Neutrino Mass: Quest Intensifies to Determine

Quest for neutrino mass measurement technologies

Physicists are intensifying their endeavors to measure neutrinos, the most enigmatic of elementary particles. Presently, a singular experiment worldwide stands a chance at conducting such measurements — the colossal, Zeppelin-shaped Karlsruhe Tritium Neutrino (KATRIN) detector situated in Germany. Nonetheless, researchers across several laboratories have been devising alternative methodologies, convening this week in Genoa, Italy, for the NuMass 2024 workshop to exchange insights.

Three teams have constructed diminutive-scale experiments, demonstrating the viability of their techniques. Another consortium is exploring an approach potentially more robust. Their aim is to construct enlarged versions of these devices capable of rivaling, or even surpassing, KATRIN.

Evidence gleaned from cosmic structures on the grandest scales suggests neutrinos possess exceptionally light masses, likely no greater than 0.12 electronvolts — a magnitude four million times smaller than that of an electron. If accurate, such assessments would place the true mass of neutrinos beyond KATRIN’s scope. Physicist Matteo Borghesi of the University of Milan-Bicocca in Italy, presenting his team’s advancements on an alternative experimental technique at the workshop, expressed concerns: “We worry that KATRIN, even though it’s a great experiment, may not be able to determine the mass. We have to be prepared.”

Neutrino mass: KATRIN’s beta decay experiment

Physicists determine the mass of neutrinos by exploiting the decay of radioactive isotopes. In particular, they study the beta decay of tritium, a radioactive form of hydrogen. During this process, a neutron in the tritium nucleus transforms into a proton, emitting an electron (or beta particle) and a neutrino (technically an antineutrino). While the neutrino escapes detection, its mass can be inferred by measuring the energy of the remaining particles.

The KATRIN experiment focuses on precisely this phenomenon. By observing the energies of electrons resulting from tritium decay, KATRIN aims to estimate the minimum energy carried away by the neutrino, which corresponds to its mass. This is achieved by analyzing the full range of electron energies and determining their stopping points within the detector.

KATRIN’s most significant achievement thus far is establishing an upper limit of 0.8 electron volts (eV) for the neutrino mass, with a sensitivity down to 0.2 eV. Therefore, for the experiment to yield a definitive measurement, the neutrino mass must fall within this range. Any result within KATRIN’s measurable range would challenge existing estimates from cosmology, suggesting the presence of “exotic, non-trivial physics,” as described by Olga Mena, a theoretical particle physicist at the Institute of Particle Physics in Valencia, Spain. This could entail the existence of undiscovered fundamental forces acting on neutrinos or modifications to Einstein’s theory of gravity.

Electron capture: Holmium-163’s potential

Physicists are exploring innovative techniques to enhance sensitivity to lighter neutrino masses and to provide complementary verification between experiments, remarks Loredana Gastaldo, a physicist at the University of Heidelberg in Germany. She highlights the timeliness of the NuMass workshop, as several alternative methods have matured sufficiently to be developed into full-fledged experiments. One such method capitalizes on the decay of holmium-163, a radioactive isotope of the rare-earth element holmium.

Unlike tritium, holmium-163 does not undergo beta decay. Instead, one of the electrons within the atom undergoes ‘capture’ by a proton in the nucleus, converting the proton into a neutron and releasing a neutrino along with photons. The captured electron creates a gap in the electron configuration of the atom, prompting rapid rearrangement of the remaining electrons and the release of energy. If the original holmium atom were incorporated into a material, this energy would be trapped, generating a minute amount of heat detectable with highly sensitive detectors.

The concept of utilizing this electron capture method originated with Álvaro de Rújula, a theoretical physicist at CERN, during his stay in Rio de Janeiro, Brazil, in 1981. Inspired by the shape of Sugarloaf Mountain, he envisioned a spectrum resembling electron capture, prompting the exploration of this approach.

Although initial attempts were abandoned, physicists revisited the idea in the late 1990s, championed by Gastaldo and physicist Angelo Nucciotti at Milan-Bicocca. Despite limited resources and recognition, both teams persevered diligently for many years.

Each group employs a distinct method to introduce holmium-163 into metal slivers embedded within sensitive heat detectors maintained at near-absolute-zero temperatures. Both teams have demonstrated high-precision energy measurements. In 2019, Gastaldo and her collaborators established an upper limit of 150 electron volts (eV) for the neutrino mass, with ongoing efforts to enhance sensitivity by an order of magnitude. Gastaldo emphasizes the significance of holmium’s inclusion in the study of neutrino mass.

Pushing the boundaries: Innovations in tritium beta decay experiments

Juliana Stachurska, a physicist at the Massachusetts Institute of Technology (MIT) in Cambridge, presented an alternative approach during the workshop. In Project 8, Stachurska and her team utilized a magnetic bottle to trap electrons from beta decay in low-density tritium gas, employing magnetic fields for containment. By analyzing radio waves, they achieved high-precision measurements of electron energy in research published recently. The team plans to transition to atomic tritium, which poses greater handling challenges but promises to reduce experimental uncertainties that have hindered previous studies, including KATRIN. Joseph Formaggio, a physicist at MIT and spokesperson for Project 8, envisions a large-scale version of the experiment capable of reaching sensitivities as low as 0.04 electron volts (eV), surpassing the stringent limits set by cosmological experiments.

Furthermore, the proposed PTOLEMY experiment plans to utilize solid tritium attached to films of graphene, an atomically thin carbon material, rather than gaseous tritium. This adjustment would allow researchers to accommodate more tritium and increase the number of radioactive emissions.

The scientific community eagerly anticipates the final results from KATRIN, according to Borghesi. Even as KATRIN nears its design sensitivity limits, researchers intend to enhance and upgrade the experiment. Magnus Schlösser, a physicist at the Karlsruhe Institute of Technology, emphasized at the workshop that “KATRIN will not close the doors after the current campaign,” highlighting the ongoing pursuit of advancements in neutrino research.

• March 4th, 2024: A previous iteration of this article erroneously stated that tritium possesses three neutrons in its nucleus. It actually contains two.

Resources

1. ^JOURNAL Castelvecchi, D. (2024). How heavy is a neutrino? Race to weigh mysterious particle heats up. Nature. [Nature]
2. ^JOURNAL Esfahani, A. A., Böser, A., Buzinsky, N., Carmona-Benitez, B., Claessens, C., De Viveiros, L., Doe, P., Fertl, M., Formaggio, J., Gaison, J., Gladstone, L., Grando, M., Guigue, M., Hartse, J., Heeger, K., Huyan, X., Johnston, C., Jones, A., Kazkaz, D., . . . Ziegler, A. (2023). Tritium Beta Spectrum and Neutrino Mass Limit from Cyclotron Radiation Emission Spectroscopy. HAL (Le Centre Pour La Communication Scientifique Directe). [HAL (Le Centre Pour La Communication Scientifique Directe)]

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