Neutrino mass has new upper limit

UNC physics researchers contributed to this measurement, which is vital to answering the underlying question on the origin of particle masses.

Inside view of a large shiny stainless steel instrument.
Inside the main spectrometer at Tritium Laboratory Karlsruhe, where at the KArlsruhe TRItium Neutrino (KATRIN) experiment is investigating the most important open issue in neutrino physics: What is the absolute mass scale of neutrinos?

A Nature Physics article published Feb. 15 features a recent discovery made by an international research team that includes scientists from the College of Arts & Sciences’ physics and astronomy department.

The team has established a new upper limit of 0.8 eV/c2 for the mass of the neutrino — the lightest known particle — a milestone that will impact future discoveries in nuclear and particle physics and cosmology.

The consequences of a massive neutrino are profound and may guide the development of an improved Standard Model of particle physics. Although their mass is tiny, the abundance of neutrinos contributes an important role in forming the universe as it appears today.

“Neutrinos were long assumed to be massless until now,” said John Wilkerson, John R. and Louise S. Parker Distinguished Professor, director of the Institute for Cosmology, Subatomic Matter and Symmetries (CoSMS) and one of three UNC-Chapel Hill participants involved in the Karlsruhe Tritium Neutrino Experiment (KATRIN). “Determining this absolute neutrino-mass scale is vital to our understanding of fundamental interactions, cosmology, astrophysics and ultimately to answering the underlying question on the origin of particle masses.”

Headshot of John Wilkerson

John Wilkerson

Research scientist Tom Caldwell was responsible for data acquisition during the experiment, and postdoctoral research associate Eric Martin also contributed.

“The updated limits on the effective anti-electron neutrino mass from KATRIN’s second physics campaign are an exciting new result, an impressive demonstration of the capabilities of the KATRIN apparatus, and the outcome of resolute, coordinated efforts from the international collaboration,” said Caldwell. “It has been a pleasure to build on the UNC group’s KATRIN DAQ efforts, driven by Mark Howe (now retired), and support the KATRIN experiment’s DAQ systems.”

“I worked on KATRIN from construction through commissioning as a graduate student, and into early data collection as a postdoc,” said Martin. “I’m thrilled to see KATRIN advancing our knowledge of neutrinos after these many years of involvement.”

To measure neutrino mass, KATRIN makes use of the beta decay of tritium, an unstable hydrogen isotope. The team was able to determine the mass of the neutrino via the measured energy of electrons released in the decay process.

But to do so required a major technological effort. The experiment houses the world’s most intense tritium source as well as a giant spectrometer to measure the energy of decay electrons with unprecedented precision. Read the abstract and full text of the published paper here.


Ghost particles

The KATRIN experiment is investigating the most important open issue in neutrino physics: What is the absolute mass scale of neutrinos?

The neutrino, “ghost particle of the universe,” is a key to open issues in science on many scales, linking the microcosm of elementary particles to the largest structures in the universe.

Excluding massless particles like photons, neutrinos are the lightest particles in the universe. Their tiny mass is a clear indication for physics beyond the Standard Model of elementary particle physics. On the largest scales, neutrinos act as “cosmic architects” and take part in shaping the visible structures in the universe, as they influence the formation and the distribution of galaxies.

It is the yet unknown mass of the neutrinos [that] could be the key to our understanding of the concept of “mass” in nature.