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Neutrinos: How Light Are they?

A more precise estimate of the particle’s upper limit has been determined after the latest attempt to weigh it.

Neutrinos, perhaps the most mysterious elementary particle, have been weighed for the first time.

Neutrinos have a mass of approximately 0.8 electron volts, according to the Karlsruhe Tritium Neutrino Experiment in Germany. Scientists have known for years that the particles should be lighter than 1 eV, but this is the first time it has been demonstrated directly. Results were published in Nature Physics on 14 February.

It was reported by KATRIN in 2019 that the previous upper limit was 1.1 eV. Until now, the experiment has only been able to put an upper limit on the mass. However, researchers claim that it might be able to make a definite measurement once it completes data collection in 2024, and is the only study of its kind worldwide.

Julia Harz, a theoretical particle physicist at the Technical University of Munich in Germany, says it would be amazing if KATRIN could determine a neutrino mass before reaching its sensitivity goal of 0.2 eV. According to her, it could provide information about how to enhance cosmological theories.

Energetic Electrons

By weighing neutrinos produced by the nuclear decay of tritium, which is a radioactive isotope of hydrogen, KATRIN measures neutrinos. Tritium nuclei transmute into helium nuclei when they eject both electrons and neutrinos (or, to be precise, antineutrinos) of equal mass. Neutrinos are lost, but electrons are channelled into a 23-metre-long, steel chamber shaped like a Zeppelin airship, where their energy is measured precisely.

While almost all the energy released during the decay of tritium is carried by the electron, some is lost with the neutrino. It is possible to calculate the particle’s mass based on the value of this shortfall.

In April and May of that year, the tritium beam was operating at one-quarter of full strength, so the KATRIN 2019 results are based on that initial run. Based on data from the first full-strength run, conducted later this year, the latest result has been published. Using these data, the upper bound is 0.9 eV, which can be lowered to 0.8 eV if combined with the earlier results.

Although the estimate of the neutrino’s mass has become more precise, it is still impossible to report a lower bound. KATRIN member Magnus Schlösser, a particle physicist at the Karlsruhe Institute of Technology, says that the data do not exclude the possibility that the mass of the atom is zero. The neutrino can’t be massless, however, as demonstrated by other evidence, particularly cosmological evidence.

The KATRIN experiment may not be able to measure the neutrino’s minimum mass, even after 2024, if its mass is less than 0.2 eV: the experiment could not detect it.

According to Schloesser, the quest is like the Spanish conquistadors’ quest for Gold City. “Looking for El Dorado is like searching for the holy grail,” he says. “You narrow your possibilities for finding it.”

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