Project 8: Reading Single Electrons by Their Cyclotron Light

For seventy years, direct measurements of the neutrino mass have followed the same approach: catch the electron emitted in a beta decay, measure its energy with the highest possible precision, and look for distortions in the high-energy endpoint of the spectrum that reveal a non-zero neutrino mass. The KATRIN experiment in Karlsruhe represents the apex of this approach with its 23-metre-long electrostatic spectrometer; its target sensitivity of 0.2 eV is set partly by the achievable energy resolution of the spectrometer and partly by the systematic uncertainty in molecular-tritium final states.

To go beyond KATRIN, a fundamentally different detection technique is needed. Project 8 at MIT and the Pacific Northwest National Laboratory is building one. Instead of filtering electrons by their kinetic energy in an electrostatic field, Project 8 measures their kinetic energy through the cyclotron radiation they emit while orbiting in a magnetic field. The technique — Cyclotron Radiation Emission Spectroscopy, or CRES — promises an order-of-magnitude improvement in energy resolution over electrostatic spectrometers. Combined with atomic (rather than molecular) tritium as the source, Project 8 targets a final neutrino-mass sensitivity of approximately 0.04 eV, into the inverted-ordering parameter space.

This post describes how CRES works, what makes it advantageous, the staged Project 8 programme, and the broader implications for neutrino-mass measurement.

How cyclotron radiation works

A charged particle moving with velocity $\vec{v}$ in a magnetic field $\vec{B}$ experiences a Lorentz force perpendicular to both. This causes it to spiral around the field line at the cyclotron frequency: $f_c=\frac{eB}{2\pi m\gamma }$ where $\gamma$ is the Lorentz factor (close to 1 for non-relativistic motion, larger for relativistic motion) and the factor of $\gamma$ in the denominator means the cyclotron frequency depends on the particle’s kinetic energy.

For an electron from tritium beta decay (kinetic energy 18.6 keV) in a 1-Tesla magnetic field:

• $\gamma \approx 1.036$ (slightly relativistic)
• $f_c\approx 27$ GHz

A 27 GHz signal is in the K-band of the radio spectrum, easily detectable with modern microwave electronics. The signal frequency depends on the electron’s kinetic energy with extreme sensitivity: a change of 1 eV in the electron energy changes $f_c$ by about 50 kHz at 27 GHz — a relative precision of 2 parts per million.

The technique therefore offers a path to single-electron energy measurement with sub-eV resolution, given that the magnetic field is sufficiently uniform and the radio-frequency signal is sufficiently long-lived to be coherently measured.

The Project 8 demonstration

Project 8 first demonstrated CRES on individual electrons in 2014, using ⁸³ᵐKr (a krypton isotope that conversion-electron-decays at 17.8 keV — close to the tritium endpoint) as a calibration source. Each emitted electron produced a clear chirped radio-frequency signal: the cyclotron frequency increased over time as the electron lost energy to its own radiation and to gas collisions, allowing direct extraction of the initial kinetic energy.

The demonstration achieved sub-eV energy resolution in 2015 publications. Subsequent improvements through 2022 brought the resolution to approximately 0.2 eV per single-electron measurement, comparable to KATRIN’s resolution but with much smaller apparatus and very different systematic uncertainties.

The technique was sufficiently mature by 2018 that Project 8 transitioned to tritium as the source, beginning the proper neutrino-mass programme.

Phase II: gaseous tritium

The current Project 8 detector — Phase II, operational since 2019 — uses a small (cm-scale) magnetic-bottle trap with gaseous tritium as the source. The trap is filled with about $10^{18}$ T₂ molecules per cubic centimetre at low temperature. Beta decay events produce electrons that are caught by the trap, spiral around the field lines, and emit cyclotron radiation that is collected by a coplanar waveguide antenna and amplified by low-noise cryogenic amplifiers.

The Phase II demonstrator is statistically limited: it accumulates electrons at a rate of approximately one per minute. Over a year of running, this corresponds to ~$10^5$ events, roughly comparable to KATRIN’s first-year statistics. The small effective volume is the principal limit.

Phase II results, expected in 2027–2028, will produce a tritium-based neutrino-mass measurement with sensitivity around 1 eV — not yet competitive with KATRIN’s current 0.45 eV bound, but a critical demonstration that the CRES approach scales from calibration sources to actual tritium.

Phase III: scaling up

The next stage, Phase III, scales the technique by deploying multiple parallel CRES traps and increasing the gas density. Target operating parameters:

• Multi-litre trap volumes (vs. cm³ for Phase II)
• Higher tritium density and longer integration times
• Improved magnetic-field uniformity for better resolution
• Atomic tritium instead of molecular T₂

The atomic tritium step is critical and challenging. Atomic tritium is a single tritium nucleus with one electron — a much simpler chemical species than T₂, but unstable because two atomic tritium atoms readily combine to form T₂. Maintaining substantial densities of atomic tritium requires either rapid wall-cooling cycles, magnetic confinement, or dilution with cooler buffer gas. None of these is fully solved at scale.

Phase III aims for 0.4 eV sensitivity by ~2030, and for 0.04 eV sensitivity at the full atomic-tritium stage in the early 2030s.

What the technique uniquely enables

CRES has two unique advantages over the electrostatic-spectrometer approach.

Better intrinsic energy resolution. Cyclotron frequency measurement is limited by the integration time and the magnetic-field uniformity. With current technology, ~10 ms integration times in well-engineered magnetic traps already give 0.5 eV resolution per electron. Continued improvements in magnetic field design, low-noise electronics, and signal-processing algorithms could reach 0.1 eV per electron. KATRIN’s electrostatic spectrometer, by contrast, has a hard intrinsic resolution limit of about 0.93 eV given its physical size and field configuration.

Atomic source compatibility. Molecular tritium decay produces a final-state distribution from molecular vibrational and rotational levels of the daughter ³He⁺—H complex. This distribution is calculable but with theoretical uncertainties at the few-meV level. With atomic tritium, the daughter is a bare ³He⁺ ion in a well-defined atomic state. The molecular-final-state systematic is eliminated entirely.

These two advantages combined mean CRES with atomic tritium has a path to ~0.04 eV — about an order of magnitude below KATRIN’s design goal.

Implications for the mass scale

If Project 8 reaches its 0.04 eV sensitivity:

Inverted ordering parameter space: The minimum effective electron-neutrino mass in the inverted ordering is approximately 0.05 eV (depending on the lightest mass eigenstate value). Project 8 sensitivity of 0.04 eV would directly probe this regime. A null result would push toward normal ordering or vanishing $m_1$.

Cosmological consistency check: The cosmological bound on $\sum m_{\nu }$ from CMB and large-scale structure is approximately 0.12 eV. Project 8 measures the kinematic effective electron-neutrino mass, which under standard mixing parameters is approximately 0.04 eV in inverted ordering and 0.009 eV in normal ordering at the cosmological minimum. Direct measurement at 0.04 eV provides an independent test of cosmology.

Tension with $0\nu \beta \beta$: If $0\nu \beta \beta$ measures Majorana effective mass at one scale and Project 8 measures kinematic effective mass at a different scale, the comparison provides constraints on the neutrino’s Majorana phases — quantities not accessible from any other measurement.

Other approaches in development

Project 8 is the most advanced cyclotron-radiation experiment, but several alternative approaches to 0.1-eV-class neutrino mass measurement exist:

• HOLMES in Italy uses ¹⁶³Ho electron-capture decay, producing a sharp X-ray spectrum that can be measured with sub-eV cryogenic bolometers. The principle is similar to tritium-endpoint physics but with different systematics.
• ECHo in Heidelberg uses the same ¹⁶³Ho approach, with detector technology that has been demonstrated and is being scaled up.
• PTOLEMY is also of interest — its main goal is detecting the cosmic neutrino background through tritium electron capture, but its high-resolution requirement (better than 0.04 eV) makes it relevant for the mass measurement as well.

These programmes are typically slower than Project 8 but provide complementary systematics. By the late 2030s, multiple sub-0.1 eV neutrino mass measurements should exist independently.

Why this matters

Direct, model-independent measurement of the absolute neutrino mass is the last unmeasured fundamental property of the Standard Model neutrino. Project 8’s path — via single-electron cyclotron radiation in atomic tritium — represents the most ambitious technological approach to this measurement currently in development.

If Project 8 succeeds at 0.04 eV: the minimum neutrino mass that can produce the inverted ordering will be directly observable.

If it discovers a non-zero mass: the absolute mass scale is fixed, with cascading implications for cosmology, $0\nu \beta \beta$ interpretation, and beyond-Standard-Model physics.

If it sets a null result at 0.04 eV: the inverted ordering becomes statistically disfavored, and the field consolidates around normal ordering with sub-meV electron-neutrino effective mass.

Either way, the measurement is foundational. Project 8 is one of those experiments that, if it works, will appear in textbooks for decades. The technique itself — single-electron CRES in cryogenic gas — is also broadly applicable beyond neutrino physics, with potential applications in dark-matter searches and precision atomic physics. Project 8’s eventual success will be both a result and a methodology for the next generation of high-precision particle-physics experiments.