Physicists are getting closer to the elusive mass of neutrinos



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Inside the large electrostatic spectrometer, at the heart of the KTRRIN experiment of Karlsruhe tritium neutrinos

The experience of Karlsruhe Tritium Neutrino (KATRIN) monitors the nuclear disintegration of tritium, hydrogen isotope.Credit: Michael Zacher

An experiment in Germany made it possible to optimally measure the maximum mass of neutrinos – light subatomic particles so difficult to measure that physicists could not estimate an upper limit of their mass.

The first results1 According to the experience of KATRIN (Karlsruhe Tritium Neutrino) in southwestern Germany, neutrinos weigh at most 1.1 electronvolts (eV). This measure is a double improvement over previous measurements of 2 eV in the upper limit. Guido Drexlin, co-spokesperson for the KATRIN collaboration, presented the results on Sept. 13 at a conference in Toyama, Japan.

Neutrinos are among the most abundant particles in the universe. They are also the lightest of all known subatomic particles that have a mass weighing about 500,000 times less than an electron. But they tend to cross the material undetected, making direct measurements extremely difficult. Physicists still have to measure the mass of a neutrino directly.

KATRIN collected data during the few weeks of its initial run in April and May. The detector monitors the nuclear disintegration of the heavy isotope of hydrogen called tritium, in which a neutron transforms into a proton and emits an electron and a neutrino. KATRIN can not catch neutrinos directly. Instead, it measures the energy range of electrons that spin inside an airship shaped chamber 23 meters long, which is the largest ultra-vacuum system. of the world. This measurement reveals the range of energies that invisible neutrinos would have, which in turn reveals the mass of particles.

The team posted a pre-print containing the results in the arXiv repository and submitted it to a journal for publication. Drexlin says the results have been 18 years of work since KATRIN's design, but they are only a first glimpse of its potential. "It shows the community that KATRIN is up and running," he says.

Over the next five years, Drexlin's collaboration involves continuous improvements in sensitivity that could allow KATRIN to actually measure the mass of a neutrino – or reduce the range of their estimates to the extent that the sensitivity of the machine permits. Cosmological observations suggest that neutrinos could be 0.1 eV or lighter.

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