arXiv subjects

Antonio Herrero-Brocal

Publications and source records attributed to Antonio Herrero-Brocal.

7 recordsLinked to original sources

arXiv2026

Non-anomalous axions: lessons from the Majoron

We show that when an anomalous and a non-anomalous global symmetries are spontaneously and simultaneously broken, the resulting Nambu--Goldstone boson is associated with the non-anomalous one. Applied to the Majoron, this implies that its underlying symmetry is $B-L$ rather than $L$, naturally resolving the domain wall problem. This result further demonstrates that effective couplings of the form $a \, F \, \tilde{F}$ do not uniquely indicate an anomalous origin, allowing the detection of non-anomalous Nambu--Goldstone bosons in searches for axion--gauge boson interactions.

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arXiv2026

Probing Dark Sector Particles Coupling to Neutrinos with Double Beta Decay

Motivated by the observation of non-zero neutrino masses and the potential for discovering physics beyond the Standard Model, numerous experiments are actively searching for neutrinoless double beta $(0νββ)$ decay. In all of these searches, a substantial amount of data on two-neutrino double beta $(2νββ)$ decay has been collected. In this work, we explore the sensitivity of current and future double beta decay experiments to a massive Majoron-like scalar particle coupled to neutrinos and potentially dark sector fermions, and compare their reach to the relevant cosmological constraints. On- and off-shell production of such a scalar leads to characteristic distortions in the emitted electron spectrum. We investigate how these distortions manifest in current and future double beta decay experiments, deriving the sensitivity to such a scenario. We project the reach of future experiments which can probe scalar-neutrino couplings of $|a_ν| \approx 2\times 10^{-6}$ for sub-MeV scalar particles and remain sensitive to off-shell production above the Q-value of double beta isotopes.

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arXiv2025

Beyond $\boldsymbol{SU(N)}$: $\boldsymbol{U(3) \times U(2)}$ as the underlying symmetry of the strong and electroweak interactions

The gauge principle is a cornerstone of particle-physics model building. Nevertheless, many constructions leave certain global $U(1)$ redundancies ungauged. In this work, we take the gauge principle to its logical extreme by promoting all $SU(N)$ symmetries to $U(N)$. We focus on a model based on local $U(3)\times U(2)$ invariance. This framework accounts for several otherwise ad hoc features of the Standard Model, including charge quantization and the observed hypercharge assignments, which emerge here as unique predictions. Furthermore, the internal consistency of the model requires the introduction of right-handed neutrinos and implies the presence of an additional $U(1)$ factor that can be identified with $B-L$, thereby naturally yielding non-zero neutrino masses. In light of these findings, we hypothesize that $U(3)\times U(2)$ constitutes the underlying symmetry of the strong and electroweak interactions. More importantly, our approach opens up novel avenues for model building, driven by this extended interpretation of the gauge principle.

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arXiv2025

The Type-I Seesaw family

We provide a comprehensive analysis of the Type-I Seesaw family of neutrino mass models, including the conventional type-I seesaw and its low-scale variants, namely the linear and inverse seesaws. We establish that all these models essentially correspond to a particular form of the type-I seesaw in the context of explicit lepton number violation. We then focus into the more interesting scenario of spontaneous lepton number violation, systematically categorizing all inequivalent minimal models. Furthermore, we identify and flesh out specific models that feature a rich majoron phenomenology and discuss some scenarios which, despite having heavy mediators and being invisible in processes such as $μ\to e γ$, predict sizable rates for decays including the majoron in the final state.

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arXiv2025

Minimal Majorana neutrino mass models

We present an extension of the Casas-Ibarra parametrization that applies to all possible Majorana neutrino mass models. This framework allows us to systematically identify minimal models, defined as those with the smallest number of free parameters. We further analyze the phenomenologically relevant combination of the Yukawa matrix, $y^\dagger y$, and show that in certain scenarios it exhibits an unexpected reduction in the number of free parameters, depending on just one real degree of freedom. Finally, the application of our results is illustrated in specific models, which can be tested or falsified due to their definite experimental predictions in heavy neutrino and charged lepton flavor violating decays.

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arXiv2025

Flavour and cosmological probes of Diracon models

We present and analyze two minimal extensions of the Standard Model featuring a spontaneously broken global, chiral, and anomaly-free $U(1)_D$ symmetry. This breaking generates naturally small Dirac neutrino masses via a seesaw mechanism and yields a physical massless Goldstone boson, the Diracon. Although both models share the same particle content and scalar potential, their distinct symmetry breaking pattern leads to remarkably different phenomenological and cosmological signatures. In the first model, the Diracon couples weakly to charged leptons but right-handed neutrinos can be efficiently produced in the early Universe, resulting in stringent constraints from the effective number of relativistic species, $ΔN_{\text{eff}}$. Conversely, in the second one, right-handed neutrino production is suppressed, and flavour-violating processes such as $μ\to e \mathcal{D}$ provide the most promising probes. These simple but elegant models showcase the complementarity between cosmological observations and low-energy flavour experiments in the search for physics beyond the Standard Model.

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arXiv2024

The majoron coupling to charged leptons

The particle spectrum of all Majorana neutrino mass models with spontaneous violation of global lepton number include a Goldstone boson, the so-called majoron. The presence of this massless pseudoscalar changes the phenomenology dramatically. In this work we derive general analytical expressions for the 1-loop coupling of the majoron to charged leptons. These can be applied to any model featuring a majoron that have a clear hierarchy of energy scales, required for an expansion in powers of the low-energy scale to be valid. We show how to use our general results by applying them to some example models, finding full agreement with previous results in several popular scenarios and deriving novel ones in other setups.

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