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Signatures of Long-Lived Heavy Neutral Leptons from Neutrinophilic Charged Higgs Pair Production at the LHC

This paper investigates displaced vertex signatures of long-lived heavy neutral leptons produced via neutrinophilic charged Higgs pair decays at the High-Luminosity LHC, finding that while single displaced vertices are observable for cross sections above $\mathcal{O}(1)$ fb, detecting two such vertices remains challenging for cross sections below $\mathcal{O}(10)$ fb.

Original author: Nobuchika Okada, Prasenjit Sanyal, Ravindra Kumar Verma

Published 2026-11-04

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CC BY 4.0

Original author: Nobuchika Okada, Prasenjit Sanyal, Ravindra Kumar Verma

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 author. For technical accuracy, refer to the original paper. Read full disclaimer

For decades, physicists have known that the universe's most abundant matter particle, the neutrino, is not massless as once thought. This discovery shattered the standard model of particle physics, the theory that successfully describes how the smallest building blocks of nature interact. To explain why these ghostly particles have mass, scientists proposed a mechanism called the "seesaw," which suggests that for every light neutrino we observe, there exists a much heavier, invisible partner. Finding these heavy partners is a holy grail of modern physics, as their existence would unlock the mystery of why neutrinos are so light and why the universe is made of matter rather than antimatter. However, these heavy partners are notoriously difficult to catch; they rarely interact with ordinary matter, and when they do, they often vanish before they can be seen, leaving no trace for our massive detectors to record.

A team of researchers has now proposed a new way to hunt for these elusive heavy particles, focusing on a specific scenario where they might leave a visible, albeit delayed, mark. In their study, they explored a model where a special type of particle, known as a charged Higgs boson, acts as a factory for these heavy neutrinos. Unlike the standard Higgs boson discovered in 2012, which gives mass to many particles, this hypothetical partner is "neutrinophilic," meaning it has a special affinity for neutrinos and interacts almost exclusively with them. The researchers calculated that if these charged Higgs bosons are created in high-energy collisions at the Large Hadron Collider, they would almost immediately decay into a heavy neutrino and a standard charged particle, like an electron or a muon. The crucial twist is that under certain conditions, the heavy neutrino produced in this process would not decay instantly. Instead, it would travel a measurable distance away from the collision point before breaking apart, creating a "displaced vertex"—a second, distinct point of decay that appears separated from the first.

The authors simulated this process to see if the upcoming high-luminosity upgrade of the Large Hadron Collider could detect such events. They found that if the production rate of these charged Higgs bosons is high enough, the signal of a single displaced vertex would be statistically significant and clearly distinguishable from background noise. This would provide a direct, observable signature of the heavy neutrino's existence and its long life. However, the study also highlighted the difficulty of a more complex scenario: detecting two such displaced vertices in the same event. The researchers determined that observing two separate decay points is extremely challenging, particularly if the production rate of the charged Higgs bosons is low, specifically below a threshold of ten femtobarns. While the detection of a single displaced vertex offers a promising path forward, the observation of two remains a steep climb for current experimental capabilities.

The significance of this work lies in its ability to bypass the usual limitations of searching for heavy neutrinos. Typically, these particles are produced in ways that make them nearly impossible to see because their interactions are too weak. By utilizing the neutrinophilic charged Higgs boson as a production mechanism, the researchers showed that these heavy particles could be created in large numbers without being suppressed. Furthermore, the study clarified that the long life of these heavy neutrinos depends on the mass of the lightest neutrino in the universe. If that lightest neutrino is sufficiently light, the heavy partner can live long enough to travel a detectable distance. The researchers also noted that this specific long-lived scenario works best in a "standard" version of the seesaw mechanism involving three heavy neutrinos, whereas a simpler version with only two heavy neutrinos would not produce the same long-lived effect.

In their analysis, the team also had to ensure their proposed model did not violate existing experimental limits. They checked the model against the known constraints on processes that change the identity of a lepton, such as a muon turning into an electron and a photon. By carefully adjusting the parameters of their model, they confirmed that the proposed interactions remain consistent with all current data while still allowing for the production of these heavy, long-lived particles. The study concludes that while the path to discovery is narrow, the signature of a single displaced vertex offers a viable and complementary avenue to probe the origin of neutrino masses. If the Large Hadron Collider can produce these charged Higgs bosons at the rates the authors suggest, the detectors may finally be able to spot the fleeting, wandering footprints of the heavy neutrinos that have long remained hidden in the shadows of particle physics.

Technical Summary: Signatures of Long-Lived Heavy Neutral Leptons from Neutrinophilic Charged Higgs Pair Production at the LHC

Problem Statement
The observation of neutrino oscillations confirms that neutrinos possess mass, a phenomenon not accounted for in the Standard Model (SM). While the Type-I seesaw mechanism offers a compelling explanation by introducing heavy right-handed neutrinos (RHNs), or Heavy Neutral Leptons (HNLs), direct collider detection remains challenging. In standard scenarios, HNL production is typically suppressed due to the small mixing between light and heavy neutrinos required to generate tiny neutrino masses. Furthermore, existing searches often focus on light HNLs (below the $W$ boson mass) where decays proceed via off-shell gauge bosons. A significant gap exists in understanding the production and detection of heavy HNLs (masses above the $W$ boson) that nonetheless exhibit long-lived behavior, a scenario theoretically possible within the standard seesaw framework if the lightest neutrino mass is sufficiently small.

Methodology
The authors investigate a specific production mechanism within the neutrinophilic Higgs doublet model. In this framework, an additional Higgs doublet ( $H_2$ ) couples exclusively to SM lepton doublets and RHNs, while the SM Higgs doublet ( $H_1$ ) generates masses for quarks and charged leptons. This structure allows for a small vacuum expectation value (VEV) for $H_2$ ( $v_2 \ll v_1$ ), naturally generating small Dirac neutrino masses without requiring extremely small Yukawa couplings, even at low seesaw scales.

The study focuses on the following process chain at the High-Luminosity LHC (HL-LHC):

Production: Pair production of neutrinophilic charged Higgs bosons ( $H^\pm$ ) via the Drell–Yan process.
Decay: Due to the neutrinophilic nature of $H_2$ and the assumed mass degeneracy among the extra Higgs bosons (suppressing bosonic decays like $H^\pm \to W^\pm H/A$ ), the charged Higgs decays dominantly into a charged lepton and an HNL ( $H^\pm \to \ell^\pm N$ ).
HNL Behavior: The authors analyze the conditions under which the produced HNLs become long-lived. They distinguish between the Standard Seesaw (three RHNs) and the Minimal Seesaw (two RHNs). They demonstrate that in the Standard Seesaw with an orthogonal matrix $O=I$ , one HNL state ( $N_1$ in Normal Hierarchy or $N_3$ in Inverted Hierarchy) can have a suppressed decay width proportional to the lightest neutrino mass, rendering it long-lived even when decaying via on-shell gauge bosons ( $N \to \ell W, \nu Z, \nu h$ ). Conversely, they show that in the Minimal Seesaw, the decay widths are not suppressed by the lightest neutrino mass, preventing long-lived scenarios.
Simulation: The authors perform dedicated simulations to identify displaced vertex signatures, considering both single displaced vertex (one HNL decays visibly, the other decays promptly or invisibly) and double displaced vertex (both HNLs decay visibly) topologies.

Key Contributions and Results

Mechanism for Long-Lived Heavy HNLs: The paper establishes that the neutrinophilic Higgs doublet framework provides a viable mechanism to produce heavy HNLs ( $m_N > m_W$ ) with macroscopic decay lengths. This is achieved without the usual suppression of production cross-sections associated with small light-heavy neutrino mixing, as the production is mediated by the charged Higgs rather than direct mixing.
Hierarchy Dependence: The study clarifies that the long-lived nature of the HNL is contingent on the Standard Seesaw mechanism with three RHNs and a specific choice of the orthogonal matrix ( $O=I$ ). In the Minimal Seesaw scenario, the necessary suppression of the decay width does not occur, making the observation of long-lived heavy HNLs in this specific channel unlikely.
Phenomenological Constraints: The authors derive bounds on the VEV of the neutrinophilic Higgs doublet ( $v_2$ ) from the lepton flavor violating process $\mu \to e\gamma$ . They confirm that the parameter space allowing for long-lived HNLs is consistent with current experimental limits.
Discovery Potential at HL-LHC:
- Single Displaced Vertex: The authors find that a high statistical significance for the observation of a single displaced vertex can be achieved for charged Higgs pair production cross-sections greater than $\mathcal{O}(1)$ fb.
- Double Displaced Vertex: The observation of two displaced vertices is identified as significantly more challenging. The authors note that this signature is difficult to achieve, particularly when the charged Higgs pair production cross-section is below $\mathcal{O}(10)$ fb, due to the lower probability of both HNLs decaying within the detector volume and the associated background suppression requirements.

Significance
The paper claims that this mechanism offers a "viable and complementary avenue" to probe the origin of neutrino masses and the structure of the seesaw mechanism. By focusing on heavy HNLs produced via charged Higgs decays, the study bypasses the production suppression typical of direct mixing-induced processes. The ability to detect heavy, long-lived HNLs via displaced vertices at the HL-LHC provides a unique test for the Standard Seesaw framework in a mass regime and production mode that are distinct from traditional searches. The work underscores that while single displaced vertex signatures are within reach for moderate cross-sections, the double displaced vertex signature remains a more stringent test, requiring higher production rates or optimized detection strategies.

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