⚛️ phenomenology

Leptogenesis with a GeV-Scale Quasi-Dirac Neutrino

This paper proposes a GeV-scale leptogenesis scenario within a type-I seesaw model featuring spontaneously broken lepton number and a $Z_2$ symmetry, demonstrating that thermal Higgs decays of quasi-Dirac neutrinos can successfully generate the observed baryon asymmetry while remaining testable at future experiments like SHiP and MATHUSLA and potentially producing a detectable stochastic gravitational-wave background from domain wall annihilation.

Original authors: Stephen F. King, Soumen Kumar Manna, Rishav Roshan, Arunansu Sil

Published 2026-10-16

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Stephen F. King, Soumen Kumar Manna, Rishav Roshan, Arunansu Sil

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe we see today is made almost entirely of matter, yet the laws of physics suggest that the Big Bang should have created equal amounts of matter and antimatter. If that had happened, the two would have annihilated each other instantly, leaving behind only a sea of light. The fact that we exist means something tipped the balance, creating a tiny surplus of matter that survived to form stars, planets, and people. For decades, physicists have searched for the mechanism that caused this imbalance. One leading idea involves neutrinos, the ghostly particles that zip through everything without interacting. If these particles have a hidden, heavier version that decays in a specific way, it could have generated the initial difference between matter and antimatter in the early universe. This process, known as leptogenesis, connects the mystery of why we are here to the origin of neutrino masses, one of the biggest unsolved puzzles in particle physics.

A team of researchers has now proposed a new way this could have happened, focusing on a specific type of heavy neutrino that is light enough to be found in upcoming experiments. In the standard version of this theory, the heavy particles required to create the imbalance are so massive that they are far beyond the reach of any current or planned machine. However, this new study suggests that the heavy particles could be much lighter, existing at a scale of a few billion electron volts, which is roughly the mass of a heavy atom nucleus. The researchers show that if these particles are nearly identical twins with a tiny, almost imperceptible difference in their mass, they can generate the necessary matter surplus through the decay of the Higgs boson, the particle responsible for giving other particles mass.

The key to this mechanism lies in the relationship between two heavy neutrinos that are so similar they act like a single particle, yet distinct enough to create a crucial effect. The researchers built a model based on a hidden symmetry in nature that, when broken, naturally creates these two nearly identical particles. In the early universe, when temperatures were incredibly high, the Higgs boson gained extra mass due to the heat of the environment. This allowed the Higgs to decay into these heavy neutrinos and ordinary leptons, a process that was previously thought impossible at such low energy scales. As the Higgs decayed, the tiny difference between the two heavy neutrinos caused the process to favor the creation of matter over antimatter. This preference was amplified by the heat of the early universe, creating a strong enough signal to explain the amount of matter we see today.

The study demonstrates that this scenario works for heavy neutrinos with masses between 0.1 and 10 GeV. This is a significant finding because it places the particles squarely within the reach of future experiments. Facilities like SHiP, MATHUSLA, and the proposed FCC-ee collider are designed to hunt for exactly these kinds of heavy, long-lived particles. The researchers calculated the specific conditions required for the universe to end up with the right amount of matter and found that the necessary parameters fall within the sensitivity range of these upcoming detectors. This means that if this theory is correct, we could potentially discover the particles responsible for our existence within the next decade, rather than waiting for a machine that might never be built.

The paper also explores what happens to the symmetry that created these particles. When this symmetry breaks, it can leave behind invisible walls in the fabric of space-time called domain walls. These walls are unstable and eventually collapse, releasing a burst of energy in the form of gravitational waves. The researchers suggest that the specific pattern of these waves could be detected by future gravitational wave observatories, offering a second way to test the theory. If the universe is filled with a background hum of these waves, it would provide independent confirmation that the symmetry breaking occurred as described. This dual approach—looking for the particles directly and listening for the gravitational echoes of their creation—makes the theory particularly robust and testable.

One of the most important aspects of this work is how it handles the requirement for the two heavy neutrinos to be nearly identical. In previous theories, this near-identical nature had to be forced into the model by hand, which felt unnatural. In this new framework, the tiny difference in mass arises automatically from the way the underlying symmetry breaks, making the theory feel more complete and grounded in fundamental principles. The researchers also compared their idea to another popular theory involving the oscillation of heavy neutrinos. They found that for the specific mass range they are studying, the oscillation theory would fail to produce enough matter, leaving the Higgs decay mechanism as the only viable option. This distinction helps narrow down where scientists should look and what they should expect to find.

The study confirms that the heavy neutrinos in this model would be very weakly interacting, meaning they would travel long distances before decaying. This behavior is exactly what experiments like SHiP and MATHUSLA are designed to catch. By mapping out the possible masses and interaction strengths, the researchers have provided a clear target for experimentalists. If these experiments find a signal in the predicted range, it would not only confirm the existence of heavy neutrinos but also solve the mystery of why the universe is made of matter. The work bridges the gap between the abstract mathematics of particle physics and the concrete reality of experimental discovery, turning a theoretical possibility into a testable prediction.

Ultimately, this research offers a hopeful path forward for understanding the origins of the universe. It suggests that the answer to one of the biggest questions in science might be hidden in particles that are light enough to be created in a laboratory. By focusing on a specific, testable range of masses and a mechanism driven by the Higgs boson, the authors have turned a broad theoretical idea into a concrete roadmap for discovery. The next step is for the experiments to begin their search, looking for the faint signatures of these heavy twins that may have set the stage for everything that exists today.

Technical Summary: Leptogenesis with a GeV-Scale Quasi-Dirac Neutrino

Problem Statement
The origin of the observed matter-antimatter asymmetry of the Universe remains a primary motivation for physics beyond the Standard Model (SM). While the type-I seesaw mechanism combined with thermal leptogenesis offers an elegant solution, the conventional scenario typically requires right-handed neutrinos (RHNs) to be extremely heavy ( $M_N \gtrsim 10^9$ GeV) to satisfy the Davidson-Ibarra bound on CP asymmetry. This mass scale renders the mechanism inaccessible to direct experimental verification. Low-scale leptogenesis (with $M_N$ at the GeV or TeV scale) is attractive for testability but faces significant challenges: the CP asymmetry is naturally suppressed, and the electroweak sphaleron processes, which convert lepton asymmetry to baryon asymmetry, become ineffective below the sphaleron decoupling temperature ( $T_{sph} \simeq 131.7$ GeV). Furthermore, resonant leptogenesis, a common solution for low-scale scenarios, requires an exceptionally small mass splitting between nearly degenerate RHNs, often introduced as an ad hoc assumption without a clear theoretical origin.

Methodology and Model Setup
The authors propose a symmetry-based framework extending the SM with two RHNs ( $N_1, N_2$ ) and a complex scalar singlet $\Phi$ . The model incorporates a global $U(1)_L$ lepton number symmetry and a discrete $Z_2$ symmetry.

Symmetry Breaking: The spontaneous breaking of $U(1)_L$ by the vacuum expectation value (VEV) of $\Phi$ ( $v_\phi$ ) generates a Dirac mass term for the RHNs, creating two exactly degenerate states in the symmetry limit.
Mass Splitting: A tiny mass splitting ( $\Delta M^0$ ) essential for resonant enhancement is generated naturally by dimension-five operators that explicitly break $U(1)_L$ while preserving $Z_2$ . This results in a quasi-Dirac neutrino with a mass splitting $\Delta M^0 = 2v_\phi^2/\Lambda$ , where $\Lambda$ is the cutoff scale.
Higgs-Decay Leptogenesis: To address the challenge of generating asymmetry below the electroweak scale, the authors utilize the Higgs-decay mechanism. In the early Universe, finite-temperature effects induce a thermal mass for the Higgs boson, kinematically allowing the decay $H \to L N$ (where $L$ is the lepton doublet) in the temperature window $T_{EW} \sim 160$ GeV $> T > T_{sph}$ .
Thermal CP Asymmetry: The CP asymmetry ( $\epsilon_{CP}$ ) arises not from vacuum decays (which are real and CP-conserving in this regime) but from the interference between tree-level and loop-level Higgs decay amplitudes. Crucially, the loop diagram acquires an absorptive part only in the presence of the thermal plasma (via scattering processes like $N + H_{bath} \to L$ ). The total CP asymmetry is driven by the interplay between the vacuum mass splitting ( $\Delta M^0$ ) and temperature-dependent thermal corrections ( $\Delta M^T$ ).

The evolution of the RHN and lepton asymmetries is studied by solving coupled Boltzmann equations, accounting for thermal decay rates and the specific kinematics of the $H \to LN$ channel.

Key Contributions and Results

Natural Origin of Mass Splitting: The paper demonstrates that the required tiny mass splitting for resonant enhancement emerges naturally from the underlying symmetry structure (spontaneous and explicit breaking of $U(1)_L$ ), avoiding ad hoc assumptions.
Successful Baryogenesis at GeV Scales: The authors show that the observed baryon asymmetry of the Universe (BAU) can be successfully generated for quasi-Dirac neutrinos with masses in the range $0.1 - 10$ GeV. The mechanism relies on a purely thermal CP asymmetry where the resonance condition is satisfied by the combined effect of vacuum and thermal mass splittings.
Parameter Space Analysis:
- The analysis identifies a viable parameter space where the Higgs-decay mechanism dominates.
- The paper compares this scenario with the Akhmedov-Rubakov-Smirnov (ARS) mechanism. It delineates a region where the ARS mechanism is ineffective (due to oscillation temperatures falling below $T_{sph}$ ), establishing Higgs decay as the sole viable mechanism in the specific low-mass-splitting regime favored by the model.
- The required active-sterile mixing angles ( $|\theta_\alpha|^2$ ) are found to be within the projected sensitivity of future experiments.
Gravitational Wave (GW) Signatures: The spontaneous breaking of the $Z_2$ symmetry leads to the formation of domain walls (DWs). The explicit breaking of this symmetry (via higher-dimensional operators) causes these walls to become unstable and annihilate. This annihilation generates a stochastic gravitational-wave background. The paper calculates the GW spectrum, showing that the peak frequency and amplitude depend on the mass splitting $\Delta M^0$ and the cutoff scale $\Lambda$ .

Significance and Claims
The paper claims to provide a theoretically well-motivated and experimentally testable framework for low-scale leptogenesis. Its primary significance lies in:

Testability: Unlike high-scale seesaw scenarios, the predicted RHNs in the $0.1-10$ GeV range are accessible to future heavy neutral lepton searches at facilities such as SHiP, MATHUSLA, and FCC-ee. The viable parameter space for successful leptogenesis overlaps significantly with the projected sensitivities of these experiments.
Theoretical Consistency: It resolves the "fine-tuning" problem of resonant leptogenesis by deriving the necessary mass splitting from symmetry breaking rather than assuming it.
Multi-Messenger Probes: The framework offers a complementary probe through stochastic gravitational waves generated by domain wall annihilation, linking the mechanism of baryogenesis to observable GW signals.
Majoron Constraints: The authors note that while the Majoron (associated with $U(1)_L$ breaking) is a candidate for dark matter in high-scale models, in this GeV-scale scenario, the parameter space required for successful Higgs-decay leptogenesis is incompatible with the Majoron being a stable dark matter candidate, though it may still leave imprints on the effective number of relativistic species ( $N_{eff}$ ).

In conclusion, the work establishes a direct phenomenological connection between the conditions for successful Higgs-decay-induced leptogenesis and the prospects for discovering the associated RHNs, suggesting that the mechanism can be tested through active-sterile neutrino mixing searches rather than relying solely on cosmological consequences.

Technical Summary: Leptogenesis with a GeV-Scale Quasi-Dirac Neutrino

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