The current world record for the most precise direct measurement of the neutrino mass belongs to KATRIN, the Karlsruhe Tritium Neutrino Experiment in Germany. KATRIN works by measuring the energies of billions of electrons from tritium beta decay, using a 200-ton vacuum spectrometer the size of a small ferry. The technique is conceptually beautiful and engineered to within an inch of its life. Its projected final sensitivity is around 0.2 electronvolts.
That is impressive. It is also, in the long run, not enough. Cosmology tentatively suggests the heaviest neutrino mass is well below 0.1 eV. To actually measure the mass — not just bound it — physicists need a detector with an order of magnitude better sensitivity than KATRIN can ever deliver.
That is what Project 8 is being built for.
A different way to weigh a neutrino
Like KATRIN, Project 8 is a tritium beta decay experiment. Tritium is the simplest convenient source of beta electrons: it decays into helium-3, emitting an electron and an antineutrino with a well-known spectrum. The precise shape of that spectrum near the endpoint depends on the neutrino mass.
KATRIN measures that shape by passing electrons through an electrostatic filter and counting how many make it through at each energy. Project 8 takes a fundamentally different approach. It measures the energy of individual electrons — one at a time — by detecting the cyclotron radio waves they emit while spiraling in a magnetic field.
This technique, called Cyclotron Radiation Emission Spectroscopy (CRES), was first proposed in 2009 by Benjamin Monreal and Joseph Formaggio. They pointed out that an electron in a magnetic field emits radio waves at a frequency proportional to its energy. If you can listen to those radio waves with sensitive receivers, you can determine the electron’s energy to remarkable precision — without ever touching it.
Why this is harder than it sounds
The radio signal from a single electron is fantastically weak. A 19-keV electron — the endpoint energy of tritium decay — in a typical lab-strength magnetic field emits radio waves at around 27 GHz, with a total radiated power of about a femtowatt. That’s roughly one billionth of a billionth of a typical mobile phone’s transmission.
Detecting that signal requires:
A strong, uniform magnetic field of around 1 tesla, ideally produced by a superconducting magnet A gas-filled trap that holds individual tritium atoms in the magnetic field for milliseconds at a time Ultra-low-noise radio receivers cooled to liquid helium temperatures Long, patient running — the experiment is rare-event physics, where each electron is precious
When all of this works, Project 8 records the radio waveform of a single electron as it spirals and slowly loses energy. The frequency tells the energy. The energy distribution of many such electrons, summed over a long run, reconstructs the beta spectrum near the endpoint.
The roadmap
Project 8 has been built in phases, each demonstrating one technical step.
Phase I (2014–2017) detected the first single-electron cyclotron radiation signal from an electron in a tabletop trap. The result, published in Physical Review Letters in 2015, was a proof of concept.
Phase II (2017–2020) scaled up to a small atomic tritium source and demonstrated frequency-based energy reconstruction on a real beta spectrum.
Phase III is the current phase. It involves a much larger detection volume, atomic (rather than molecular) tritium to eliminate a final systematic uncertainty, and the first competitive limit on the neutrino mass — projected to be comparable to or better than KATRIN’s, but in a fundamentally different way.
Phase IV, the final design, aims for a sensitivity of around 40 milli-electronvolts — about an order of magnitude below KATRIN’s final projection, and right at the boundary where the experiment might actually measure the mass rather than bound it.
Why atomic tritium matters
This is one of the experiment’s harder engineering challenges and one of its scientific differentiators.
Tritium gas is normally molecular: T₂, two tritium atoms bonded together. Molecular tritium has internal vibrational and rotational states that smear out the beta-decay endpoint by about 0.36 eV — a hard systematic floor that no spectroscopic experiment can beat without changing the source.
Atomic tritium — single tritium atoms, not bound to anything — has no internal structure to broaden the endpoint. Producing and confining atomic tritium long enough to study its decays has been a years-long engineering project for the Project 8 collaboration, primarily at the University of Washington in Seattle.
What’s at stake
If neutrinos really do sit at masses around 50 meV, as cosmology suggests, Project 8 should resolve that mass directly. We would, for the first time in history, know the absolute mass of a neutrino — not bound it from above, not infer it from cosmological models, but measure it.
If neutrinos turn out to be lighter than Project 8 can resolve, the experiment will produce an extraordinary upper limit but won’t see the mass itself. In that case, the next generation of cosmological surveys would carry more weight than direct measurements.
Either way, Project 8 is the kind of long-shot precision experiment that fundamental physics depends on. KATRIN’s spectrometer, like its predecessors, exploited a clever idea taken to engineering perfection. Project 8 is attempting something different: replacing the clever idea with a new one, and starting the engineering over.
The first results from Phase III are expected in the next few years. The Phase IV design is now being finalized. By the early 2030s, if all goes well, the neutrino’s mass may stop being an open question.
For the experiment Project 8 is racing, see KATRIN narrows the upper limit on neutrino mass once again. For the basic physics of how oscillation tells us neutrinos must have mass in the first place, see How neutrino oscillation works.
Frequently asked
What is Project 8?
Project 8 is a next-generation neutrino mass measurement experiment based at the University of Washington in Seattle. It uses a novel technique called Cyclotron Radiation Emission Spectroscopy (CRES) — listening to the radio waves emitted by individual electrons spiraling in a magnetic field — to measure the energy spectrum of tritium beta decay near its endpoint, where the shape depends on the neutrino mass.
How does CRES differ from KATRIN's approach?
KATRIN uses a giant electrostatic spectrometer to filter electrons by energy, measuring millions at a time. Project 8 instead traps a small number of tritium decay electrons in a magnetic field and detects the cyclotron radio emission from each one individually. The technique allows in-principle scaling to atomic tritium sources and avoids some of the systematics that limit KATRIN.
What sensitivity does Project 8 target?
The full-scale Project 8 aims for sensitivity around 40 meV on the effective electron-neutrino mass — well into the regime where, if neutrinos are in the inverted ordering, a signal must appear. This is roughly five times better than KATRIN's projected final sensitivity.
What stage is Project 8 in?
The experiment is in staged development. Phase I demonstrated the CRES technique. Phase II measured the tritium spectrum at small scale. Phase III is building a larger atomic-tritium source, and Phase IV will be the final neutrino-mass measurement. Full-sensitivity results are expected in the late 2020s to early 2030s.
Why measure neutrino mass directly?
Because oscillation only constrains mass differences, not absolute masses. The cosmological route (CMB-S4) measures the sum of masses with high precision but depends on cosmological model assumptions. Direct laboratory measurement is model-independent and provides the cleanest test. Combining all three approaches (oscillation + direct + cosmological) is what will pin down the absolute neutrino mass scale completely.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, June 23). Project 8: the next-generation neutrino mass measurement, one electron at a time. Neutrino Times. https://neutrino-times.com/articles/project-8-next-generation-neutrino-mass/
Chicago
Neutrino Times Editorial Team. "Project 8: the next-generation neutrino mass measurement, one electron at a time." Neutrino Times, June 23, 2025. https://neutrino-times.com/articles/project-8-next-generation-neutrino-mass/.
MLA
Neutrino Times Editorial Team. "Project 8: the next-generation neutrino mass measurement, one electron at a time." Neutrino Times, 23 Jun. 2025, https://neutrino-times.com/articles/project-8-next-generation-neutrino-mass/.
BibTeX
@misc{neutrino-times-project-8-next-generation-neutrino-mass,
author = {Neutrino Times Editorial Team},
title = {Project 8: the next-generation neutrino mass measurement, one electron at a time},
howpublished = {Neutrino Times},
year = {2025},
month = {jun},
url = {https://neutrino-times.com/articles/project-8-next-generation-neutrino-mass/},
note = {Accessed: 2025-06-23}
} RIS
TY - GEN TI - Project 8: the next-generation neutrino mass measurement, one electron at a time AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-06-23 PB - Neutrino Times UR - https://neutrino-times.com/articles/project-8-next-generation-neutrino-mass/ ER -