76 research outputs found
Ultrahigh energy neutrino interactions and weak-scale string theories
It has been suggested that ultrahigh energy neutrinos can acquire
cross-sections approaching hadronic size if the string scale is as low as 1-10
TeV. In this case, the vertical air showers observed with energies above the
Greisen-Zatsepin-Kuzmin cutoff at E approximately 6x10^{19} eV could be
initiated by neutrinos which are the only known primaries able to travel long
distances unimpeded. We have calculated the neutrino-nucleon cross-section due
to the exchange of Kaluza-Klein excitations of the graviton in a field
theoretical framework. We have found that the neutrino-nucleon cross section
and the transferred energy per interaction are too small to explain vertical
showers even in the most optimistic scenario.Comment: 7 pages, 3 figures; discussion of the sensitivity of Auger/Owl
shortened, matches version to be publishe
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
Three Family Type IIB Orientifold String Vacua with Non-Abelian Wilson Lines
We address the implementation of non-Abelian Wilson lines in D=4 N=1 Type IIB
orientifold constructions. We present an explicit three-family example with the
gauge group (U(4)xU(2)xSU(2)xSU(2))^2x(U(6)xSp(4))^2 and give the particle
spectrum and the trilinear superpotential. Emphasizing the new subtleties
associated with the introduction of non-Abelian Wilson lines, we show that the
Abelian gauge anomalies are cancelled by the Green-Schwarz-type mechanism, and
calculate the Fayet-Iliopoulos terms and gauge coupling corrections. The
analysis thus sets a stage for further investigations of the phenomenological
implications of this model.Comment: 23 pages, Latex; minor changes, final version to appear in JHE
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/
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
Baryon Asymmetry and Neutrino Mixing
In theories where is a spontaneously broken local symmetry, the
cosmological baryon asymmetry can be generated by the out-of-equilibrium decay
of heavy Majorana neutrinos. We study this mechanism assuming a similar pattern
of mixings and masses for leptons and quarks, as suggested by SO(10)
unification. This implies that is broken at the unification scale
\Lambda_{\mbox{\scriptsize GUT}}\sim 10^{16} GeV, if m_{\n_\m} \sim 3\cdot
10^{-3}eV as preferred by the MSW explanation of the solar neutrino deficit.
The observed value of the baryon asymmetry, , is then
obtained without any fine tuning of parameters.Comment: latex2e, 10 pages, 3 figures, uses epsfi
Baryogenesis by Brane-Collision
We present a new scenario for baryogenesis in the context of heterotic
brane-world models. The baryon asymmetry of the universe is generated at a
small-instanton phase transition which is initiated by a moving brane colliding
with the observable boundary. We demonstrate, in the context of a simple model,
that reasonable values for the baryon asymmetry can be obtained. As a byproduct
we find a new class of moving-brane cosmological solutions in the presence of a
perfect fluid.Comment: 20 pages, Latex, 2 eps-figure
Leptogenesis with four gauge singlets
We consider a generic type of leptogenesis model which can successfully
produce the correct value of the observed baryon number to entropy ratio. The
main feature of this model is that it is a simple TeV scale model, a scale
accessible in near future machines, with a minimal particle content. Both
supersymmetric and non-supersymmetric versions of the model are feasible. This
model also gives left-handed neutrino masses compatible with all current data
from direct and indirect neutrino experiments.Comment: 13 pages, 9 PS figures, REVTe
Leptogenesis in the Light of Super-Kamiokande Data and a Realistic String Model
We discuss leptogenesis in the light of indications of neutrino masses and
mixings from Super-Kamiokande and other data on atmospheric neutrinos, as well
as the solar neutrino deficit. Neutrino masses and mixings consistent with
these data may produce in a natural and generic way a lepton asymmetry that is
suffient to provide the observed baryon asymmetry, after processing via
non-perturbative electroweak effects. We illustrate this discussion in the
framework of the string-derived flipped SU(5) model, using particle assignments
and choices of vacuum parameters that are known to give realistic masses to
quarks and charged leptons. We display one scenario for neutrino masses that
also accommodates leptogenesis.Comment: 18 pages, 1 figur
Leptogenesis in a Hybrid Texture Neutrino Mass Model
We investigate the CP asymmetry for a hybrid texture of the neutrino mass
matrix predicted by family symmetry in the context of the type-I seesaw
mechanism and examine its consequences for leptogenesis. We, also, calculate
the resulting Baryon Asymmetry of the Universe (BAU) for this texture.Comment: Accepted for publication in Mod. Phys. Lett.
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