573 research outputs found
Leptogenesis in SO(10) models with a left-right symmetric seesaw mechanism
We study leptogenesis in supersymmetric SO(10) models with a left-right
symmetric seesaw mechanism, including flavour effects and the contribution of
the next-to-lightest right-handed neutrino. Assuming M_D = M_u and hierarchical
light neutrino masses, we find that successful leptogenesis is possible for 4
out of the 8 right-handed neutrino mass spectra that are compatible with the
observed neutrino data. An accurate description of charged fermion masses
appears to be an important ingredient in the analysis.Comment: Submitted for the SUSY07 proceedings, 4 pages, 9 figure
Infrared Behavior of High-Temperature QCD
The damping rate \gamma_t(p) of on-shell transverse gluons with ultrasoft
momentum p is calculated in the context of next-to-leading-order
hard-thermal-loop-summed perturbation of high-temperature QCD. It is obtained
in an expansion to second order in p. The first coefficient is recovered but
that of order p^2 is found divergent in the infrared. Divergences from
light-like momenta do also occur but are circumvented. Our result and method
are critically discussed, particularly regarding a Ward identity obtained in
the literature. When enforcing the equality between \gamma_t(0) and
\gamma_l(0), a rough estimate of the magnetic mass is obtained. Carrying a
similar calculation in the context of scalar quantum electrodynamics shows that
the early ultrasoft-momentum expansion we make has little to do with the
infrared sensitivity of the result.Comment: REVTEX4, 55 page
Heavy-to-Light Meson Transitions in QCD
I discuss QCD sum rules determinations of the form factors governing the
decay . For some of these form factors the computed
dependence on the momentum transferred does not agree with the expectation from
the nearest pole dominance hypothesis. Relations are observed among the form
factors, that seem to be compatible with equations recently derived by B.Stech.
The measurement of a number of color suppressed nonleptonic B decay rates could
shed light on the accuracy of the calculation of these form factors and on the
factorization approximation.Comment: LaTex, 7 pages, 2 figures (files included). Talk given at the 6th
International Symposium on Heavy Flavours, Pisa, 6-10 June 199
Nonminimal supersymmetric standard model with lepton number violation
We carry out a detailed analysis of the nonminimal supersymmetric standard
model with lepton number violation. The model contains a unique trilinear
lepton number violating term in the superpotential which can give rise to
neutrino masses at the tree level. We search for the gauged discrete symmetries
realized by cyclic groups which preserve the structure of the associated
trilinear superpotential of this model, and which satisfy the constraints of
the anomaly cancellation. The implications of this trilinear lepton number
violating term in the superpotential and the associated soft supersymmetry
breaking term on the phenomenology of the light neutrino masses and mixing is
studied in detail. We evaluate the tree and loop level contributions to the
neutrino mass matrix in this model. We search for possible suppression
mechanism which could explain large hierarchies and maximal mixing angles.Comment: Latex file, 43 pages, 2 figure
Finite-size effects on multibody neutrino exchange
The effect of multibody massless neutrino exchanges between neutrons inside a
finite-size neutron star is studied. We use an effective Lagrangian, which
incorporates the effect of the neutrons on the neutrinos. Following Schwinger,
it is shown that the total interaction energy density is computed by comparing
the zero point energy of the neutrino sea with and without the star. It has
already been shown that in an infinite-size star the total energy due to
neutrino exchange vanishes exactly. The opposite claim that massless neutrino
exchange would produce a huge energy is due to an improper summation of an
infrared-divergent quantity. The same vanishing of the total energy has been
proved exactly in the case of a finite star in a one-dimensional toy model.
Here we study the three-dimensional case. We first consider the effect of a
sharp star border, assumed to be a plane. We find that there is a non-
vanishing of the zero point energy density difference between the inside and
the outside due to the refraction index at the border and the consequent
non-penetrating waves. An analytical and numerical calculation for the case of
a spherical star with a sharp border confirms that the preceding border effect
is the dominant one. The total result is shown to be infrared-safe, thus
confirming that there is no need to assume a neutrino mass. The ultraviolet
cut-offs, which correspond in some sense to the matching of the effective
theory with the exact one, are discussed. Finally the energy due to long
distance neutrino exchange is of the order of , i.e. negligible with respect to the neutron mass density.Comment: Latex file (Revtex), 34 pages, 8 postscripted figure
Soft leptogenesis in the inverse seesaw model
We consider leptogenesis induced by soft supersymmetry breaking terms ("soft
leptogenesis"), in the context of the inverse seesaw mechanism. In this model
there are lepton number (L) conserving and L-violating soft
supersymmetry-breaking B-terms involving the singlet sneutrinos which, together
with the -- generically small-- L-violating parameter responsible of the
neutrino mass, give a small mass splitting between the four singlet sneutrino
states of a single generation. In combination with the trilinear soft
supersymmetry breaking terms they also provide new CP violating phases needed
to generate a lepton asymmetry in the singlet sneutrino decays. We obtain that
in this scenario the lepton asymmetry is proportional to the L-conserving soft
supersymmetry-breaking B-term, and it is not suppressed by the L-violating
parameters. Consequently we find that, as in the standard see-saw case, this
mechanism can lead to sucessful leptogenesis only for relatively small value of
the relevant soft bilinear coupling. The right-handed neutrino masses can be
sufficiently low to elude the gravitino problem. Also the corresponding Yukawa
couplings involving the lightest of the right-handed neutrinos are constrained
to be \sum |Y_{1k}|^2\lesssim 10^{-7} which generically implies that the
neutrino mass spectrum has to be strongly hierarchical.Comment: 28 pages, 1 figure; some references added; final version to appear in
JHE
CP violation in scatterings, three body processes and the Boltzmann equations for leptogenesis
We obtain the Boltzmann equations for leptogenesis including decay and
scattering processes with two and three body initial or final states. We
present an explicit computation of the CP violating scattering asymmetries. We
analyze their possible impact in leptogenesis, and we discuss the validity of
their approximate expressions in terms of the decay asymmetry. In scenarios in
which the initial heavy neutrino density vanishes, the inclusion of CP
asymmetries in scatterings can enforce a cancellation between the lepton
asymmetry generated at early times and the asymmetry produced at later times.
We argue that a sizeable amount of washout is crucial for spoiling this
cancellation, and we show that in the regimes in which the washouts are
particularly weak, the inclusion of CP violation in scatterings yields a
reduction in the final value of the lepton asymmetry. In the strong washout
regimes the inclusion of CP violation in scatterings still leads to a
significant enhancement of the lepton asymmetry at high temperatures; however,
due to the independence from the early conditions that is characteristic of
these regimes, the final value of the lepton asymmetry remains approximately
unchanged.Comment: 24 pages, 6 figures. One appendix added. Some numerical results and
corresponding figures (mainly fig. 3) corrected. Final version to be
published in JHE
Comment on ``Relativistic kinetic equations for electromagnetic, scalar and pseudoscalar interactions''
It is found that the extra quantum constraints to the spinor components of
the equal-time Wigner function given in a recent paper by Zhuang and Heinz
should vanish identically. We point out here the origin of the error and give
an interpretation of the result. However, the principal idea of obtaining a
complete equal-time transport theory by energy averaging the covariant theory
remains valid. The classical transport equation for the spin density is also
found to be incorrect. We give here the correct form of that equation and
discuss briefly its structure.Comment: 5 pages LaTe
Supersymmetric Leptogenesis
We study leptogenesis in the supersymmetric standard model plus the seesaw.
We identify important qualitative differences that characterize supersymmetric
leptogenesis with respect to the non-supersymmetric case. The lepton number
asymmetries in fermions and scalars do not equilibrate, and are related via a
non-vanishing gaugino chemical potential. Due to the presence of new anomalous
symmetries, electroweak sphalerons couple to winos and higgsinos, and QCD
sphalerons couple to gluinos, thus modifying the corresponding chemical
equilibrium conditions. A new constraint on particles chemical potentials
corresponding to an exactly conserved -charge, that also involves the number
density asymmetry of the heavy sneutrinos, appears. These new ingredients
determine the matrices that mix up the density asymmetries of the
lepton flavours and of the heavy sneutrinos. We explain why in all temperature
ranges the particle thermodynamic system is characterized by the same number of
independent quantities. Numerical differences with respect to usual treatment
remain at the level.Comment: 30 pages, 2 figures. Typos corrected, one reference added. Version
published in JCA
Leptonic CP violation: zero, maximal or between the two extremes
Discovery of the CP-violation in the lepton sector is one of the challenges
of the particle physics. We search for possible principles, symmetries and
phenomenological relations that can lead to particular values of the
CP-violating Dirac phase, . In this connection we discuss two extreme
cases: the zero phase, , and the maximal CP-violation, , and relate them to the peculiar pattern of the neutrino mixing. The
maximal CP-violation can be related to the reflection
symmetry. We study various aspects of this symmetry and introduce a generalized
reflection symmetry that can lead to an arbitrary phase that depends on the
parameter of the symmetry transformation. The generalized reflection symmetry
predicts a simple relation between the Dirac and Majorana phases. We also
consider the possibility of certain relations between the CP-violating phases
in the quark and lepton sectors.Comment: 34 pages, no figures; v3: version appeared in JHE
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