arXiv subjects

Michele Lucente

Publications and source records attributed to Michele Lucente.

29 records · Page 2Linked to original sources

arXiv2018

Neutrino masses, leptogenesis and dark matter from small lepton number violation?

We consider the possibility of simultaneously addressing the baryon asymmetry of the Universe, the dark matter problem and the neutrino mass generation in minimal extensions of the Standard Model via sterile fermions with (small) total lepton number violation. Within the framework of Inverse and Linear Seesaw models, the small lepton number violating parameters set the mass scale of the active neutrinos, the efficiency of leptogenesis through a small mass splitting between pairs of sterile fermions as well as the mass scale of a sterile neutrino dark matter candidate. We provide an improved parametrization of these seesaw models taking into account existing experimental constraints and derive a linearized system of Boltzmann equations to describe the leptogenesis process, which allows for an efficient investigation of the parameter space. This in particular enables us to perform a systematic study of the strong washout regime of leptogenesis. Our study reveals that one can have a successful leptogenesis at the temperature of the electroweak scale through oscillations between two sterile states with a natural origin of the (necessary) strong degeneracy in their mass spectrum. The minimal model however requires a non-standard cosmological history to account for the relic dark matter. Finally, we discuss the prospect for neutrinoless double beta decay and for testing, in future experiments, the values of mass and different active-sterile mixings required for successful leptogenesis.

hep-ph↗

arXiv2017

Probing Dark Matter Long-lived Mediators with Solar $γ$ rays

We show that solar $γ$-ray observations can provide a complementary probe of Dark Matter in scenarios where the interactions with the Standard Model proceed via long-lived mediators. For illustration we consider a simplified model which provides solar $γ$-ray fluxes observable with the next generation $γ$-ray telescopes, while complying with the existing experimental constraints. Our results suggest that solar $γ$-ray fluxes can be orders of magnitude larger than the ones from the Galactic center, while being subject to low backgrounds.

hep-ph↗

arXiv2017

Solar $γ$-rays as a Complementary Probe of Dark Matter

We show that observations of solar $γ$-rays offer a novel probe of dark matter in scenarios where interactions with the visible sector proceed via a long-lived mediator. As a proof of principle, we demonstrate that there exists a class of models which yield solar $γ$-ray fluxes observable with the next generation of $γ$-ray telescopes, while being allowed by a variety of current experimental constraints. The parameter space allowed by big bang nucleosynthesis and beam dump experiments naturally leads to mediator lifetimes sufficient to produce observable solar $γ$-ray signals. The model allows for solar $γ$-ray fluxes up to orders of magnitude larger compared to dwarf spheroidal galaxies, without reaching equilibrium between dark matter annihilation and capture rate. Our results suggest that solar $γ$-ray observations are complementary, and in some cases superior, to existing and future dark matter detection efforts.

astro-ph.HE↗

arXiv2017

Lepton number violation as a key to low-scale leptogenesis

We explore the possibility of having a successful leptogenesis through oscillations between new sterile fermion states added to the Standard Model field content in a well motivated framework, naturally giving rise to the required mass splitting between the sterile states through a small total lepton number violation. We propose a framework with only two sterile states forming a pseudo-Dirac state, in which their mass difference as well as the smallness of the neutrino masses are due to two sources of lepton number violation with $ΔL=2$, corresponding to an Inverse Seesaw framework extended by a Linear Seesaw mass term. We also explore the pure Inverse Seesaw mechanism in its minimal version, requiring at least four new sterile states in order to comply with neutrino data. Our analytical and numerical studies reveal that one can have a successful leptogenesis at the temperature of the electroweak scale through oscillations between the two sterile states with a "natural" origin of the strong degeneracy in their mass spectrum. We also revisit the analytical expression of the baryon asymmetry of the Universe in the weak washout regime of this framework.

hep-ph↗

arXiv2016

Implication of Sterile Fermions in Particle Physics and Cosmology

The neutrino mass generation mechanism, the nature of dark matter and the origin of the baryon asymmetry of the Universe are three compelling questions that cannot be accounted for in the Standard Model of particle physics. In this thesis we focus on all these issues by providing a possible solution in terms of a minimal extension of the Standard Model, consisting in the addition of a set of sterile fermions to the field content of the theory. Sterile fermions are gauge singlet fields, that can interact via mixing with the active neutrinos. We focus on the Inverse Seesaw mechanism, which is characterised by a low (TeV or lower) new physics scale and that can be tested in current and future experimental facilities. We present the model building analysis that points towards the minimal realisations of the mechanism, and the phenomenological study in order to accommodate light neutrino masses and to impose all the relevant experimental constraints in the model, as well as the expected experimental signatures. We show the viability of the sterile neutrino hypothesis as dark matter component, together with the characteristic features of this scenario in the minimal Inverse Seesaw mechanism. The possibility of successfully accounting for the baryon asymmetry in a testable realisation of the leptogenesis mechanism is also addressed. On the other hand it is important to look for manifestations of sterile fermions in laboratory experiments. We address this point by making predictions for the expected rates of rare lepton number violating decays of vector bosons, that can be mediated by sterile fermions, as well as by studying the impact of sterile fermions on global fit of electroweak precision data.

hep-ph↗

arXiv2016

Lepton number symmetry as a way to testable leptogenesis

We propose a minimal and motivated extension of the Standard Model characterised by an approximate lepton number conservation, which is able to simultaneously generate neutrino masses and to account for a successful baryogenesis via leptogenesis. The sterile fermions involved in the leptogenesis process have masses at the GeV scale. We determine the viable parameter space that complies with both the neutrino and baryogenesis phenomenology, and analyse the different regimes for the generation of a lepton asymmetry in the early Universe (weak and strong-washout) in order to determine their testability in future experimental facilities.

hep-ph↗

arXiv2015

Loop level constraints on Seesaw neutrino mixing

We perform a detailed study of the importance of loop corrections when deriving bounds on heavy-active neutrino mixing in the context of general Seesaw mechanisms with extra heavy right-handed neutrinos. We find that, for low-scale Seesaws with an approximate $B-L$ symmetry characterized by electroweak scale Majorana masses and large Yukawas, loop corrections could indeed become relevant in a small part of the parameter space. Previous results in the literature showed that a partial cancellation between these important loop corrections and the tree level contributions could relax some constraints and lead to qualitatively different results upon their inclusion. However, we find that this cancellation can only take place in presence of large violations of the $B-L$ symmetry, that lead to unacceptably large contributions to the light neutrino masses at loop level. Thus, when we restrict our analysis of the key observables to an approximate $B-L$ symmetry so as to recover the correct values for neutrino masses, we always find loop corrections to be negligible in the regions of the parameter space preferred by data.

hep-ph↗

arXiv2015

Leptogenesis in natural low-scale seesaw mechanisms

We explore the hypothesis of having an approximate lepton number conservation as a way to achieve a successful leptogenesis in low-scale seesaw mechanisms. The smallness of the active neutrino masses, as well as a strong degeneracy in the mass spectrum of the heavy sterile states, are both consequence of the assumed approximate symmetry. We propose a minimal extension of the Standard Model in order to implement the idea, and perform an analytical and numerical study to determine the viable solutions in the model and the testability of this leptogenesis scenario in future experiments.

hep-ph↗

arXiv2015

Lepton flavor violating decays of vector quarkonia and of the $Z$ boson

We address the impact of sterile fermions on the lepton flavor violating decays of quarkonia as well as of the $Z$ boson. We compute the relevant Wilson coefficients and show that the ${\rm B}(V\to\ell_α\ell_β)$, where $V=ϕ,ψ^{(n)}$, $Υ^{(n)},Z$ can be significantly enhanced in the case of large sterile fermion masses and a non-negligible active-sterile mixing. We illustrate that feature in a specific minimal realization of the inverse seesaw mechanism, known as $(2,3)$-ISS, and in an effective model in which the presence of non-standard sterile fermions is parameterized by means of one heavy sterile (Majorana) neutrino.

hep-ph↗

arXiv2014

Dark Matter in the minimal Inverse Seesaw mechanism

We consider the possibility of simultaneously addressing the dark matter problem and neutrino mass generation in the minimal inverse seesaw realisation. The Standard Model is extended by two right-handed neutrinos and three sterile fermionic states, leading to three light active neutrino mass eigenstates, two pairs of (heavy) pseudo-Dirac mass eigenstates and one (mostly) sterile state with mass around the keV, possibly providing a dark matter candidate, and accounting for the recently observed and still unidentified monochromatic 3.5 keV line in galaxy cluster spectra. The conventional production mechanism through oscillation from active neutrinos can account only for $\sim 43\%$ of the observed relic density. This can be slightly increased to $\sim 48\%$ when including effects of entropy injection from the decay of light (with mass below 20 GeV) pseudo-Dirac neutrinos. The correct relic density can be achieved through freeze-in from the decay of heavy (above the Higgs mass) pseudo-Dirac neutrinos. This production is only effective for a limited range of masses, such that the decay occurs not too far from the electroweak phase transition. We thus propose a simple extension of the inverse seesaw framework, with an extra scalar singlet coupling to both the Higgs and the sterile neutrinos, which allows to achieve the correct dark matter abundance in a broader region of the parameter space, in particular in the low mass region for the pseudo-Dirac neutrinos.

hep-ph↗

arXiv2014

Looking for the minimal inverse seesaw realisation

In this work we consider a simple extension of the Standard Model involving additional fermionic singlets and assume an underlying inverse seesaw mechanism (with one or more right-handed neutrinos and one or more sterile fermions) for neutrino mass generation. Under the assumption that both sterile states and right-handed neutrinos are present, our goal is to determine which is the minimal inverse seesaw realisation that accounts for neutrino data while at the same time complying with all experimental requirements (electroweak precision tests and laboratory constraints). This study aims at identifying the minimal inverse seesaw realisation for the 3-flavour and for the 3 +~more-mixing schemes, the latter giving an explanation for the reactor anomalies and/or providing a possible candidate for the dark matter of the Universe. Based on a perturbative approach, our generic study shows that in the class of inverse seesaw models giving rise to a 3-flavour flavour mixing scheme, only two mass scales are relevant (the light neutrino mass scale, $m_ν$ and the mass of the right-handed neutrinos, $M_R$) while in the case of a 3 + 1-mixing scheme, an additional mass scale ($μ$ $\in[m_ν,M_R]$) is required. For each of the two obtained inverse seesaw frameworks, we conduct a thorough numerical analysis, providing predictions for the hierarchy of the light neutrino spectrum and for the effective mass in neutrinoless double beta decay.

hep-ph↗