs found
Flavour physics without flavour symmetries
We quantitatively analyze a quark-lepton flavour model derived from a
six-dimensional supersymmetric theory with gauge symmetry,
compactified on an orbifold with magnetic flux. Two bulk -plets
charged under the provide the three quark-lepton generations whereas two
uncharged -plets yield two Higgs doublets. At the orbifold fixed
points mass matrices are generated with rank one or two. Moreover, the zero
modes mix with heavy vectorlike split multiplets. The model possesses no
flavour symmetries. Nevertheless, there exist a number of relations between
Yukawa couplings, remnants of the underlying GUT symmetry and the wave function
profiles of the zero modes, which lead to a prediction of the light neutrino
mass scale, eV and heavy Majorana neutrino masses in
the range from GeV to GeV. The model successfully includes
thermal leptogenesis.Comment: Minor additions; Published versio
Proton decay in flux compactifications
We study proton decay in a six-dimensional orbifold GUT model with gauge
group . Magnetic flux in the compact dimensions
determines the multiplicity of quark-lepton generations, and it also breaks
supersymmetry by giving universal GUT scale masses to scalar quarks and
leptons. The model can successfully account for quark and lepton masses and
mixings. Our analysis of proton decay leads to the conclusion that the proton
lifetime must be close to the current experimental lower bound. Moreover, we
find that the branching ratios for the decay channels
and are of similar size, in fact the latter one can
even be dominant. This is due to flavour non-diagonal couplings of heavy vector
bosons together with large off-diagonal Higgs couplings, which appears to be a
generic feature of flux compactifications.Comment: 26 pages, 3 figures, 2 table
The Neutrino Mass Window for Baryogenesis
Interactions of heavy Majorana neutrinos in the thermal phase of the early
universe may be the origin of the cosmological matter-antimatter asymmetry.
This mechanism of baryogenesis implies stringent constraints on light and heavy
Majorana neutrino masses. We derive an improved upper bound on the CP asymmetry
in heavy neutrino decays which, together with the kinetic equations, yields an
upper bound on all light neutrino masses of 0.1 eV. Lepton number changing
processes at temperatures above the temperature T_B of baryogenesis can erase
other, pre-existing contributions to the baryon asymmetry. We find that these
washout processes become very efficient if the effective neutrino mass
\tilde{m}_1 is larger than m_* \simeq 10^{-3} eV. All memory of the initial
conditions is then erased. Hence, for neutrino masses in the range from (\Delta
m^2_sol)^{1/2} \simeq 8*10^{-3} eV to (\Delta m^2_atm)^{1/2} \simeq 5*10^{-2}
eV, which is suggested by neutrino oscillations, leptogenesis emerges as the
unique source of the cosmological matter-antimatter asymmetry.Comment: 29 pages, 12 figures include
Some Aspects of Thermal Leptogenesis
Properties of neutrinos may be the origin of the matter-antimatter asymmetry
of the universe. In the seesaw model for neutrino masses this leads to
important constraints on the properties of light and heavy neutrinos. In
particular, an upper bound on the light neutrino masses of 0.1 eV can be
derived. We review the present status of thermal leptogenesis with emphasis on
the theoretical uncertainties and discuss some implications for lepton and
quark mass hierarchies, CP violation and dark matter. We also comment on the
`leptogenesis conspiracy', the remarkable fact that neutrino masses may lie in
the range where leptogenesis works best.Comment: 23 pages, 5 figures, submitted to the Focus on Neutrino Physics issue
of the New Journal of Physics, edited by F. Halzen, M. Lindner and A. Suzuk
Cosmic Microwave Background, Matter-Antimatter Asymmetry and Neutrino Masses
We study the implications of thermal leptogenesis for neutrino parameters.
Assuming that decays of N_1, the lightest of the heavy Majorana neutrinos,
initiate baryogenesis, we show that the final baryon asymmetry is determined by
only four parameters: the CP asymmetry epsilon_1, the heavy neutrino mass M_1,
the effective light neutrino mass \tilde{m}_1, and the quadratic mean \bar{m}
of the light neutrino masses. Imposing the CMB measurement of the baryon
asymmetry as constraint on the neutrino parameters, we show, in a model
independent way, that quasi-degenerate neutrinos are incompatible with thermal
leptogenesis. For maximal CP asymmetry epsilon_1, and neutrino masses in the
range from (\Delta m^2_{sol})^{1/2} to (\Delta m^2_{atm})^{1/2}, the
baryogenesis temperature is T_B = O(10^{10}) GeV.Comment: 28 pages, 14 figures included; v2: erratum added, M_1 lower bound in
the strong wash-out regime (see Eq. (63)) relaxed by a factor 2/
Stau-catalyzed Li Production in Big-Bang Nucleosynthesis
If the gravitino mass is in the region from a few GeV to a few 10's GeV, the
scalar lepton X such as stau is most likely the next lightest supersymmetry
particle. The negatively charged and long-lived X^- may form a Coulomb bound
state (A X) with a nucleus A and may affect the big-bang nucleosynthesis
through catalyzed fusion process. We calculate a production cross section of
Li6 from the catalyzed fusion (He4 X^-) + d \to Li6 + X^- by solving the
Schr\"{o}dinger equation exactly for three-body system of He4, d, and X. We
utilize the state-of-the-art coupled-channel method, which is known to be very
accurate to describe other three-body systems in nuclear and atomic reactions.
The importance of the use of appropriate nuclear potential and the exact
treatment of the quantum tunneling in the fusion process are emphasized. We
find that the astrophysical S-factor at the Gamow peak corresponding to T=10
keV is 0.038 MeV barn. This leads to the Li6 abundance from the catalyzed
process as Li6|_{CBBN}\simeq 4.3\times 10^{-11} (D/2.8\times 10^{-5})
([n_{X^-}/s]/10^{-16}) in the limit of long lifetime of X. Particle physics
implication of this result is also discussed.Comment: 16 pages, 7 figure
Non equilibrium dynamics of mixing, oscillations and equilibration: a model study
The non-equilibrium dynamics of mixing, oscillations and equilibration is
studied in a field theory of flavored neutral mesons that effectively models
two flavors of mixed neutrinos, in interaction with other mesons that represent
a thermal bath of hadrons or quarks and charged leptons. This model describes
the general features of neutrino mixing and relaxation via charged currents in
a medium. The reduced density matrix and the non-equilibrium effective action
that describes the propagation of neutrinos is obtained by integrating out the
bath degrees of freedom. We obtain the dispersion relations, mixing angles and
relaxation rates of ``neutrino'' quasiparticles. The dispersion relations and
mixing angles are of the same form as those of neutrinos in the medium, and the
relaxation rates are given by where
are the relaxation rates of the flavor fields in
\emph{absence} of mixing, and is the mixing angle in the medium.
A Weisskopf-Wigner approximation that describes the asymptotic time evolution
in terms of a non-hermitian Hamiltonian is derived. At long time
``neutrinos'' equilibrate with the bath. The equilibrium
density matrix is nearly diagonal in the basis of eigenstates of an
\emph{effective Hamiltonian that includes self-energy corrections in the
medium}. The equilibration of ``sterile neutrinos'' via active-sterile mixing
is discussed.Comment: 28 pages, 3 figures, version to appear in PR
Leptogenesis and Low-energy Observables
We relate leptogenesis in a class of theories to low-energy experimental
observables: quark and lepton masses and mixings. With reasonable assumptions
motivated by grand unification, one can show that the CP-asymmetry parameter
takes a universal form. Furthermore the dilution mass is related to the light
neutrino masses. Overall, these models offer a natural explanation for a lepton
asymmetry in the early universe.Comment: 10 pages, revised discussion on light neutrino masse
Neutrino masses and mixing angles from leptoquark interactions
In this paper we show that the mixing between leptoquarks (LQ's) from
different multiplets can generate a non-trivial Majorana mass matrix
for neutrinos through one loop self energy diagrams. Such mixing can arise from
gauge invariant and renormalizable LQ-Higgs interaction terms after EW symmetry
breaking. We use the experimental indication on neutrino oscillation to find
constraints on specific combinations of LQ couplings to quark-lepton pairs and
to the SM higgs boson. These constraints are compared with the ones from
.Comment: The expressions for majorana mass matrix of neutrinos have been
corrected so that they are symmetric. Final version to be published in
Physical Review
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