We present the first complete field-theoretic realization of the finite one-loop T4-3-i topology for Majorana neutrino mass. Here, T4-3-i denotes a one-loop realization of the Weinberg operator in which a fermion links the two external lepton--Higgs pairs. If this fermion is a Majorana singlet or triplet, the same interactions generate a tree-level type-I or type-III seesaw contribution, respectively, so that the loop is not the leading source of neutrino mass. This lower-order contribution is removed by taking the mediator to be a Dirac fermion $N$, placing lepton-number violation in a separate Majorana fermion $\psi$ inside the loop, and imposing an exact $Z_2$ symmetry that keeps the new scalars inert and stabilizes the lightest odd state. We classify the allowed electroweak charge assignments and study the minimal singlet-doublet realization, denoted T4-3-i-B1, which contains one Dirac fermion, one Majorana fermion, an inert scalar doublet, and an inert scalar singlet. The resulting rank-two neutrino mass matrix predicts one massless neutrino. We confront both normal and inverted neutrino-mass orderings with neutrino-oscillation and cosmological data, charged-lepton flavor violation, including $\mu-e$ conversion, electroweak precision observables, $h\to\gamma\gamma$, theoretical consistency conditions, the relic abundance, and direct-detection limits. Both fermionic and scalar dark matter are viable. The fermionic candidate has only a loop-induced Higgs coupling and consequently a strongly suppressed spin-independent scattering rate, whereas the scalar candidate couples through a tree-level Higgs portal and can lie above the neutrino floor while remaining compatible with current limits. In both cases, coannihilation with inert scalars is essential for reproducing the observed relic abundance.
We consider a model which provides an explanation of the origin of light neutrino masses, the baryon asymmetry of the Universe -- via leptogenesis -- and explains the observed dark matter relic abundance with several components. In this scenario, Majorana masses of the active neutrinos are produced by the inverse seesaw (ISS) mechanism with the lepton-number-violating mass parameter being dynamically generated at three loops with a novel topology. This model is based on an extension of the Standard Model gauge symmetry by a global $U(1)'$ and a discrete $\mathbb{Z}_2\otimes \mathbb{Z}_3$. The latter, which is responsible for the radiative origin of the ISS lepton-number-violating parameter, survives the spontaneous breaking of the global $U(1)'$ and, at the same time, ensures the stabilization of the dark sector. The lightest particles carrying non-trivial residual charges are stable, becoming potentially viable dark matter candidates. The model complies with bounds and constraints from neutrino data, collider and high-intensity charged lepton flavor-violating observables, as well as dark matter relic density and direct detection. To efficiently explore the model's high-dimensional parameter space and identify phenomenologically viable regions, we perform a global numerical scan using the MultiNest algorithm.
A. Abada, Nicol'as Bernal, A. E. C. Hernández et al.· 0 citations
We revisit the traditional Type I, II and III Seesaw mechanisms in the presence of a complex scalar field charged under a global $U(1)$ symmetry that can be identified with lepton number and the Peccei-Quinn symmetry. After symmetry breaking, the radial mode becomes heavy while the angular mode appears as an axion-like particle, traditionally dubbed the Majoron. We construct the effective field theory obtained after integrating out the heavy states and analyse two matching orders: first removing the radial mode and then the Seesaw fields, and vice versa. Both procedures yield the same low-energy Lagrangian containing only Standard Model fields and the Majoron. Because a single vacuum expectation value fixes the mediator masses, the radial mode and every Majoron coupling, these models predict relations among observables rather than their individual size, and it is these relations that are testable. Indeed, the invisible Higgs width is locked to the universal suppression of the Higgs couplings, while the Majoron-lepton coupling is fixed by the measured non-unitarity of the leptonic mixing matrix, and the two independently give comparable lower bounds on the same lepton-number breaking scale, of order $1$-$10$ TeV. Neutrinoless double beta decay with Majoron emission, by contrast, has no sensitivity in this class of models. The framework is thus falsifiable even when the new states lie far beyond experimental reach.
We study the possibility of an inverse first-order electroweak phase transition (IFOEWPT) and observable gravitational waves (GW) in a radiative neutrino mass model of scotogenic type where singlet-doublet (SD) fermions, the lightest of whom is the dark matter (DM) candidate, generate the necessary seesaw at one-loop level. Considering the possibility of light neutrinos being Dirac for simplicity and additional detection prospects, we extend the standard model (SM) with two generations of $SU(2)_L$ singlet and doublet fermions, one singlet scalar, and three right-handed neutrinos (RHNs). While RHNs provide the right chiral parts of light Dirac neutrinos, the SD fermions and the scalar singlet facilitate the one-loop neutrino mass diagram. The neutral component of the lighter SD fermion, stabilized under a residual $Z_2$ symmetry plays the role of DM while the heavier SD fermions strongly couple to the Higgs leading to an IFOEWPT where the Universe undergoes two different first-order phase transition as it goes from the symmetric to the final broken Higgs phase. We constrain the parameter space from the requirements of generating the correct neutrino mass, DM relic as well as IFOEWPT while incorporating the existing constraints from different experiments. The final allowed parameter space of the model can be probed at collider, direct-detection, GW and cosmic microwave background (CMB) experiments in near future.
D. Borah, Indrajit Saha, S. K. Sahoo et al.· 0 citations
Dynamical dark energy offers an alternative to a cosmological constant with distinct observational signatures. However, the small energy density scale, Hubble-sized mass, and Planckian excursions make simple models fine-tuned and unnatural. In this work, we show that a weak version of the axion, identified with the phase field of the anomalous $U(1)_{B+L}$ of the Standard Model, can generate the scale hierarchies expected for dark energy. The axion potential is controlled by sources of explicit baryon and lepton number violation and is radiatively stable. We show that the leading contribution comes from two inequivalent Weinberg operators, one $B+L$-conserving and one $B+L$-violating, which generate the axion potential. We propose a flavor selection rule based on a spontaneously broken $S_3$ permutation symmetry in the lepton sector that simultaneously removes the quadratic divergence and dominant temperature-dependent contributions. The resulting potential first appears at quartic order in the axion-dependent neutrino masses and, for the observed departure from tribimaximal mixing, its amplitude is parametrically close to the dark-energy density. The dominant uncertainty comes from $\delta_{\rm CP}$ and $\theta_{23}$, so experiments like Hyper-K and DUNE can directly test the model in the future. Cosmologically, the field behaves as thawing quintessence for $f$ close to $M_{pl}$, stays frozen by Hubble friction until late times and never enters the adiabatic regime.
We study leptogenesis via out-of-equilibrium decays of right-handed neutrinos in a parity symmetric extension of the Standard Model with gauge group $SU(3)_c \times SU(2)_L \times SU(2)_R \times U(1)_X$, spontaneously broken to the Standard Model gauge group at a scale $v_R$. We focus on the minimal Higgs realization, in which the strong CP problem is resolved without invoking additional symmetries. In this framework, lepton number is violated through Yukawa interactions, while neutrino masses arise only at higher loop order, a feature that permits the CP-violating quantum corrections to right-handed neutrino decays to be parametrically large. For thermal production of right-handed neutrinos, we find that successful leptogenesis requires $v_R \gtrsim 6\times 10^{12}$ GeV. Non-thermal production, however, relaxes this bound dramatically, requiring only $v_R \gtrsim 10$ TeV, a scale accessible to collider searches for new particles and searches for rare processes.
We compute the one-loop effects of vector-like fermions (VLFs) on the Higgs-strahlung cross-section, the flagship precision observable of future $e^+e^-$ Higgs factories such as FCC-ee. We consider four benchmark extensions of the Standard Model (SM): two in which a VL quark (VLQ) or VL lepton (VLL) doublet, together with its singlet partners, couples to the Higgs boson through purely internal Yukawa interactions, and two in which a single VL singlet mixes directly with the third-generation charged lepton or neutrino. Working throughout in the on-shell renormalization scheme and imposing perturbativity bounds on the relevant couplings under renormalization-group (RG) running, we compute both the universal self-energy and the non-universal vertex contributions to the Higgs-strahlung amplitude at one loop, together with the oblique parameters $S$ and $T$ and the $H\to\gamma\gamma$ rate as complementary cross-checks. In the two Yukawa-driven scenarios, we find that a sizable fraction of the parameter space still allowed by current LHC searches produces shifts in $\sigma(ZH)$ at or above the per-mille sensitivity envisioned for FCC-ee, offering a radiative probe of these couplings. In the two mixing-driven scenarios, existing electroweak-precision bounds on the relevant mixing angles already preclude an observable effect, although we characterize the underlying loop dynamics in full generality for future reference. These results identify the VL Yukawa couplings, rather than any residual mixing with the SM fermions, as the more promising target for Higgs-strahlung precision measurements at a future $e^+e^-$ collider.
C. Marzo, Vin'icius Padovani, Daniele Rizzo· 0 citations
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