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Physicists want to ramp up their efforts to weigh neutrinos, which are perhaps the most mysterious of all elementary particles.

Currently, only one experiment in the world has a shot at making such a measurement — the massive, Zeppelin-shaped Karlsruhe Tritium Neutrino (KATRIN) detector in Germany. But researchers at a handful of other laboratories have been developing alternative approaches, and this week they gathered in Genoa, Italy, to compare notes at a workshop called NuMass 2024.

Three teams say they have built small-scale experiments showing that their techniques could work. Another group is working on an approach that could be even more powerful. The researchers hope to build scaled-up versions of these devices that could eventually compete with KATRIN, or even improve on it.

Observations of cosmic structure at the largest scales suggest that neutrinos are extremely light, with masses of, at most, 0.12 electronvolts — four million times smaller than the mass of an electron. If correct, such estimates would put the neutrino’s true mass out of KATRIN’s reach. “We worry that KATRIN, even though it’s a great experiment, may not be able to determine the mass,” says physicist Matteo Borghesi of the University of Milan-Bicocca in Italy, who presented his team’s progress on an alternative experimental technique at the workshop. “We have to be prepared.”

Tiny masses

To weigh neutrinos, physicists use the decay of radioactive isotopes. The neutrinos produced in such decays escape undetected, but their mass can be calculated by measuring the energy of the remaining particles.

KATRIN uses the ‘beta decay’ of tritium, a heavy, radioactive isotope of hydrogen. When tritium decays, one of the three neutrons in its nucleus is converted into a proton, ejecting an electron (also called a beta particle) and a neutrino (or, to be precise, a particle with the same mass called an antineutrino). The decay releases a total amount of energy that is well known, and most of that energy is carried away by the electron and the neutrino, in the form of kinetic energy as well as the energy trapped in the masses of the two particles. The neutrino can come out with a range of possible energies, but at minimum it must carry the amount contained in its mass. KATRIN aims to estimate that minimum by measuring the full range of energies of the corresponding electrons, which it can determine according to where the electrons stop in the Zeppelin-shaped structure.

So far, KATRIN’s best result has been to set an upper bound of 0.8 eV for the mass of the neutrino, and its best possible sensitivity is 0.2 eV. Therefore, when the KATRIN collaboration releases its final results later this year, it will be able to make a definite measurement only if the mass is between 0.2 and 0.8 eV. Such a result would be in striking disagreement with estimates from cosmology, says Olga Mena, a theoretical particle physicist at the Institute of Particle Physics in Valencia, Spain. For the neutrino’s mass to be in the range that KATRIN can measure would take “exotic, non-trivial physics”, Mena says, such as previously unknown fundamental forces affecting neutrinos, or changes to Einstein’s theory of gravity.

Electron capture

Physicists want to develop techniques that could ultimately push the sensitivity to lighter masses, as well as providing cross checks between experiments. The NuMass workshop comes at an interesting time for the field, says physicist Loredana Gastaldo at the University of Heidelberg in Germany, because some of these alternatives have now matured to the point that they could be turned into fully fledged experiments. One option takes advantage of the decay of holmium-163, a radioactive isotope of the rare-earth element holmium.

Unlike tritium, holmium-163 doesn’t undergo beta decay. Instead, one of the electrons in the atom gets ‘captured’ by a proton in its nucleus. This converts the proton into a neutron, releasing a neutrino and photons. The captured electron leaves behind a gap in the configuration of the atom’s electrons, and the other electrons quickly rearrange themselves, releasing energy. If the original holmium atom were embedded in a material, all of that energy would remain trapped, producing a tiny amount of heat that can be measured with a sensitive enough detector.

The idea of using this approach, called electron capture, first came to Álvaro de Rújula, a theoretical physicist at CERN, the European particle-physics laboratory outside Geneva, Switzerland, during a stay in Rio de Janeiro, Brazil, in 1981. He was on the beach in the Copacabana neighbourhood, he says, when he suddenly got inspiration from looking at the shape of the nearby Sugarloaf Mountain, which has “the shape of the electron-capture spectrum” (a graph that shows the range of energies that can be measured as leftover heat from the decay).

Physicists abandoned the idea after some initial attempts, but it was taken up again in the late 1990s by Gastaldo and by another physicist, Angelo Nucciotti at Milan-Bicocca. Although both teams were considerably underfunded and understaffed, they worked “heroically” and with little recognition for many years, says de Rújula.

Each of the two groups takes a different approach to inject holmium-163 into slivers of metal embedded in sensitive heat detectors that are kept at temperatures close to absolute zero. Both teams have shown they can measure the energy with high precision. In 2019, Gastaldo and her collaborators placed an upper bound of 150 eV on the mass of the neutrino, and they are currently working on improving that by a factor of 10. “We can now show that holmium is in the game, too,” says Gastaldo.

Alternative methods

Another approach was described at the workshop by Juliana Stachurska, a physicist at the Massachusetts Institute of Technology (MIT) in Cambridge. In an experiment called Project 8, she and her collaborators put low-density tritium gas in a magnetic bottle, which traps the electrons from beta decay using magnetic fields. In work published last year1, the researchers showed that they could measure the energy of the electrons with high precision by analysing radio waves. The team plans to switch to atomic tritium, which is more challenging to handle but would remove some experimental uncertainties that have limited the precision of previous experiments, including KATRIN. “Nobody has ever done atomic tritium before,” says Stachurska.

Project 8’s Phase II apparatus at the University of Washington.

Project 8 experiment is trialing a new technique to measure the mass of the neutrino.Credit: Alec Lindman

MIT physicist JosephFormaggio, a spokesperson for Project 8, says he hopes to one day build a large-scale version of the experiment that could get the sensitivity down to 0.04 eV — small enough to beat the stringent limits from cosmological experiments.

Even further down the line, a proposed experiment called PTOLEMY plans to use solid, rather than gaseous, tritium attached to films of an atomically thin carbon material called graphene. This would enable researchers to pack in much more tritium and get a higher number of radioactive emissions.

For now, the community is eagerly awaiting the final results from KATRIN, says Borghesi. Even after that experiment reaches the limits of its design sensitivity, the researchers involved plan to press on and upgrade it. Magnus Schlösser, a physicist at the Karlsruhe Institute of Technology, says his main message at the workshop was that “KATRIN will not close the doors after the current campaign”.

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