
Arbelaez, C., Carcamo Hernandez, A. E., Cepedello, R., Hirsch, M., & Kovalenko, S. (2019). Radiative typeI seesaw neutrino masses. Phys. Rev. D, 100(11), 115021–7pp.
Abstract: We discuss a radiative typeI seesaw. In these models, the radiative generation of Dirac neutrino masses allows to explain the smallness of the observed neutrino mass scale for rather light righthanded neutrino masses in a type1 seesaw. We first present the general idea in a modelindependent way. This allows us to estimate the typical scale of righthanded neutrino mass as a function of the number of loops. We then present two example models, at the one and twoloop level, which we use to discuss neutrino masses and leptonflavorviolating constraints in more detail. For the twoloop example, righthanded neutrino masses must lie below 100 GeV, thus making this class of models testable in heavy neutral lepton searches.



Arbelaez, C., Carcamo Hernandez, A. E., Cepedello, R., Kovalenko, S., & Schmidt, I. (2020). Sequentially loop suppressed fermion masses from a single discrete symmetry. J. High Energy Phys., 06(6), 043–24pp.
Abstract: We propose a systematic and renormalizable sequential loop suppression mechanism to generate the hierarchy of the Standard Model fermion masses from one discrete symmetry. The discrete symmetry is sequentially softly broken in order to generate oneloop level masses for the bottom, charm, tau and muon leptons and twoloop level masses for the lightest Standard Model charged fermions. The tiny masses for the light active neutrinos are produced from radiative typeI seesaw mechanism, where the Dirac mass terms are effectively generated at twoloop level.



Arbelaez, C., Cepedello, R., Fonseca, R. M., & Hirsch, M. (2020). (g2) anomalies and neutrino mass. Phys. Rev. D, 102(7), 075005–14pp.
Abstract: Motivated by the experimentally observed deviations from standard model predictions, we calculate the anomalous magnetic moments a(alpha) = (g – 2)(alpha) for a = e, μin a neutrino mass model originally proposed by Babu, Nandi, and Tavartkiladze (BNT). We discuss two variants of the model: the original model, and a minimally extended version with an additional hyperchargezero triplet scalar. While the original BNT model can explain a(mu), only the variant with the triplet scalar can explain both experimental anomalies. The heavy fermions of the model can be produced at the highluminosity LHC, and in the part of parameter space where the model explains the experimental anomalies it predicts certain specific decay patterns for the exotic fermions.



Arbelaez, C., Cepedello, R., Helo, J. C., Hirsch, M., & Kovalenko, S. (2022). How many 1loop neutrino mass models are there? J. High Energy Phys., 08(8), 023–29pp.
Abstract: It is wellknown that at treelevel the d = 5 Weinberg operator can be generated in exactly three different ways, the famous seesaw models. In this paper we study the related question of how many phenomenologically consistent 1loop models one can construct at d=5. First, we discuss that there are two possible classes of 1loop neutrino mass models, that allow avoiding stable charged relics: (i) models with dark matter candidates and (ii) models with “exits”. Here, we define “exits” as particles that can decay into standard model fields. Considering 1loop models with new scalars and fermions, we find in the dark matter class a total of (115+203) models, while in the exit class we find (38+368) models. Here, 115 is the number of DM models, which require a stabilizing symmetry, while 203 is the number of models which contain a dark matter candidate, which maybe accidentally stable. In the exit class the 38 refers to models, for which one (or two) of the internal particles in the loop is a SM field, while the 368 models contain only fields beyond the SM (BSM) in the neutrino mass diagram. We then study the RGE evolution of the gauge couplings in all our 1loop models. Many of the models in our list lead to Landau poles in some gauge coupling at rather low energies and there is exactly one model which unifies the gauge couplings at energies above 10(15) GeV in a numerically acceptable way.



Arbelaez, C., Cottin, G., Helo, J. C., & Hirsch, M. (2020). Longlived charged particles and multilepton signatures from neutrino mass models. Phys. Rev. D, 101(9), 095033–13pp.
Abstract: Lepton number violation (LNV) is usually searched for by the LHC collaborations using the samesign dilepton plus jet signature. In this paper, we discuss multilepton signals of LNV that can arise with experimentally interesting rates in certain loop models of neutrino mass generation. Interestingly, in such models, the observed smallness of the active neutrino masses, together with the high multiplicity of the final states, leads in large parts of the viable parameter space of such models to the prediction of longlived charged particles, which leave highly ionizing tracks in the detectors. We focus on one particular oneloop neutrino mass model in this class and discuss its LHC phenomenology in some detail.



Arbelaez, C., Dib, C., MonsalvezPozo, K., & Schmidt, I. (2021). QuasiDirac neutrinos in the linear seesaw model. J. High Energy Phys., 07(7), 154–22pp.
Abstract: We implement a minimal linear seesaw model (LSM) for addressing the QuasiDirac (QD) behaviour of heavy neutrinos, focusing on the mass regime of MN less than or similar to MW. Here we show that for relatively low neutrino masses, covering the few GeV range, the samesign to oppositesign dilepton ratio, Rll, can be anywhere between 0 and 1, thus signaling a QuasiDirac regime. Particular values of Rll are controlled by the width of the QD neutrino and its mass splitting, the latter being equal to the lightneutrino mass m(nu) in the LSM scenario. The current upper bound on m(nu 1) together with the projected sensitivities of current and future UN l(2) experimental measurements, set stringent constraints on our lowscale QD mass regime. Some experimental prospects of testing the model by LHC displaced vertex searches are also discussed.



Arbelaez, C., Fonseca, R. M., Romao, J. C., & Hirsch, M. (2013). Supersymmetric SO(10)inspired GUTs with sliding scales. Phys. Rev. D, 87(7), 075010–19pp.
Abstract: We construct lists of supersymmetric models with extended gauge groups at intermediate steps, all of which are inspired by SO(10) unification. We consider three different kinds of setups: (i) the model has exactly one additional intermediate scale with a leftright (LR) symmetric group; (ii) SO(10) is broken to the LR group via an intermediate PatiSalam scale; and (iii) the LR group is broken into SU(3)(c) X SU(2)(L) X U(1)(R) X U(1)(BL), before breaking to the standard model (SM) group. We use sets of conditions, which we call the “sliding mechanism,” which yield unification with the extended gauge group(s) allowed at arbitrary intermediate energy scales. All models thus can have new gauge bosons within the reach of the LHC, in principle. We apply additional conditions, such as perturbative unification, renormalizability and anomaly cancellation and find that, despite these requirements, for the ansatz (i) with only one additional scale still around 50 different variants exist that can have a LR symmetry below 10 TeV. For the more complicated schemes (ii) and (iii) literally thousands of possible variants exist, and for scheme (ii) we have also found variants with very low PatiSalam scales. We also discuss possible experimental tests of the models from measurements of supersymmetry masses. Assuming mSugra boundary conditions we calculate certain combinations of soft terms, called “invariants,” for the different classes of models. Values for all the invariants can be classified into a small number of sets, which contain information about the class of models and, in principle, the scale of beyondminimal supersymmetric extension of the Standard Model physics, even in case the extended gauge group is broken at an energy beyond the reach of the LHC.



Arbelaez, C., Gonzalez, M., Hirsch, M., & Kovalenko, S. G. (2016). QCD corrections and longrange mechanisms of neutrinoless double beta decay. Phys. Rev. D, 94(9), 096014–5pp.
Abstract: Recently it has been demonstrated that QCD corrections are numerically important for shortrange mechanisms (SRM) of neutrinoless double beta decay (0 nu beta beta) mediated by heavy particle exchange. This is due to the effect of color mismatch for certain effective operators, which leads to mixing between different operators with vastly different nuclear matrix elements (NMEs). In this note we analyze the QCD corrections for longrange mechanisms (LRM), due to diagrams with lightneutrino exchange between a Standard Model (VA)chi(VA) and a beyond the SM lepton number violating vertex. We argue that in contrast to the SRM in the LRM case, there is no operator mixing from colormismatched operators. This is due to a combined effect of the nuclear shortrange correlations and color invariance. As a result, the QCD corrections to the LRM amount to an effect no more than 60%, depending on the operator in question. Although less crucial, taken into account QCD running makes theoretical predictions for 0 nu beta betadecay more robust also for LRM diagrams. We derive the current experimental constraints on the Wilson coefficients for all LRM effective operators.



Arbelaez, C., Gonzalez, M., Kovalenko, S. G., & Hirsch, M. (2017). QCDimproved limits from neutrinoless double beta decay. Phys. Rev. D, 96(1), 015010–12pp.
Abstract: We analyze the impact of QCD corrections on limits derived from neutrinoless double beta decay (0 nu beta beta ). As demonstrated previously, the effect of the color mismatch arising from loops with gluons linking the quarks from different colorsinglet currents participating in the effective operators has a dramatic impact on the predictions for some particular Wilson coefficients. Here, we consider all possible contributions from heavy particle exchange, i.e. the socalled shortrange mechanism of 0 nu beta beta decay. All highscale models (HSM) in this class match at some scale around a similar to few TeV with the corresponding effective theory, containing a certain set of effective dimension9 operators. Many of these HSM receive contributions from more than one of the basic operators and we calculate limits on these models using the latest experimental data. We also show with one nontrivial example, how to derive limits on more complicated models, in which many different Feynman diagrams contribute to 0 nu beta beta decay, using our general method.



Arbelaez, C., Helo, J. C., & Hirsch, M. (2019). Longlived heavy particles in neutrino mass models. Phys. Rev. D, 100(5), 055001–15pp.
Abstract: All extensions of the standard model that generate Majorana neutrino masses at the electroweak scale introduce some heavy mediators, either fermions and/or scalars, weakly coupled to leptons. Here, by “heavy,” we mean implicitly the mass range between a few 100 GeV up to, say, roughly 2 TeV, such that these particles can be searched for at the LHC. We study decay widths of these mediators for several different treelevel neutrino mass models. The models we consider range from the simplest d = 5 seesaw up to d = 11 neutrino mass models. For each of the models, we identify the most interesting parts of the parameter space, where the heavy mediator fields are particularly long lived and can decay with experimentally measurable decay lengths. One has to distinguish two different scenarios, depending on whether fermions or scalars are the lighter of the heavy particles. For fermions, we find that the decay lengths correlate with the inverse of the overall neutrino mass scale. Thus, since no lower limit on the lightest neutrino mass exists, nearly arbitrarily long decay lengths can be obtained for the case in which fermions are the lighter of the heavy particles. For charged scalars, on the other hand, there exists a maximum value for the decay length in these models. This maximum value depends on the model and on the electric charge of the scalar under consideration but can at most be of the order of a few millimeters. Interestingly, independent of the model, this maximum occurs always in a region of parameter space, where leptonic and gauge boson final states have similar branching ratios, i.e., where the observation of lepton numberviolating final states from scalar decays is possible.

