Exploring Hypercharge Breaking Scenarios as a Solution to Baryon Asymmetry in the Universe

February 21, 2025 feature

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by Ingrid Fadelli , Phys.org

The Standard Model (SM), the main physics framework describing elementary particles and the forces driving them, outlines key patterns in physical interactions referred to as gauge symmetries. One of the symmetries it describes is the so-called U(1)Y hypercharge: a gauge symmetry that contributes to the electric charge of particles before electromagnetic and weak forces become distinct (i.e., before the electroweak phase transition).

Researchers at Universidad Autónoma de Madrid's Theoretical Physics Department (DFT) and Instituto de Física Teórica (IFT) recently carried out a study investigating how the conditions present in the early universe could prompt the spontaneous breaking of this gauge symmetry, linking this phenomenon to certain models of neutrino mass generation known as radiative neutrino mass models. Their paper, published in Physical Review Letters, specifically builds on a theoretical framework called the Zee-Babu model, an extension of the SM explaining neutrino mass generation.

'In the SM, the spontaneously broken electroweak gauge symmetry, which governs the electromagnetic and weak interactions of nature, was restored in the universe's first instants, when the universe's temperature was higher than the electroweak energy scale,' Prof. Jose Miguel No, Luca Merlo, Alvaro Lozano-Onrubia and Sergio López-Zurdo told Phys.org.

'In the last few years, one of us (Prof. No) has been exploring the possibility that this symmetry could have remained broken in the early universe, as this could have important implications for the origin of the primordial matter-antimatter asymmetry (the so-called 'baryon asymmetry of the universe'). Yet, it was very difficult to find extensions of the SM which would feature such an effect.'

While attending a theoretical physics workshop in Munich, Prof. No started discussing the phenomenology of some particle physics models that generate masses for SM neutrinos with Prof. Kaladi Babu from Oklahoma State University. Prof. Babu, a world expert on these frameworks, known as radiative neutrino mass models, offered valuable insight that paved the way for the team's present study.

'This conversation led to the realization that such models had the required ingredients to keep the hypercharge gauge symmetry—one of the two symmetries which comprise the electroweak gauge symmetry—broken at very high temperatures,' said the researchers.

'Back at IFT in Madrid, we all sat down together and confirmed that these neutrino mass models could feature early universe hypercharge breaking, potentially offering a new perspective on the connection between neutrino masses and the baryon asymmetry.'

The SM has a simple scalar sector that only contains the Higgs field responsible for generating the mass of massive particles described by the model. This simple scalar sector suggests the universe has a fairly basic thermal history, with SM gauge symmetries being exact at high temperatures. Still, the electroweak symmetry spontaneously breaks at low temperatures, a phenomenon that is well-established experimentally.

'In certain extensions of the SM with a richer scalar sector, however, the universe's thermal history would have been more involved/sophisticated,' explained No, Merlo, Lozano-Onrubia and López-Zurdo. 'In particular, we showed that extensions where this richer scalar sector is linked to the generation of neutrino masses, the SM hypercharge gauge symmetry can indeed be broken at high temperatures, while in the SM it is not.'

The researchers showed that the breaking of the hypercharge symmetry in the Early universe could explain the observed imbalance between matter and antimatter in the universe, which is known as the baryon asymmetry problem. In their paper, No, Merlo, Lozano-Onrubia and López-Zurdo outline a nonconventional mechanism that could have contributed to the formation of today's matter-dominated universe.

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'First of all, our study underlines the relevance of considering the quantum contributions in these analyses, as our results crucially depend on properly including them,' said the researchers.

'We showed that particle physics models with a non-minimal scalar sector are very promising scenarios to simultaneously explain several of the unresolved problems of the SM, in our case, the origin of neutrino masses and the generation of the baryon asymmetry of the universe.'

The recent work by this research team could soon open interesting possibilities for the study of neutrino physics and the origins of matter. In the future, it could contribute to the development of alternative models of the universe that offer better explanations for its matter-antimatter asymmetry.

'At the same time, our work paves the way for alternative, new ways of explaining the origin of the baryon asymmetry of the universe that eventually leads to our very existence,' said the researchers.

The recent analyses by No, Merlo, Lozano-Onrubia and López-Zurdo specifically apply to a specific theoretical framework of neutrino mass generation known as the Zee-Babu model. As part of their current research, the researchers are trying to determine whether their results only apply to this model or could be extended to a broader range of theories describing rich scalar sectors.

'Moreover, this 'inverse symmetry breaking' phenomenon (that a certain symmetry, that would be thought to be exact in the early universe, was actually broken then) could provide a solution to other problems of the SM, such as the 'strong-CP' problem—why the strong interactions appear to conserve the discrete CP symmetry,' added the researchers. 'This is a possibility that we are exploring in a further analysis we are conducting.'

More information: S. López-Zurdo et al, Early Universe Hypercharge Breaking and Neutrino Mass Generation, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.041802.

Journal information: Physical Review Letters

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