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KATRIN
KATRIN
KATRIN
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Transport of the main spectrometer to the Karlsruhe Institute of Technology.

KATRIN is a German acronym (Karlsruhe Tritium Neutrino Experiment) for an undertaking to measure the mass of the electron antineutrino with sub-eV precision by examining the spectrum of electrons emitted from the beta decay of tritium. The experiment is a recognized CERN experiment (RE14).[1][2] The core of the apparatus is a 200-ton spectrometer.

In 2015, the commissioning measurements on this spectrometer were completed, successfully verifying its basic vacuum, transmission and background properties.[3] The experiment began running tests in October 2016. The inauguration took place 11 June 2018, with the first tritium measurements by the experiment (the so-called First Tritium or FT 2-week engineering run in mid-2018). The projected experiment duration at the time was 5 years. The first science measurements (so-called first campaign) took place 10 April 2019.[4]

In February 2022, the experiment announced an upper limit of mν < 0.8 eV c–2 at 90% confidence level.[5][6] In April 2025 this result was improved to mν < 0.45 eV c–2 at the same confidence level.[7]

Construction and assembly

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Illustration of KATRIN beamline and its main components.[5]

The spectrometer was built by MAN DWE GmbH in Deggendorf. Although only 350 km from Karlsruhe, the tank's size made land transport impossible.[8] Instead, it was shipped in autumn 2006 by water, down the Danube to the Black Sea, through the Mediterranean Sea and Atlantic Ocean to Rotterdam, then up the Rhine to Karlsruhe. This 8600 km long detour limited land travel to only the final 7 km from the Leopoldshafen docks to the laboratory.

The construction proceeded well with several of the major components on-site by 2010. The main spectrometer test program was scheduled for 2013 and the complete system integration for 2014.[9] The experiment is located at the former Forschungszentrum Karlsruhe, now Campus Nord of the Karlsruhe Institute of Technology.

Experiment

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Energy spectrum of the electrons emitted in tritium beta decay. Three graphs for different neutrino masses are shown. These graphs differ only in the range near the high-energetic end-point; the intersection with the abscissa depends on the neutrino mass. In the KATRIN experiment the spectrum around this end-point is measured with high precision to obtain the neutrino mass.
Timeline of neutrino mass measurements by different experiments.[5]

The beta decay of tritium is one of the least energetic beta decays. The electron and the neutrino which are emitted share only 18.6 keV of energy between them. KATRIN is designed to produce a very accurate spectrum of the numbers of electrons emitted with energies very close to this total energy (only a few eV away), which correspond to very low energy neutrinos. If the neutrino is a massless particle, there is no lower bound to the energy the neutrino can carry, so the electron energy spectrum should extend all the way to the 18.6 keV limit. On the other hand, if the neutrino has mass, then it must always carry away at least the amount of energy equivalent to its mass by E = m c ², and the electron spectrum should drop off short of the total energy limit and have a different shape.

In most beta decay events, the electron and the neutrino carry away roughly equal amounts of energy. The events of interest to KATRIN, in which the electron takes almost all the energy and the neutrino almost none, are very rare, occurring roughly once in a trillion decays. In order to filter out the common events so the detector is not overwhelmed, the electrons must pass through an electric potential that stops all electrons below a certain threshold, which is set a few eV below the total energy limit. Only electrons that have enough energy to pass through the potential are counted.

Results

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First results from the first measurement campaign (10 April – 13 May 2019) were published 13 September 2019. They put the upper bound of electron neutrino mass to 1.1 eV.[10][11]

As of September 2019, the experiment hopes to achieve 3 measuring campaigns, each comprising 65 days of active measurement, in a year. The experiment reckons it needs 1000 days of measurement to reach target sensitivity of 0.2 eV (upper limit for neutrino mass). Thus the final results are expected in 5–6 years.

The February 2022 upper limit is mν < 0.8 eV c–2 at 90% CL in combination with the previous campaign.[5][6]

Importance

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The precise mass of the neutrino is important not only for particle physics, but also for cosmology. The observation of neutrino oscillation is strong evidence in favor of massive neutrinos, but gives only a weak lower bound.[12]

Along with the possible observation of neutrinoless double beta decay, KATRIN is one of the neutrino experiments most likely to yield significant results in the near future.

[edit]

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

KATRIN

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from Grokipedia
The KArlsruhe TRItium Neutrino (KATRIN) experiment is a precision physics facility located at the in , dedicated to measuring the effective antineutrino mass with sub-volt sensitivity through kinematic analysis of β-decay . Spanning a 70-meter , it employs a gaseous source, electrostatic filters, and a high-resolution spectrometer to detect near the decay endpoint, enabling model-independent determination of the mass scale. KATRIN builds on earlier experiments like and Troitsk, which established an upper mass limit of about 2 eV/c², by aiming for an unprecedented sensitivity of 0.2 eV/c² at 90% confidence level over 1,000 days of measurement time. Commissioned in 2018 after over a of development involving more than 150 scientists from international institutions, the experiment uses superconducting magnets and systems to minimize and achieve sub-eV resolution. Its results are pivotal for probing the of , as —once thought massless—play a crucial role in cosmic , matter-antimatter asymmetry, and potential extensions like sterile . As of April 2025, KATRIN's analysis of 259 days of data has set a new world-record upper limit on the neutrino mass of 0.45 eV/c² at 90% confidence level, tightening the previous KATRIN upper limit by a factor of almost two and ruling out masses above this threshold. Ongoing measurements through the end of 2025, enhanced by AI-driven data processing, seek to further constrain the mass to below 0.3 eV/c², while future upgrades like the detector will explore and keV-scale candidates. These advancements underscore KATRIN's status as a cornerstone in research, bridging particle physics with .

Background and Motivation

Neutrino Physics Context

, once assumed to be massless particles in the of , were shown to possess non-zero masses through the observation of flavor oscillations in experiments such as and the . These oscillations demonstrate that s change flavors as they propagate, a phenomenon that requires the existence of mass eigenstates with differing masses, thereby confirming that the neutrino masses are non-zero. However, oscillation experiments measure only the squared mass differences (Δm²) between flavors, providing no direct information on the absolute mass scale of the s. In cosmology, the sum of the three neutrino masses (Σm_ν) influences the formation of large-scale and the (CMB) through its effects on matter clustering and radiation density. Recent cosmological observations, including data from the DESI survey as of 2025, combined with Planck, , and surveys, impose upper limits on this sum as tight as Σm_ν < 0.064 eV at 95% confidence level in the standard ΛCDM model, though bounds vary (0.05–0.12 eV) depending on datasets and model assumptions. These tight constraints are approaching or below the minimal sum required by oscillations (~0.06 eV for normal ), prompting discussions of potential tensions or extensions to the . These cosmological constraints highlight the tension with the minimal sum required by oscillations (around 0.06 eV for the normal ), underscoring the need for direct measurements to resolve the absolute scale and its implications for the universe's expansion history. The distinction between relative mass differences from oscillations and the absolute mass scale is crucial for determining the mass —whether the lightest is the flavor or one of the others—which remains unresolved. data yield |Δm²_{21}| ≈ 7.5 × 10^{-5} eV² and |Δm²_{32}| ≈ 2.5 × 10^{-3} eV², but without the overall scale, the cannot be fixed, affecting models of leptogenesis and grand unified theories. Prior to more sensitive experiments, indirect limits from beta decay provided the tightest direct bounds: the experiment reported an upper limit of m_ν < 2.3 eV/c² (95% CL) in , while the Troitsk experiment obtained m_ν < 2.0 eV/c² (95% CL) around the same period. These results from the and established the feasibility of kinematic methods but fell short of probing cosmologically relevant scales. The KATRIN experiment addresses this gap by aiming for a direct sensitivity to the absolute mass below 0.2 eV.

Project History

The KATRIN experiment originated from a proposal submitted in by an international collaboration led by the (KIT), building on prior tritium beta-decay experiments at and Troitsk to pursue a direct, model-independent measurement of the mass. A was published that year, followed by the formal founding of the KATRIN Collaboration in June , which initially comprised over 100 researchers from institutions in , , the , the , and beyond. The collaboration's design efforts culminated in the KATRIN Design Report of 2004, which outlined the experiment's technical specifications and received approval from funding bodies that year, marking the transition from planning to implementation. Funding for KATRIN was secured primarily through the German Federal Ministry of Education and Research (BMBF), the via programs like the Fusion Technology initiative, and contributions from international partners such as the and national agencies in participating countries, with a total project cost of approximately €60 million. Construction commenced in 2005, following the ordering of key components, including the Windowless Gaseous Tritium Source (WGTS) and the main spectrometer vessel. A significant milestone was reached in 2006 with the delivery and initial setup of the pre-spectrometer and main spectrometer at KIT's North, where is housed adjacent to the Tritium Laboratory Karlsruhe (TLK). By 2012, the core apparatus had achieved full assembly, though subsequent commissioning addressed technical challenges, including systems and tritium handling protocols. The KATRIN Collaboration has grown to approximately 150 scientists, engineers, and technicians from more than 20 institutions across six countries, including KIT, the , the , and UK universities, fostering expertise in neutrino physics, cryogenics, and precision . In 2007, KATRIN was recognized as a CERN experiment under the designation RE14, enabling access to 's technical resources and expertise for development and validation. After years of testing and , the experiment initiated its first science run with in April 2019, commencing a multi-year data-taking campaign aimed at achieving sub-eV sensitivity.

Scientific Principle

Tritium Beta Decay

Tritium, the radioactive isotope of hydrogen with atomic mass 3, undergoes beta-minus decay through the reaction 3H→3He+e−+νˉe^3\mathrm{H} \to ^3\mathrm{He} + e^- + \bar{\nu}_e, where an electron and an electron antineutrino are emitted along with the helium-3 nucleus. This process releases a total kinetic energy, known as the Q-value, of 18.592071(22) eV, which is shared among the electron, antineutrino, and recoiling nucleus. The decay is a superallowed Fermi transition, characterized by a nuclear matrix element that is nearly constant and independent of the electron energy, minimizing uncertainties in the spectral shape from nuclear structure effects. Additionally, tritium's short half-life of 12.32 years enables high decay rates and substantial event statistics in experiments. Its low Q-value is particularly advantageous for neutrino mass measurements, as it allows the use of high-resolution spectrometers to probe the spectrum near the endpoint with sufficient relative precision. The shape of the electron energy spectrum provides the basis for extracting the effective electron antineutrino mass mνm_\nu. In the relativistic formulation, the differential decay rate near the endpoint is proportional to the phase-space integral, given by pE(E0−E)(E0−E)2−mν2c4p E (E_0 - E) \sqrt{(E_0 - E)^2 - m_\nu^2 c^4}
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