KATRIN: Charting the present and the future of neutrino mass measurements

Representing the world’s most sensitive direct neutrino mass experiment, the Karlsruhe Tritium Neutrino (KATRIN) experiment is set to evolve from a single-purpose neutrino mass experiment into a broader research infrastructure

For almost a century, neutrinos have challenged our understanding of nature. They are among the most abundant particles in the Universe, passing through Earth and each of us by the trillions every second, yet interacting so weakly with matter that they are extraordinarily difficult to study. One of the greatest mysteries surrounding neutrinos is their mass. While neutrino oscillation experiments have demonstrated beyond doubt that neutrinos possess mass, the absolute value of that mass remains unknown. This represents one of the few experimentally established phenomena that cannot be accommodated within the Standard Model of particle physics in its original form.

Determining the neutrino mass is therefore not simply a matter of measuring another particle property. It addresses fundamental questions in particle physics and cosmology alike. The neutrino mass influences the evolution of large-scale structures in the Universe and provides clues about physics beyond the Standard Model. For these reasons, physicists have pursued increasingly sensitive neutrino mass measurements for almost seven decades.

KATRIN: The world’s most sensitive neutrino scale

Today, the Karlsruhe Tritium Neutrino (KATRIN) experiment represents by far the world’s most sensitive direct neutrino mass experiment. Located at the Tritium Laboratory Karlsruhe (TLK) on the campus of the Karlsruhe Institute of Technology (KIT) in Germany, KATRIN combines an intense, ultra-stable tritium source with the largest electrostatic spectrometer ever constructed. The unique tritium infrastructure at TLK makes Karlsruhe the only location worldwide where such measurements can currently be performed.

KATRIN determines the neutrino mass by studying the beta decay of tritium. During the decay, the available energy is shared between the emitted electron and the neutrino. If the neutrino has mass, the shape of the electron energy spectrum changes ever so slightly near its endpoint at approximately 18.6 keV. The effect is extraordinarily subtle. Only the final few electronvolts of the spectrum carry direct information about the neutrino mass. Measuring this tiny distortion requires exceptional source stability, ultra-high vacuum, and eV-scale energy resolution.

The heart of the experiment is KATRIN’s gigantic electrostatic spectrometer – a stainless-steel vacuum vessel measuring approximately 25 m in length and 10 m in diameter – which has been sent onto the famous shipping route, starting from Deggendorf to Karlsruhe via a more than 8,000 km long journey from Danube via Black Sea, the Mediterranean up the river Rhine. Electrons produced in the world’s strongest windowless gaseous tritium source are magnetically guided over a beamline of about 70 m toward the spectrometer. There, the so-called MAC-E filter (Magnetic Adiabatic Collimation combined with an Electrostatic filter) acts as a high-pass energy filter. By precisely adjusting the retarding potential, only electrons above a selected energy threshold are transmitted to the detector. Repeating this measurement at many different thresholds allows KATRIN to reconstruct the shape of the beta spectrum near its endpoint with unrivalled precision.

Completing the first precision measurement campaign

Scientific data taking began in 2019, marking the start of the most sensitive direct neutrino mass search ever undertaken. Over the following six years, KATRIN accumulated a data set through a series of increasingly refined measurement campaigns. The final neutrino mass scan concluded in October 2025, bringing the primary data-taking programme for the absolute neutrino mass measurement to completion.

The subsequent months have been devoted to the equally demanding task of further understanding and quantifying every possible source of systematic uncertainty. Between the end of February and the beginning of August 2026, dedicated systematic measurement campaigns have characterised the detector response, source properties, electromagnetic fields, and many other effects that could influence the final result.

Together with the enormous accumulated statistics, these studies place KATRIN on track to achieve its target sensitivity of approximately 0.3eV/c² for the effective electron neutrino mass.

Extracting the maximum scientific value from this unique data set requires extensive and independent validation. Two analysis teams within the collaboration are currently performing parallel blind analyses using different analysis frameworks and statistical approaches. At the same time, dedicated working groups continue refining the understanding of the remaining systematic effects to ensure that the final neutrino mass result reaches the highest possible level of confidence.

Behind these scientific achievements stands an equally remarkable technological accomplishment. Since the start of its operations, KATRIN has processed approximately 47 kg of tritium, representing one of the largest throughputs of radioactive tritium ever handled in a controlled experimental facility. Hundreds of tritium transfers, isotope separation campaigns, and exhaust processing operations have been successfully carried out within the TLK infrastructure. Together, these efforts enabled more than 1,400 days of stable tritium operation, providing the foundation for KATRIN’s unprecedented statistical sensitivity.

Looking beyond the neutrino mass

With the completion of the neutrino mass programme, KATRIN now enters an exciting new scientific phase. Rather than focusing exclusively on the tiny endpoint region of the beta spectrum, the experiment will search over a wider range for possible signatures of new particles that could help explain one of the greatest mysteries in modern physics: the nature of dark matter.

The primary target of this programme is the search for so-called sterile neutrinos (hypothetical heavier neutrino states that do not participate in the Standard Model’s weak interaction but could mix slightly with ordinary neutrinos). Such particles are compelling dark matter candidates and would leave characteristic ‘kinks’ in the tritium beta spectrum if produced during radioactive decay.

To enable this search, KATRIN is undergoing a major detector upgrade. Beginning in autumn 2026, the current focal-plane detector will be replaced by a new high-resolution differential detector coined TRISTAN. Its enhancement allows KATRIN to scan essentially the entire tritium beta spectrum for the tiny spectral distortions expected from sterile neutrinos over a wide mass range of more than 10 keV. Note, that in the current setup, this range was limited to a few 10 eVs. The dedicated sterile neutrino measurement campaign is planned for 2027 and 2028 and will significantly expand KATRIN’s scientific reach beyond the determination of the absolute neutrino mass.

Preparing the next generation

Although KATRIN represents the state of the art in direct neutrino mass measurements, the quest for even higher sensitivity continues. Future experiments will need to improve upon KATRIN’s performance by at least another order of magnitude to probe neutrino masses well below 50 meV. Achieving this ambitious goal requires entirely new detector technologies, source concepts, and analysis methods.

In the coming years, KATRIN and the Tritium Laboratory Karlsruhe will therefore increasingly serve as a technology development and demonstration platform for these next-generation experiments, such as KATRIN++.

Among the most promising detector concepts are ultra-high-resolution cryogenic quantum sensor arrays operating at millikelvin (mK) temperatures and single electron time-of-flight techniques capable of measuring electron energies with dramatically improved precision and allowing for high statistics.

Another major challenge arises from the tritium source itself. KATRIN employs molecular tritium (T2), whose beta decay leaves the daughter molecule in a variety of excited rotational and vibrational states. These unavoidable molecular excitations broaden the beta spectrum and fundamentally limit the achievable energy resolution while introducing additional systematic uncertainties.

To overcome this limitation, researchers at TLK are developing methods to produce, cool, and magnetically trap large quantities of atomic tritium. Demonstrating such an atomic tritium source would constitute a crucial technological milestone for future neutrino experiments and provide a substantially cleaner beta spectrum than molecular tritium can offer.

This ambitious programme is being pursued in close collaboration with other international efforts exploring tritium-based neutrino physics, including QTNM in the United Kingdom, Project 8 in the United States, PTOLEMY in Italy and other expert groups from the University of Turku, Finland. Together, these collaborations are laying the technological foundations for the next generation of neutrino experiments.

A new era for precision neutrino physics

The coming years will see KATRIN evolve from a neutrino mass experiment into a versatile research platform addressing some of the most profound questions in modern physics: the existence of sterile neutrinos, the nature of dark matter, and the technologies required to measure the neutrino mass with even greater precision. In doing so, KATRIN and the Tritium Laboratory Karlsruhe will continue to shape the international roadmap for precision neutrino physics, ensuring that Karlsruhe remains at the forefront of this quest for decades to come.

Please note, this article will also appear in the 27th edition of our quarterly publication.