Casas-Ibarra Parametrisation Research Articles

Probing heavy triplet leptons of the type-III seesaw mechanism at future muon colliders

We study the search potential of heavy triplet leptons in a type-III seesaw mechanism at future high-energy and high-luminosity muon colliders. The impact of up-to-date neutrino oscillation results is taken into account on neutrino parameters and the consequent decay patterns of heavy leptons in a type-III seesaw. The three heavy leptons in the Casas-Ibarra parametrization with diagonal unity matrix are distinguishable in their decay modes. We consider the pair production of charged triplet leptons ${E}^{+}{E}^{\ensuremath{-}}$ through both ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ annihilation and vector boson fusion (VBF) processes. The leading-order framework of electroweak parton distribution functions is utilized to calculate the VBF cross sections. The charged triplet leptons can be probed for masses above $\ensuremath{\sim}1\text{ }\text{ }\mathrm{TeV}$. The ${E}^{+}{E}^{\ensuremath{-}}\ensuremath{\rightarrow}ZZ{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ channel can be further utilized to fully reconstruct the three triplet leptons and distinguish neutrino mass patterns. The pair production of heavy neutrinos $N$ and the associated production ${E}^{\ifmmode\pm\else\textpm\fi{}}N$ are only induced by VBF processes and lead to a lepton-number-violating (LNV) signature. We also study the search potential of LNV processes at future high-energy muon colliders with $\sqrt{s}=30\text{ }\text{ }\mathrm{TeV}$.

Type-III see-saw: Phenomenological implications of the information lost in decoupling from high-energy to low-energy

The type-III see-saw seems to explain the very minuteness of neutrino masses readily and naturally. The high-energy see-saw theories usually involve larger number of effective parameters than the physical and measurable parameters appearing in the low-energy neutrino phenomenology. Casas-Ibarra parametrisation facilitates to encode the information lost in integrating the heavy fermions out in an arbitrary complex orthogonal matrix. The CMS collaboration has already searched for triplet fermions in the type-III see-saw model with only one generation of triplet fermion flavour democratically decaying into SM leptons. We reinterpret this CMS search in the context of a realistic type-III see-saw model with two or three generations of triplet fermions, and endeavour to comprehend the implications of the foregoing matrix on the 95% CL lower limit on the mass of the triplet fermions. We also discuss the phenomenological implication of the aforesaid matrix in view of charged lepton flavour violating observables and displaced decays of the triplet fermions at colliders.

Type-I seesaw with eV-scale neutrinos

We consider seesaw type-I models including at least one (mostly-)sterile neutrino with mass at the eV scale. Three distinct situations are found, where the presence of light extra neutrinos is naturally justified by an approximately conserved lepton number symmetry. To analyse these scenarios consistently, it is crucial to employ an exact parametrisation of the full mixing matrix. We provide additional exact results, including generalised versions of the seesaw relation and of the Casas-Ibarra parametrisation, valid for every scale of seesaw. We find that the existence of a light sterile neutrino imposes an upper bound on the lightest neutrino mass. We further assess the impact of light sterile states on short- and long-baseline neutrino oscillation experiments, emphasise future detection prospects, and address CP Violation in this framework via the analysis of CP asymmetries and construction of weak basis invariants. The proposed models can accommodate enough active-sterile mixing to play a role in the explanation of short-baseline anomalies.

A Frequentist analysis of three right-handed neutrinos with GAMBIT

The extension of the Standard Model by right-handed neutrinos can not only explain the active neutrino masses via the seesaw mechanism, it is also able solve a number of long standing problems in cosmology. Especially, masses below the TeV scale are of particular interest as they can lead to a plethora of signatures in experimental searches. We present the first full frequentist analysis of the extension of the Standard Model by three right-handed neutrinos, with masses between 60 MeV and 500 GeV, using the Global and Modular BSM (beyond the Standard Model) Inference Tool GAMBIT. Our analysis is based on the Casas-Ibarra parametrisation and includes a large range of experimental constraints: active neutrino mixing, indirect constraints from, e.g., electroweak precision observables and lepton universality, and numerous direct searches for right-handed neutrinos. To study their overall effect, we derive combined profile likelihood results for the phenomenologically most relevant parameter projections. Furthermore, we discuss the role of (marginally) statistically preferred regions in the parameter space. Finally, we explore the flavour mixing pattern of the three right-handed neutrinos for different values of the lightest neutrino mass. Our results comprise the most comprehensive assessment of the model with three right-handed neutrinos model below the TeV scale so far, and provide a robust ground for exploring the impact of future constraints or detections.

A neutrino mass model with hidden U(1) gauge symmetry

We propose a realisation of inverse seesaw model controlled by hidden U(1) gauge symmetry, and discuss the impact of a bosonic dark matter (DM) candidate by imposing a Z2 parity. We present the detail of scalar spectra and apply the Casas-Ibarra parametrisation to fit the neutrino oscillation data. For this model, the allowed region is extracted to explain the observed relic density and the muon g−2 discrepancy, satisfying flavor constraints with DM involved. We interpret the DM annihilation into f¯f including all SM charged fermions and investigate the direct detection to place the bound on DM-Higgs coupling. Finally the LHC DM production is explored in light of charged lepton pair signature plus missing transverse energy.

Same-sign trilepton signal for stop quark in the presence of sneutrino dark matter

We have explored a minimal supersymmetric standard model scenario extended by one pair of gauge singlets per generation. In the model light neutrino masses and their mixings are generated via inverse seesaw mechanism. In such a scenario, a right-handed sneutrino can be the lightest supersymmetric particle and a cold Dark Matter (DM) candidate. If Casas-Ibarra parametrisation is imposed on the Dirac neutrino Yukawa coupling matrix ($Y_{\nu}$) to fit the neutrino oscillation data, the resulting $Y_{\nu}$ is highly constrained from the lepton flavor violating (LFV) decay constraints. The smallness of $Y_{\nu}$ requires the sneutrino DM to co-annihilate with other sparticle(s) in order to satisfy DM relic density constraint. We have studied sneutrino co-annihilation with wino and observed that this sneutrino-wino compressed parameter space gives rise to a novel same-sign trilepton signal for the stop quark, which is more effective than the conventional stop search channels in the present framework. We have shown that the choice of neutrino mass hierarchy strongly affects the signal event rate, making it easier to probe the scenario with inverted mass hierarchy.

Radiative corrections to light neutrino masses in low scale type I seesaw scenarios and neutrinoless double beta decay

We perform a detailed analysis of the one-loop corrections to the light neutrino mass matrix within low scale type I seesaw extensions of the Standard Model and their implications in experimental searches for neutrinoless double beta decay. We show that a sizable contribution to the effective Majorana neutrino mass from the exchange of heavy Majorana neutrinos is always possible, provided one requires a fine-tuned cancellation between the tree-level and one-loop contribution to the light neutrino masses. We quantify the level of fine-tuning as a function of the seesaw parameters and introduce a generalisation of the Casas-Ibarra parametrization of the neutrino Yukawa matrix, which easily allows to include the one-loop corrections to the light neutrino masses.

A model with a viable dark matter candidate and massive neutrinos

By extending the SM with two scalar singlets, a vector-like fermion doublet and a fermion singlet, all odd under a Z2 symmetry, it is possible to explain the correct dark matter relic density and also fulfill the main constraints for neutrino physics. In this work, we only consider the case of fermionic dark matter. We study the parameter space, first of all taking into account the relic density constraint, which gives us a set of parameters that can be used as inputs for the Casas-Ibarra parametrization in order to get the remaining parameters that also match neutrino physics. We also analyze possible constraints from lepton flavor violation processes such as μ→eγ.

Minimal3+2sterile neutrino model at LBNE

In this paper we examine the sensitivity of the Long Baseline Neutrino Oscillation Experiment to the inclusion of two new sterile neutrino flavors with masses in the eV range. We implement a model with a modified Casas-Ibarra parametrization which can accommodate medium scale mass eigenstates and introduces a new complex mixing angle. We explore the new mixing angle parameter space and demonstrate how LBNE can be used to either provide evidence for or rule out a particular model of sterile neutrinos. Certain three-flavor CP-violation scenarios cannot be distinguished from the sterile neutrinos. Constraints from the Daya Bay reactor experiment and T2K are used to help lift this degeneracy.

Casas–Ibarra parametrization and leptogenesis

The Casas–Ibarra parametrization is a description of the Dirac neutrino mass matrix MD in terms of the neutrino mixing matrix V, an orthogonal matrix O and the diagonal mass matrices of light and heavy Majorana neutrinos in the type-I seesaw mechanism. Because MD†MD is apparently independent of V but dependent on O in this parametrization, a number of authors have claimed that unflavored leptogenesis has nothing to do with CP violation at low energies. Here we question this logic by clarifying the physical meaning of O. We establish a clear relationship between O and the observable quantities, and find that O does depend on V. We show that both unflavored leptogenesis and flavored leptogenesis have no direct connection with low-energy CP violation.