The three known neutrino flavors — electron, muon, and tau — interact only via the weak nuclear force and gravity. They have tiny but non-zero masses. They oscillate between flavors. And, in most theoretical models of how they got their masses, they have partner particles that have never been observed.
These hypothetical partners are called heavy neutral leptons (HNLs), or sometimes heavy right-handed neutrinos, or sterile neutrinos in the heavy mass range. They are the missing relatives that the see-saw mechanism predicts must exist if neutrinos have Majorana masses from grand-unified-theory dynamics.
A substantial experimental program has emerged over the past decade to look for them. The LHC experiments have searched. Dedicated proposed experiments like SHiP are being developed. The constraints are tightening. The answer to whether they exist within experimental reach remains open.
What an HNL would look like
In the simplest see-saw picture, an HNL is a fermion that mixes with one of the three active neutrinos through a small mixing angle denoted U. The mixing has two consequences.
The HNL inherits weak interactions. Through its mixing with ordinary neutrinos, the HNL can interact with W and Z bosons in muted versions of the standard weak interaction. The interaction strength is suppressed by U² relative to that of ordinary neutrinos.
The HNL is unstable. It decays through its weak interactions, producing Standard Model particles. The lifetime depends on the mass and the mixing — heavier HNLs decay faster, and more weakly-mixed HNLs decay slower.
At masses around 1 GeV, an HNL with typical see-saw-motivated mixing has a lifetime that is macroscopic — long enough that it travels measurable distances before decaying. The decay products might be a muon plus a charged pion, or an electron plus other Standard Model fermions, depending on the kinematics.
In a particle detector, an HNL produced in a hadron collision and decaying after some distance would appear as a displaced vertex — a set of tracks emerging from a point that is not the original collision vertex. Displaced vertices are rare in ordinary Standard Model physics, so they are a relatively clean signature for searches.
How HNLs are produced
The dominant production mode at a collider depends on the HNL mass.
Below about 5 GeV. HNLs are produced in the decays of mesons containing heavy quarks. The decays B → e^± + HNL or B → μ^± + HNL, where the HNL plays the role normally played by an ordinary neutrino, occur at small rates set by the HNL mixing. Searches in this regime are conducted at LHCb, NA62, T2K, and other dedicated experiments.
5 GeV to 100 GeV. HNLs can be produced through the conversion of W bosons. A W produced in a collision can decay W → ℓ^± + HNL, where ℓ is a charged lepton. The HNL then travels some distance and decays. Searches in this range have been conducted at LHC experiments and at LEP in its day.
Above 100 GeV. Direct production via Drell-Yan-like processes (qq → HNL + ν) becomes possible. The HNL decay products are typically detectable at conventional collider detectors. Searches in this range are an active LHC program.
Each search constrains the HNL mixing parameter U as a function of mass. Combined limits cover a swath of parameter space from about 1 GeV up to about 1 TeV, with sensitivity peaking at intermediate masses where production rates are high and lifetimes are convenient for detection.
What current searches have found
So far, no HNL signal has been confirmed.
LHC experiments. ATLAS and CMS have published searches for prompt HNL decays at high mass and displaced-vertex HNL decays at lower mass. Constraints on the mixing parameter |U_e|², |U_μ|², and |U_τ|² have reached values of order 10⁻⁶ at the most sensitive masses, with weaker limits at the edges of the mass range.
LHCb. A dedicated displaced-vertex search by LHCb has constrained HNLs that mix with muons and electrons in the range of a few hundred MeV to a few GeV.
Beam-dump experiments. Older experiments like CHARM, PS191, and BEBC, plus the active NA62 program, have constrained low-mass HNLs in a complementary parameter region.
Tau-neutrino mixing. Limits on HNL mixing with the tau flavor are generally weaker than the corresponding limits for electron and muon mixing, because tau leptons are harder to identify in collider environments.
The combined picture is that substantial regions of parameter space remain unprobed, particularly at GeV-scale masses with small mixing — exactly the range that the see-saw mechanism predicts most naturally if neutrino mass scales are to come out right.
The SHiP experiment
The most ambitious dedicated HNL search currently in development is SHiP (Search for Hidden Particles), a proposed CERN experiment that would use a high-intensity proton beam from the SPS to search for long-lived weakly-coupled particles produced in interactions with a fixed beam dump.
The SHiP detector would consist of a large evacuated decay volume — about 50 meters long — instrumented to detect the decay products of any long-lived particles that drift in. The proton beam dump would produce vast numbers of mesons, including the heavy mesons whose rare decays could produce HNLs.
In its proposed run plan, SHiP would accumulate the equivalent of about 2 × 10²⁰ protons on target — enough to probe HNL mixing parameters as small as |U|² ≈ 10⁻⁹ at favorable masses. This is several orders of magnitude beyond what LHC experiments can currently reach.
SHiP has been through several iterations of design review and is now expected to be approved for construction in the late 2020s, with operations possible in the early 2030s. If approved on the current schedule, it would be one of the most sensitive HNL searches anywhere.
HNLs and leptogenesis
The HNL searches matter beyond their direct goal because of the leptogenesis connection. In the standard see-saw picture, heavy right-handed neutrinos in the very early universe decay asymmetrically — slightly more lepton-producing decays than antilepton-producing decays — generating the small lepton asymmetry that, through sphaleron processes, becomes the matter-antimatter asymmetry we observe today.
The leptogenesis scenario does not require HNLs in any particular mass range. The simplest version uses HNLs near the grand-unification scale, around 10¹⁰ GeV — far above any conceivable collider energy. But variant scenarios called resonant leptogenesis and ARS leptogenesis can operate with much lighter HNLs, sometimes as low as a GeV or two — exactly the mass range that SHiP and current LHC searches probe.
If an HNL is discovered in the GeV range with the right mixing, it would not just be a particle-physics discovery — it would be an experimental verification of one of the leading theoretical scenarios for why the universe contains matter at all.
What HNLs aren’t quite
Some terminology in the field is unsettled. The same particle is called by several names depending on which subfield is using it.
Sterile neutrino is the common term in oscillation physics when the particle’s mass is small (eV-scale or below) and the goal is to explain anomalies like LSND/MiniBooNE or the gallium anomaly or the reactor antineutrino anomaly. Sterile neutrinos in this context are very weakly mixed and probe a particular kind of low-energy phenomenology.
Heavy neutral lepton is the common term in collider searches and beam-dump experiments when the particle’s mass is in the GeV-to-TeV range and the goal is to find a discrete particle through its decay products.
Right-handed neutrino is the theory-side term that emphasizes the particle’s role in see-saw and leptogenesis. It typically refers to states with masses well above the electroweak scale, although the same fundamental object can be invoked at lower masses too.
These are, in many models, the same kind of particle — a fermion with no Standard Model gauge interactions except through small mixings with ordinary neutrinos. Different mass ranges produce different phenomenology, but the underlying theoretical object is the same.
The next decade
The next several years should produce significant progress in HNL searches.
LHC Run 4 will increase the LHC’s integrated luminosity by about a factor of three, tightening limits on heavier HNLs.
SHiP approval and construction would open a new search regime at GeV masses with much smaller mixings than currently accessible.
FCC-ee, a proposed circular electron-positron collider at CERN, could produce an extraordinarily large sample of Z bosons. If it is built, the Z decays could constrain HNL mixings to extraordinary precision — possibly to the level needed to test the see-saw mechanism’s natural parameter range directly.
Long-baseline neutrino experiments like DUNE and Hyper-K may also have indirect HNL sensitivity through their precision oscillation measurements.
If any of these programs discovers an HNL, the implications would be substantial. If they do not, the lower bounds on HNL mixing will continue to tighten, and the see-saw scenario will be increasingly constrained.
A search worth doing
Heavy neutral leptons may or may not exist within experimentally accessible reach. The simplest see-saw scenarios place them above the LHC’s energy. But variant scenarios — including some of the most theoretically motivated ones for leptogenesis — predict masses and mixings exactly where current and proposed experiments can look.
The search is one of the most consequential ongoing programs in particle physics. A discovery would simultaneously explain neutrino mass, the matter-antimatter asymmetry, and potentially several other puzzles. A continued non-discovery sharpens the constraints on the kind of new physics that can exist.
Either outcome will reshape the picture of fundamental physics. The waiting continues, and the parameter space is narrowing.
For the theoretical framework, see The see-saw mechanism. For the cosmological connection, see Leptogenesis. For the lighter-mass sterile-neutrino counterpart, see Sterile neutrinos: a stubborn maybe. For the basics of neutrino flavor mixing, see How neutrino oscillation works.
Frequently asked
What is a heavy neutral lepton?
A heavy neutral lepton (HNL), sometimes called a heavy right-handed neutrino, is a hypothetical particle that mixes with the ordinary neutrinos through Yukawa couplings to the Higgs field, but has a mass much larger than the eV-scale active neutrinos. In see-saw models, HNLs explain why ordinary neutrinos are so light. Their masses range from GeV to far above TeV, depending on the model.
Why search for them at colliders?
If an HNL has a mass below a few hundred GeV and a substantial enough mixing with ordinary neutrinos, it can be produced in particle collisions and detected through its eventual decay. Because HNLs are typically long-lived, they often travel a measurable distance before decaying — producing displaced vertices that stand out cleanly against backgrounds.
Where do existing searches stand?
The LHC experiments (ATLAS, CMS, LHCb) have set constraints on HNL mixing for masses from about 1 GeV to about 1 TeV. Older experiments like NA62, BEBC, and CHARM have constrained lighter HNLs. Most of the parameter space that is theoretically natural for explaining ordinary neutrino masses remains unprobed, but the constraints are tightening with each year of LHC running.
What is SHiP?
SHiP (Search for Hidden Particles) is a proposed CERN experiment specifically optimized to detect long-lived neutral particles produced in proton collisions. The detector would sit downstream of a beam dump at the SPS accelerator, looking for HNL decays in a large evacuated decay volume. SHiP could probe HNL parameter space relevant to keV-to-GeV sterile neutrinos that current LHC detectors miss.
What if HNLs are too heavy or too weakly mixed to detect?
Then their effects appear only indirectly — through their contribution to the see-saw mechanism that gives ordinary neutrinos their mass, and through their role in leptogenesis. Cosmological observations and neutrinoless double-beta decay can probe heavy-HNL scenarios indirectly. The direct collider searches focus on the lower-mass, more-mixed corner of parameter space where direct detection is possible.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, November 21). Heavy neutral leptons: searching for the missing relatives of the neutrino. Neutrino Times. https://neutrino-times.com/articles/heavy-neutral-leptons-collider-searches/
Chicago
Neutrino Times Editorial Team. "Heavy neutral leptons: searching for the missing relatives of the neutrino." Neutrino Times, November 21, 2025. https://neutrino-times.com/articles/heavy-neutral-leptons-collider-searches/.
MLA
Neutrino Times Editorial Team. "Heavy neutral leptons: searching for the missing relatives of the neutrino." Neutrino Times, 21 Nov. 2025, https://neutrino-times.com/articles/heavy-neutral-leptons-collider-searches/.
BibTeX
@misc{neutrino-times-heavy-neutral-leptons-collider-searches,
author = {Neutrino Times Editorial Team},
title = {Heavy neutral leptons: searching for the missing relatives of the neutrino},
howpublished = {Neutrino Times},
year = {2025},
month = {nov},
url = {https://neutrino-times.com/articles/heavy-neutral-leptons-collider-searches/},
note = {Accessed: 2025-11-21}
} RIS
TY - GEN TI - Heavy neutral leptons: searching for the missing relatives of the neutrino AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-11-21 PB - Neutrino Times UR - https://neutrino-times.com/articles/heavy-neutral-leptons-collider-searches/ ER -