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Lepton Number Violation at the LHC in Radiative Neutrino Mass Models with Leptoquarks

This paper investigates the potential for the High-Luminosity LHC to detect lepton number violation via same-sign dilepton plus jet signatures in a radiative neutrino mass model with leptoquarks, finding that it can probe leptoquark masses up to approximately 1.5 TeV and unambiguously establish the Majorana nature of neutrinos.

Original authors: K. S. Babu, Rahool Kumar Barman, Dorival Gonçalves

Published 2026-09-23

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CC BY 4.0

Original authors: K. S. Babu, Rahool Kumar Barman, Dorival Gonçalves

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

Nature hides its deepest secrets in the smallest particles. Among these, the neutrino is perhaps the most elusive, a ghostly particle that passes through the Earth by the trillions every second without leaving a trace. For decades, physicists have known that neutrinos have mass, but the origin of that mass remains one of the great unsolved mysteries of modern physics. The standard model of particle physics, which successfully describes almost everything we see, suggests that neutrinos should be massless. To explain why they have mass, scientists have proposed that neutrinos might be their own antiparticles, a property known as being Majorana particles. If this is true, it would mean that a fundamental rule of the universe, called lepton number conservation, can be broken. This violation would be a smoking gun for new physics, revealing a mechanism that generates mass in a way we have never seen before.

A team of researchers has now turned their attention to a specific theoretical framework that could explain this mystery while offering a way to test it at the world's most powerful particle collider. They focused on a model that replaces the usual particles responsible for generating neutrino mass with a different kind of particle called a leptoquark. Unlike the known particles, leptoquarks are hypothetical objects that can transform a quark, which makes up protons and neutrons, into a lepton, such as an electron or a muon. In this specific version of the theory, the mass of the neutrino is not created instantly but is built up slowly through a complex loop of interactions, a process that happens only once in a quantum sense. The researchers wanted to know if this specific setup could leave a detectable signature at the Large Hadron Collider, the massive machine in Switzerland that smashes protons together at nearly the speed of light.

The team, led by physicists at Oklahoma State University and the University of Tokyo, set out to simulate what would happen if these leptoquarks existed and were produced in the collisions at the collider. They were looking for a very rare and specific event: the creation of two charged particles with the same electric charge, such as two electrons or two muons, appearing together with a spray of other particles called jets. In the normal world, particles are created in pairs with opposite charges to balance the books. Finding two of the same charge would be a clear sign that the rule of lepton number conservation had been broken by two units. This is the exact kind of event that would unambiguously establish that neutrinos are their own antiparticles. The researchers built a detailed computer model of their theory, incorporating all the known rules of particle physics and the strict limits set by previous experiments. They had to ensure their model did not contradict existing data, such as the precise measurements of how neutrinos oscillate or the fact that protons do not decay rapidly.

After carefully mapping out the allowed possibilities for the mass and behavior of these leptoquarks, the team ran simulations to see how often these same-sign particle pairs would appear. They found that the most likely way to produce this signal is through the strong nuclear force, which is the same force that holds atomic nuclei together. This force can create pairs of leptoquarks in huge numbers compared to other methods. Once created, these heavy particles would decay into the signature two same-sign leptons and jets. The researchers then applied the strict selection criteria used by the major detectors at the collider to filter out background noise and false alarms. They calculated how many of these rare events would be visible if the collider were to run at its full future potential, known as the High-Luminosity LHC, which will collect vastly more data than it has so far.

The results of their analysis suggest that this specific type of leptoquark is within reach of the next generation of experiments. If these particles exist with masses up to about 1.5 TeV, the High-Luminosity LHC should be able to detect the signal of two same-sign leptons and jets with high confidence. This would not only confirm the existence of these new particles but also provide direct evidence that lepton number is violated in nature. Such a discovery would be a monumental step forward, confirming that neutrinos are indeed Majorana particles and revealing the mechanism behind their tiny masses. The study demonstrates that even if the new particles are heavy and the interactions are subtle, the sheer power of the collider, combined with the unique signature of this process, offers a clear path to uncovering one of the universe's most fundamental secrets.

Technical Summary: Lepton Number Violation at the LHC in Radiative Neutrino Mass Models with Leptoquarks

Problem Statement
The fundamental nature of neutrino mass—specifically whether neutrinos are Majorana or Dirac particles—remains an open question in particle physics. While neutrinoless double beta decay ( $0\nu\beta\beta$ ) is the primary low-energy probe for Lepton Number Violation (LNV) by two units ( $|\Delta L|=2$ ), it is limited to the electron flavor channel. The "black-box theorem" establishes that any process violating lepton number by two units necessarily induces a Majorana mass for neutrinos. However, collider experiments offer a broader scope, potentially accessing mixed lepton flavors ( $e^\pm\mu^\pm$ , $\mu^\pm\mu^\pm$ , etc.).

This paper investigates the prospects for observing $|\Delta L|=2$ signatures at the High-Luminosity LHC (HL-LHC) within a specific theoretical framework: the Leptoquark (LQ) variant of the Zee Model. In this model, Majorana neutrino masses arise radiatively at the one-loop level. The authors aim to determine if the specific particle content and interactions of this model allow for observable LNV signals ( $pp \to \ell^\pm\ell'^\pm + \text{jets}$ ) that are consistent with current experimental constraints, including neutrino oscillation data, charged lepton flavor violation (cLFV), electroweak precision tests, and existing LHC leptoquark searches.

Methodology
The study employs a comprehensive phenomenological analysis combining theoretical modeling, constraint mapping, and collider simulation:

Model Framework: The authors extend the Standard Model (SM) by introducing an $SU(2)_L$ doublet leptoquark $\Omega(3, 2, 1/6)$ and a singlet leptoquark $\chi(3, 1, -1/3)$ . The model includes a trilinear scalar coupling in the Higgs potential that induces mixing between the $\omega_{-1/3}$ and $\chi_{-1/3}$ components, resulting in physical mass eigenstates $X_1$ and $X_2$ . This mixing, combined with specific Yukawa interactions, generates neutrino masses at one-loop and facilitates $|\Delta L|=2$ processes.
Constraint Analysis: The parameter space is rigorously constrained by:
- Neutrino Masses: Reproducing the observed mass-squared differences and mixing angles (assuming normal ordering and a fixed lightest mass $m_1 = 0.04$ eV).
- Low-Energy Observables: Limits on charged lepton flavor violating decays (e.g., $\mu \to e\gamma$ , $\tau \to \ell\gamma$ ) and coherent $\mu-e$ conversion in nuclei.
- Electroweak Precision: Constraints on the oblique parameters $S$ , $T$ , and $U$ , with particular attention to the isospin-breaking effects on $\Delta T$ .
- LHC Bounds: Existing exclusion limits on leptoquark pair production cross-sections from ATLAS and CMS at $\sqrt{s}=13$ TeV.
Signal Generation and Simulation:
- Production mechanisms for the LNV signature $pp \to \ell^\pm\ell'^\pm + \text{jets}$ are categorized into three classes: QCD pair production, Electroweak (EW) pair production, and single leptoquark production.
- Signal events are simulated using MadGraph5_aMC@NLO interfaced with Pythia8 for parton showering and hadronization. The model is implemented via FeynRules.
- Detector effects are simulated using Delphes3 with a default HL-LHC detector card.
- Background estimates are derived from existing CMS same-sign dilepton plus jets searches ( $137 \text{ fb}^{-1}$ ) and rescaled to the HL-LHC integrated luminosity of $3 \text{ ab}^{-1}$ .
- Event selection criteria include requirements on lepton $p_T$ , jet multiplicity, invariant mass vetoes, missing transverse energy, and the scalar sum of transverse momenta ( $H_T$ ).

Key Contributions and Results

Allowed Parameter Space: The analysis identifies a narrow region of parameter space consistent with all current constraints. The most stringent constraint arises from the oblique parameter $\Delta T$ , which forces the mass splitting between the leptoquark eigenstates ( $\Delta m = m_{\omega_{2/3}} - m_{X_1}$ ) to be small. Consequently, viable models require leptoquark masses $m_{LQ} \gtrsim 1.4$ TeV.
Dominant Production Mechanism: The study finds that QCD pair production of leptoquarks overwhelmingly dominates the LNV signal cross-section compared to EW pair production and single leptoquark production. This is due to the strong coupling nature of QCD and the large gluon parton distribution functions. Single production channels are suppressed by small Yukawa couplings required to satisfy low-energy flavor constraints.
Collider Sensitivity:
- The paper demonstrates that the HL-LHC ( $3 \text{ ab}^{-1}$ ) can probe leptoquark masses up to $m_{LQ} \sim 1.5$ TeV via the $pp \to \ell^\pm\ell'^\pm + \text{jets}$ channel.
- For a representative benchmark point ( $m_{\omega_{2/3}} = 1.4$ TeV, $\Delta m = 10$ GeV, $\theta_{LQ} = 0.2$ ), a $2\sigma$ sensitivity is achievable with an integrated luminosity of approximately $790 \text{ fb}^{-1}$ .
- The analysis confirms that a sizable LNV signal is compatible with the small neutrino masses observed in oscillation experiments, as the LNV cross-section depends primarily on the leptoquark masses (QCD production) rather than the Yukawa couplings that determine the neutrino mass scale.

Significance and Claims
The paper claims that the observation of the same-sign dilepton plus jets signature at the HL-LHC would constitute direct evidence for the Majorana nature of neutrinos, independent of the specific mechanism of mass generation, in accordance with the generalized black-box theorem.

Unlike $0\nu\beta\beta$ searches which are restricted to the electron channel, this LHC analysis highlights the unique capability of colliders to probe LNV in mixed lepton flavor channels ( $e^\pm\mu^\pm$ ). The authors conclude that the leptoquark variant of the Zee Model provides a concrete, testable scenario where radiative neutrino mass generation is inextricably linked to observable LNV signatures at high energies. The study establishes that the HL-LHC has the potential to either discover this specific mechanism or significantly constrain the parameter space of radiative neutrino mass models involving leptoquarks.

Technical Summary: Lepton Number Violation at the LHC in Radiative Neutrino Mass Models with Leptoquarks

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