KATRIN is a German acronym (KarlsruheTritiumNeutrino Experiment) for an undertaking to measure themass of theelectron antineutrino with sub-eV precision by examining the spectrum ofelectrons emitted from thebeta decay oftritium. The experiment is a recognizedCERN experiment (RE14).[1][2] The core of the apparatus is a 200-tonspectrometer.
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]
The spectrometer was built byMAN DWE GmbH inDeggendorf. Although only 350 km fromKarlsruhe, the tank's size made land transport impossible.[8] Instead, it was shipped in autumn 2006 by water, down theDanube to theBlack Sea, through theMediterranean Sea andAtlantic Ocean toRotterdam, then up theRhine to Karlsruhe. This 8600 km long detour limited land travel to only the final 7 km from theLeopoldshafen 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 theKarlsruhe Institute of Technology.
Thebeta decay of tritium is one of the least energeticbeta decays. Theelectron and theneutrino which are emitted share only18.6 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 energyneutrinos. 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 byE = m c ², and the electron spectrum should drop off short of the total energy limit and have a different shape.
In mostbeta 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 atrillion decays. In order to filter out the common events so the detector is not overwhelmed, the electrons must pass through anelectric potential that stops all electrons below a certain threshold, which is set a feweV below the total energy limit. Only electrons that have enough energy to pass through the potential are counted.
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]
The precise mass of the neutrino is important not only for particle physics, but also forcosmology. The observation ofneutrino oscillation is strong evidence in favor of massive neutrinos, but gives only a weak lower bound.[12]
Along with the possible observation of neutrinolessdouble beta decay, KATRIN is one of the neutrino experiments most likely to yield significant results in the near future.
49°05′45″N8°26′10″E / 49.09583°N 8.43611°E /49.09583; 8.43611