⚛️ phenomenology

Solar Constraints on Heavy Neutral Leptons with $\nu_\tau$ Mixing

This paper utilizes solar neutrino data from the SOHO observatory to derive significantly improved constraints on the mixing between heavy neutral leptons and tau neutrinos in the MeV mass range, surpassing existing terrestrial limits by more than an order of magnitude.

Original authors: Vedran Brdar, Samiur R. Mir, Xun-Jie Xu

Published 2026-08-21

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CC BY 4.0

Original authors: Vedran Brdar, Samiur R. Mir, Xun-Jie Xu

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

Deep within the standard model of particle physics, the universe is populated by a family of particles known as neutrinos. These are ghostly, nearly massless entities that stream through everything, rarely interacting with the matter around them. For decades, scientists have known that there are three distinct types, or "flavors," of these particles: electron neutrinos, muon neutrinos, and tau neutrinos. While the first two have been studied extensively in laboratories on Earth, the third, the tau neutrino, remains a mystery in many ways. This is partly because creating them requires immense energy that is difficult to generate in a lab, leaving a significant gap in our understanding of how they might behave. Theoretical physics suggests that these known neutrinos might have a heavier, invisible cousin called a heavy neutral lepton. If such particles exist, they could mix with the known neutrinos, altering their behavior in ways that might explain why the universe has more matter than antimatter. However, because these heavy cousins are so elusive, scientists have struggled to find them, particularly those that might mix with the tau neutrino.

A new study turns its gaze away from Earth-bound laboratories and toward the Sun to solve this puzzle. The researchers realized that the Sun acts as a natural, high-energy factory that produces a vast number of electron neutrinos. As these particles travel from the Sun's core to its surface, a phenomenon called oscillation causes some of them to transform into tau neutrinos. While the Sun does not produce tau neutrinos directly in its core, this transformation creates a steady stream of them moving outward. The team proposed that if heavy neutral leptons exist and mix with these tau neutrinos, the tau neutrinos could collide with protons inside the Sun and kick up these heavy particles. Once created, these heavy leptons would escape the Sun's gravity and travel through the solar system. Because they are unstable, they would eventually decay, breaking apart into electrons and positrons (the antimatter version of electrons) that could be detected by instruments orbiting our planet.

To test this idea, the researchers analyzed data collected by the Solar and Heliospheric Observatory, a space-based telescope that has been monitoring the Sun for decades. They focused on a specific period when solar activity was low, known as the "Quiet Sun," to ensure that any strange signals they found were not just background noise from solar storms. The team calculated exactly what the signal would look like if heavy neutral leptons with a mass of about 5 million electron volts were decaying in the space between the Sun and Earth. They then compared their theoretical predictions against the actual measurements of electrons and positrons recorded by the observatory. The data showed no signs of the extra burst of particles that would be expected if these heavy leptons were abundant.

The absence of this signal allowed the scientists to set strict new limits on the existence of these particles. They determined that if heavy neutral leptons mixing with tau neutrinos do exist in the mass range of roughly 2 to 20 million electron volts, their interaction with known neutrinos must be incredibly weak. Specifically, the strength of this mixing was constrained to be less than one part in one hundred. This finding is significant because it improves upon the best previous limits from Earth-based experiments by more than ten times in this specific mass range. By using the Sun as a natural accelerator and space telescopes as detectors, the study has effectively ruled out a large swath of possibilities for where these elusive particles might hide, proving that the Sun is a powerful tool for exploring the frontiers of particle physics.

Technical Summary: Solar Constraints on Heavy Neutral Leptons with $\nu_\tau$ Mixing

Problem Statement
Heavy Neutral Leptons (HNLs), gauge-singlet fermions that mix with active neutrinos, are a minimal extension of the Standard Model (SM). While the mixing of HNLs with electron ( $\nu_e$ ) and muon ( $\nu_\mu$ ) neutrinos is tightly constrained ( $|U_{eN}|^2, |U_{\mu N}|^2 \lesssim \mathcal{O}(10^{-6})$ ) for MeV-scale masses, the mixing with tau neutrinos ( $|U_{\tau N}|^2$ ) remains largely unconstrained. Existing laboratory limits for $|U_{\tau N}|^2$ in the MeV mass range are weak, typically at the $\mathcal{O}(10^{-1})$ level. This disparity arises because terrestrial sources (nuclear beta decays, meson decays) rarely produce $\nu_\tau$ , making the direct production of $\tau$ -flavored HNLs difficult. This work addresses the lack of constraints in the $\nu_\tau$ sector by leveraging the Sun as a source of $\nu_\tau$ via neutrino oscillations.

Methodology
The authors propose a two-step mechanism for HNL production in the Sun, illustrated in Fig. 1:

Oscillation: Electron neutrinos ( $\nu_e$ ) produced in the solar core oscillate into $\nu_\tau$ during propagation through the solar interior.
Scattering: The resulting $\nu_\tau$ flux scatters off solar protons (and nuclei) via the process $\nu_\tau + p \to N + p$ , producing HNLs ( $N$ ) through mixing. Unlike $\nu_e$ -flavored HNLs, which can be produced directly at the source, $\nu_\tau$ -flavored HNLs require this scattering step because oscillations alone cannot create new mass eigenstates.

The differential flux of produced HNLs, $d\Phi_N^{(0)}/dE$ , is calculated using the standard solar profile and the adiabatic approximation for flavor transition probabilities ( $P_{e\tau}$ ). The flux is proportional to the squared mixing matrix element $|U_{\tau N}|^2$ and the solar $\nu_e$ flux.

Once produced, HNLs escape the Sun and decay in the heliosphere. For masses in the $\mathcal{O}(10)$ MeV range, the dominant visible decay channel is $N \to e^+ e^- \nu_\tau$ . The resulting electron-positron ( $e^\pm$ ) flux is modeled by solving a diffusion transport equation (Eq. 4) within the heliospheric magnetic field. The diffusion coefficient is assumed to be $D \approx 2 \times 10^{22} \text{ cm}^2 \text{ s}^{-1}$ . The steady-state $e^\pm$ number density is derived using Green's functions, accounting for the HNL decay length and the observation point at 1 AU.

Key Contributions and Analysis

First Calculation of Solar $\nu_\tau$ -Induced HNL Flux: The paper provides the first calculation of the HNL flux generated specifically by the upscattering of solar $\nu_\tau$ on protons, a mechanism previously unexplored for constraining $\nu_\tau$ mixing.
Diffusion Modeling: The authors model the propagation of decay-produced $e^\pm$ through the heliosphere, distinguishing this from direct detection of HNLs. The signal is characterized by the differential intensity $J_{obs}$ , which is compared against space-based data.
Data Utilization: The analysis utilizes MeV-electron data collected by the Solar and Heliospheric Observatory (SOHO) via the EPHIN instrument. The data covers a 13-month period (June 2007–July 2008) during the declining phase of Solar Cycle 23, a "Quiet Sun" period where background solar wind is minimized.
Statistical Framework: Constraints are derived using a $\chi^2$ test statistic comparing the expected signal (HNL decay $e^\pm$ ) plus background against the observed SOHO flux. The authors conservatively assume the observed flux is entirely background, treating any HNL contribution as an excess. A 20% relative uncertainty is applied to the data.

Results
Using SOHO data, the authors derive 90% Confidence Level (CL) constraints on the parameter space of HNL mass ( $m_N$ ) and mixing ( $|U_{\tau N}|^2$ ).

Sensitivity: For HNL masses around $m_N \sim 5$ MeV, the derived limit reaches $|U_{\tau N}|^2 < 10^{-2}$ .
Comparison: These constraints improve upon existing terrestrial limits (from atmospheric neutrino oscillations, Earth upscattering with Borexino/Super-Kamiokande, and the CHARM experiment) by more than an order of magnitude in the 1–20 MeV mass range.
Benchmark Points: The paper illustrates the exclusion power by showing that a mixing value of $|U_{\tau N}|^2 = 0.1$ is excluded by the data, while $|U_{\tau N}|^2 = 0.004$ remains allowed.

Significance and Claims
The paper claims that solar neutrinos, combined with space-based observations, constitute a novel and robust probe for HNLs with dominant $\nu_\tau$ mixing. The study establishes that the Sun acts as a copious source of $\nu_\tau$ -flavored HNLs via oscillation and scattering, a mechanism that overcomes the production difficulties faced by terrestrial experiments. The resulting constraints represent the leading probe for low-mass HNLs with $\nu_\tau$ mixing, opening a new avenue for searching for physics beyond the Standard Model in a previously unconstrained parameter space. The authors note that while Big Bang Nucleosynthesis and supernova cooling provide complementary constraints, they are subject to cosmological model dependencies or astrophysical uncertainties, whereas the solar method offers a direct probe in this specific mass range.

Technical Summary: Solar Constraints on Heavy Neutral Leptons with ντ\nu_\tauντ​ Mixing

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Technical Summary: Solar Constraints on Heavy Neutral Leptons with $\nu_\tau$ Mixing