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Escudero, M., Lopez-Pavon, J., Rius, N., & Sandner, S. (2020). Relaxing cosmological neutrino mass bounds with unstable neutrinos. J. High Energy Phys., 12(12), 119–44pp.
Abstract: At present, cosmological observations set the most stringent bound on the neutrino mass scale. Within the standard cosmological model (Lambda CDM), the Planck collaboration reports Sigma m(v)< 0.12 eV at 95 % CL. This bound, taken at face value, excludes many neutrino mass models. However, unstable neutrinos, with lifetimes shorter than the age of the universe <tau>(nu) less than or similar to t(U), represent a particle physics avenue to relax this constraint. Motivated by this fact, we present a taxonomy of neutrino decay modes, categorizing them in terms of particle content and final decay products. Taking into account the relevant phenomenological bounds, our analysis shows that 2-body decaying neutrinos into BSM particles are a promising option to relax cosmological neutrino mass bounds. We then build a simple extension of the type I seesaw scenario by adding one sterile state nu (4) and a Goldstone boson phi, in which nu (i)-> nu (4)phi decays can loosen the neutrino mass bounds up to Sigma m(v) similar to 1 eV, without spoiling the light neutrino mass generation mechanism. Remarkably, this is possible for a large range of the right-handed neutrino masses, from the electroweak up to the GUT scale. We successfully implement this idea in the context of minimal neutrino mass models based on a U(1)(mu-tau) flavor symmetry, which are otherwise in tension with the current bound on Sigma m(v).
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Hernandez, P., Kekic, M., Lopez-Pavon, J., Racker, J., & Salvado, J. (2016). Testable baryogenesis is in seesaw models. J. High Energy Phys., 08(8), 157–29pp.
Abstract: We revisit the production of baryon asymmetries in the minimal type I seesaw model with heavy Majorana singlets in the GeV range. In particular we include “washout” effects from scattering processes with gauge bosons, Higgs decays and inverse decays, besides the dominant top scatterings. We show that in the minimal model with two singlets, and for an inverted light neutrino ordering, future measurements from SHiP and neutrinoless double beta decay could in principle provide sufficient information to predict the matter-antimatter asymmetry in the universe. We also show that SHiP measurements could provide very valuable information on the PMNS CP phases.
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Donini, A., Hernandez, P., Lopez-Pavon, J., Maltoni, M., & Schwetz, T. (2012). The minimal 3+2 neutrino model versus oscillation anomalies. J. High Energy Phys., 07(7), 161–20pp.
Abstract: We study the constraints imposed by neutrino oscillation experiments on the minimal extension of the Standard Model that can explain neutrino masses, which requires the addition of just two singlet Weyl fermions. The most general renormalizable couplings of this model imply generically four massive neutrino mass eigenstates while one remains massless: it is therefore a minimal 3+2 model. The possibility to account for the confirmed solar, atmospheric and long-baseline oscillations, together with the LSND/MiniBooNE and reactor anomalies is addressed. We find that the minimal model can fit oscillation data including the anomalies better than the standard 3 nu model and similarly to the 3 + 2 phenomenological models, even though the number of free parameters is much smaller than in the latter. Accounting for the anomalies in the minimal model favours a normal hierarchy of the light states and requires a large reactor angle, in agreement with recent measurements. Our analysis of the model employs a new parametrization of seesaw models that extends the Casas-Ibarra one to regimes where higher order corrections in the light-heavy mixings are significant.
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Abdullahi, A. M. et al, & Lopez-Pavon, J. (2023). The present and future status of heavy neutral leptons. J. Phys. G, 50(2), 020501–100pp.
Abstract: The existence of nonzero neutrino masses points to the likely existence of multiple Standard Model neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as heavy neutral leptons (HNLs). In this white paper, we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding of key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.
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Caputo, A., Hernandez, P., Kekic, M., Lopez-Pavon, J., & Salvado, J. (2017). The seesaw path to leptonic CP violation. Eur. Phys. J. C, 77(4), 258–7pp.
Abstract: Future experiments such as SHiP and highintensity e(+)e(-) colliders will have a superb sensitivity to heavy Majorana neutrinos with masses below M-Z. We show that the measurement of the mixing to electrons and muons of one such state could establish the existence of CP violating phases in the neutrino mixing matrix, in the context of low-scale seesaw models. We quantify in the minimal model the CP reach of these future experiments, and demonstrate that CP violating phases in the mixing matrix could be established at 5 sigma CL in a very significant fraction of parameter space.
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Caputo, A., Hernandez, P., Lopez-Pavon, J., & Salvado, J. (2017). The seesaw portal in testable models of neutrino masses. J. High Energy Phys., 06(6), 112–20pp.
Abstract: A Standard Model extension with two Majorana neutrinos can explain the measured neutrino masses and mixings, and also account for the matter-antimatter asymmetry in a region of parameter space that could be testable in future experiments. The testability of the model relies to some extent on its minimality. In this paper we address the possibility that the model might be extended by extra generic new physics which we parametrize in terms of a low-energy effective theory. We consider the effects of the operators of the lowest dimensionality, d = 5, and evaluate the upper bounds on the coefficients so that the predictions of the minimal model are robust. One of the operators gives a new production mechanism for the heavy neutrinos at LHC via higgs decays. The higgs can decay to a pair of such neutrinos that, being long-lived, leave a powerful signal of two displaced vertices. We estimate the LHC reach to this process.
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