The most recent attempt to weigh the elusive particle yielded a more accurate estimate of its upper limit.

Physicists have made progress in determining the mass of the neutrino, one of the most enigmatic of all elementary particles.

Neutrinos have a maximum mass of 0.8 electron volts, according to the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany. Researchers have known for a long time that the particles should be lighter than 1 eV, but this is the first time it has been shown in a direct test. The findings were published in Nature Physics1 on February 14th.

KATRIN reported a prior upper limit of 1.1 eV in 20192. Only an upper limit on the mass has been established so far by the experiment. However, experts believe that after it completes data collection in 2024, it may be able to produce a definitive measurement, and that it is the world’s only experiment capable of doing so.

“It would be really interesting if the KATRIN experiment could identify a neutrino mass before attaining their sensitivity objective of 0.2 eV,” Julia Harz, a theoretical particle physicist at the Technical University of Munich in Germany, says. She adds that it might provide recommendations on how to develop cosmological ideas in particular.

Electrons with high energy

The neutrinos generated by the nuclear disintegration of tritium, a radioactive isotope of hydrogen, are weighed by KATRIN. An electron and a neutrino are ejected when a tritium nucleus transmutes into a helium nucleus (or, more accurately, a particle with an equal mass called an antineutrino). Although the neutrino is lost, the electron is channeled into a 23-metre-long steel vacuum chamber designed like a Zeppelin airship, where its energy is carefully measured.

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The electron transports practically all of the energy released by the breakdown of tritium, although the neutrino loses some. The mass of the particle may be calculated using the value of this deficit.

The findings from KATRIN in 2019 were based on an early run of the experiment in April and May of that year, when the tritium beam was just a fourth of its peak power. The most recent result is based on information from the first full-strength run, which occurred later in 2019. When paired with the preceding findings, these data predict an upper limit of 0.9 eV, which drops to 0.8 eV.

Despite the tightening of the estimate, a lower limit for the neutrino’s mass cannot be reported. According to KATRIN member Magnus Schlösser, a particle physicist at the Karlsruhe Institute of Technology, the statistics do not rule out the possibility that the mass is zero. However, additional data, including cosmic measurements, suggests that the neutrino cannot be massless.

Even after 2024, it’s likely that KATRIN will be unable to determine the neutrino’s lowest mass: if the mass is less than 0.2 eV, it may lay beyond the sensitivity of the experiment.

Schlösser connects the quest to the hunt for a fabled city of riches by the Spanish conquistadors. He compares it to searching for El Dorado. “You narrow the range of possibilities for where you may locate it.”

PHOTO:The Karlsruhe Tritium Neutrino (KATRIN) experiment has produced the most precise measurement of the neutrino’s mass yet. CREDIT: Markus Breig

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