56 research outputs found
Flavor effects in thermal leptogenesis
We review recent developments in leptogenesis on flavor effects. Their
account discloses an important connection between the matter-antimatter
asymmetry of the Universe and CP violation at low energies. Besides, they
modify the upper bound on the neutrino masses holding in the unflavored case.
In this respect, it is important to identify the exact condition for flavor
effects to be relevant and for the `fully flavored' Boltzmann equations to be
valid.Comment: 3 pages, no figure ; contribution to the proceedings of NOW 2006,
Conca Specchiulla, Sep. 9-16, 200
The minimal scenario of leptogenesis
We review the main features and results of thermal leptogenesis within the
type I seesaw mechanism, the minimal extension of the Standard Model explaining
neutrino masses and mixing. After presenting the simplest approach, the vanilla
scenario, we discuss various important developments in recent years, such as
the inclusion of lepton and heavy neutrino flavour effects, a description
beyond a hierarchical heavy neutrino mass spectrum and an improved kinetic
description within the density matrix and the closed-time-path formalisms. We
also discuss how leptogenesis can ultimately represent an important
phenomenological tool to test the seesaw mechanism and the underlying model of
new physics.Comment: 37 pages, 4 figures; invited review chapter for the "Focus on the
Origin of Matter" issue published in the New Journal of Physic
Reconciling leptogenesis with observable mu --> e gamma rates
We perform a detailed analysis of thermal leptogenesis in the framework of
seesaw models which approximately conserve lepton number. These models are
known to allow for large Yukawa couplings and a low seesaw scale in agreement
with neutrino mass constraints, and hence to lead to large lepton flavour
violating rates that can be probed experimentally. Although large Yukawa
couplings lead to (inverse) decay rates much larger than the Hubble expansion
rate, we show that the leptogenesis washout induced is generically small if the
mass splitting between the right-handed neutrinos is small enough. As a result,
large lepton flavour violating rates are compatible with successful
leptogenesis. We emphasize that this scenario does not require any particular
flavour structure. A small splitting is natural and radiatively stable in this
context because it is protected by the lepton number symmetry.Comment: One section added, a few equations and sentences rewritten,
references added, results unchange
On the Role of Low-Energy CP Violation in Leptogenesis
The link between low-energy CP violation and leptogenesis became more
accessible with the understanding of flavor effects. However, a definite
well-motivated model where such a link occurs was still lacking. Adjoint SU(5)
is a simple grand unified theory where neutrino masses are generated through
the Type I and Type III seesaw mechanisms, and the lepton asymmetry is
generated by the fermionic triplet responsible for the Type III seesaw. We
focus exclusively on the case of inverted hierarchy for neutrinos, and we show
that successful flavored leptogenesis in this theory strongly points towards
low-energy CP violation. Moreover, since the range of allowed masses for the
triplet is very restricted, we find that the discovery at the LHC of new states
present in the theory, together with proton decay and unification of gauge
couplings, can conspire to provide a hint in favor of leptogenesis.Comment: 12 pages, 6 figure
Diffuse gamma-ray constraints on dark matter revisited. I: the impact of subhalos
We make a detailed analysis of the indirect diffuse gamma-ray signals from
dark matter annihilation in the Galaxy. We include the prompt emission, as well
as the emission from inverse Compton scattering whenever the annihilation
products contain light leptons. We consider both the contribution from the
smooth dark matter halo and that from substructures. The main parameters for
the latter are the mass function index and the minimal subhalo mass. We use
recent results from N-body simulations to set the most reasonable range of
parameters, and find that the signal can be boosted by a factor ranging from 2
to 15 towards the Galactic poles, slightly more towards the Galactic
anticenter, with an important dependence on the subhalo mass index. This
uncertainty is however much less than that of the extragalactic signal studied
in the literature. We derive upper bounds on the dark matter annihilation cross
section using the isotropic gamma-ray emission measured by Fermi-LAT, for two
directions in the sky, the Galactic anticenter and the Galactic pole(s). The
former represents the lowest irreducible signal from dark matter annihilation,
and the latter is robust as the astrophysical background, dominated by the
hadronic contribution, is rather well established in that direction. Finally,
we show how the knowledge of the minimal subhalo mass, which formally depends
on the dark matter particle interactions with normal matter, can be used to
derive the mass function index.Comment: 32 pages. V2: changes to match the version accepted for publication
in JCA
Leptogenesis with heavy neutrino flavours: from density matrix to Boltzmann equations
Leptogenesis with heavy neutrino flavours is discussed within a density
matrix formalism. We write the density matrix equation, describing the
generation of the matter-antimatter asymmetry, for an arbitrary choice of the
right-handed (RH) neutrino masses. For hierarchical RH neutrino masses lying in
the fully flavoured regimes, this reduces to multiple-stage Boltzmann
equations. In this case we recover and extend results previously derived within
a quantum state collapse description. We confirm the generic existence of
phantom terms. However, taking into account the effect of gauge interactions,
we show that they are washed out at the production with a wash-out rate that is
halved compared to that one acting on the total asymmetry. In the N_1-dominated
scenario they cancel without contributing to the final baryon asymmetry. In
other scenarios they do not in general and they have to be taken into account.
We also confirm that there is a (orthogonal) component in the asymmetry
produced by the heavier RH neutrinos which completely escapes the washout from
the lighter RH neutrinos and show that phantom terms additionally contribute to
it. The other (parallel) component is washed out with the usual exponential
factor, even for weak washout. Finally, as an illustration, we study the two RH
neutrino model in the light of the above findings, showing that phantom terms
can contribute to the final asymmetry also in this case.Comment: 39 pages, 5 figures; v2: added Appendix and discussion in 2.2 on
lepton-antilepton flavour compositions difference (eq. (28) is a new result);
footnotes 4 and 5; new effect of gauge interactions in inducing wash-out of
phantom terms but with halved wash-out rate compared to the total asymmetry
wash-out rate; matches JCAP versio
The Contribution of Blazars to the Extragalactic Diffuse Gamma-ray Background and Their Future Spatial Resolution
We examine the constraints on the luminosity-dependent density evolution
model for the evolution of blazars given the observed spectrum of the diffuse
gamma-ray background (DGRB), blazar source-count distribution, and the blazar
spectral energy distribution sequence model, which relates the observed the
blazar spectrum to its luminosity. We show that the DGRB observed by the Large
Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced
entirely by gamma-ray emission from blazars and nonblazar active galactic
nuclei, and that our blazar evolution model is consistent with and constrained
by the spectrum of the DGRB and flux source-count distribution function of
blazars observed by Fermi-LAT. Our results are consistent with previous work
that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model
includes only three free parameters, and forecasts that >~ 95% of the flux from
blazars will be resolved into point sources by Fermi-LAT with 5 years of
observation, with a corresponding reduction of the flux in the DGRB by a factor
of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's
sensitivity to dark matter annihilation photons.Comment: 13 pages, 7 figures; v3: minor changes, matches version to appear in
Phys. Rev.
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