Carnegie Mellon University
September 16, 2019

CMU Professor Assists International Experiment in Pinning Down Elusive Neutrino Mass

Ben Panko

An international team of scientists has announced a breakthrough in its quest to measure the mass of the neutrino, one of the most abundant, yet elusive, elementary particles in our universe.

At the 2019 Topics in Astroparticle and Underground Physics conference in Toyama, Japan, leaders from the KATRIN experiment reported Friday that the estimated range for the rest mass of the neutrino is between 0.02 and 1 electron volts, or eV. These inaugural results obtained earlier this year by the Karlsruhe Tritium Neutrino experiment — or KATRIN — cut the mass range for the neutrino by more than half by lowering the upper limit of the neutrino's mass from 2 eV to 1 eV.

"Neutrino mass is a big hole in our understanding of particle physics," said Diana Parno, an assistant professor at Carnegie Mellon University’s Department of Physics and an analysis co-coordinator for the worldwide collaboration behind KATRIN. "This result addresses that empty spot."

The KATRIN experiment is based at the Karlsruhe Institute of Technology in Germany and involves researchers at 20 research institutions around the globe. In addition to Carnegie Mellon, KATRIN member institutions in the United States are:

  • The University of Washington, led by Hamish Robertson, a professor emeritus
  • The University of North Carolina at Chapel Hill, led by John Wilkerson, a professor of physics and astronomy
  • The Massachusetts Institute of Technology, led by Joseph Formaggio, a professor of physics
  • The Lawrence Berkeley National Laboratory, led by Alan Poon, deputy director of the Nuclear Science Division
  • Case Western Reserve University, led by Benjamin Monreal, an associate professor of physics

Neutrinos are abundant. They are one of the most common of the known fundamental particles in our universe, second only to photons. "Collectively, their mass has shaped the way that the universe has formed," Parno said.

Yet neutrinos are also elusive. They are neutral particles with no charge and they interact with other matter only through the aptly named "weak interaction," which means that opportunities to detect neutrinos and measure their mass are both rare and difficult.

The KATRIN discovery stems from direct, high-precision measurements of how a rare type of electron-neutrino pair share energy. This approach is the same as neutrino mass experiments from the 1990s and early 2000s in Mainz, Germany, and Troitsk, Russia, both of which set the previous upper limit of the mass at 2 eV. The heart of the KATRIN experiment is the source that generates electron-neutrino pairs: gaseous tritium, a highly radioactive isotope of hydrogen. As the tritium nucleus undergoes radioactive decay, it emits a pair of particles: one electron and one neutrino, both sharing 18,560 eV of energy.

KATRIN scientists cannot directly measure the neutrinos, but they can measure electrons, and try to calculate neutrino properties based on electron properties.

Most of the electron-neutrino pairs emitted by the tritium share their energy load equally. But in rare cases, the electron takes nearly all the energy — leaving only a tiny amount for the neutrino. Those rare pairs are what KATRIN scientists are after because — thanks to E = mc2 — scientists know that the minuscule amount of energy left for the neutrino must include its rest mass. If KATRIN can accurately measure the electron's energy, they can calculate the neutrino's energy and therefore its mass.

The tritium source generates about 25 billion electron-neutrino pairs each second, only a fraction of which are pairs where the electron takes nearly all the decay energy. The KATRIN facility in Karlsruhe uses a complex series of magnets to channel the electron away from the tritium source and toward an electrostatic spectrometer, which measures the energy of the electrons with high precision. An electric potential within the spectrometer creates an "energy gradient" that electrons must "climb" in order to pass through the spectrometer for detection. Adjusting the electric potential allows scientists to study the rare, high-energy electrons that carry information concerning the neutrino mass. 

Parno's team, including Ph.D. students Larisa Thorne and Ana Paula Vizcaya Hernández and postdoctoral research fellow Yung-Ruey Yen, contributes primarily to analysis for KATRIN, with special attention to backgrounds and to fitting, and assists in analysis coordination for the experiment. Vizcaya Hernández has been invaluable in demonstrating control of ion backgrounds through simulations and test measurements, Parno notes, and Thorne has worked to create models of how KATRIN's systems work so its results can be better understood. Professor Emeritus of Physics Gregg Franklin, who advised Parno's 2011 Ph.D. in physics from Carnegie Mellon, also contributed to the modeling effort.

Parno and her group have been traveling overseas regularly to work on maintenance and upgrades for KATRIN's detector, a fitting task since she oversaw its 2011 disassembly and shipment in 28 crates from the University of Washington to Germany.

"We only broke 2 things," Parno joked, both of which were luckily easily fixed.

Carnegie Mellon also hosted a 2017 workshop for the KATRIN analysis team.

Other U.S. institutions have made broad contributions to KATRIN, including providing the electron-detector system — the "eye" of KATRIN — which looks into the heart of the spectrometer, an instrument built at the University of Washington. The University of North Carolina at Chapel Hill led the development of the detector’s data acquisition system, the "brains" of KATRIN. MIT's contribution was the design and development of the simulation software used to model the response of KATRIN. The Lawrence Berkeley National Laboratory, along with the UW, provided much of the analysis framework. The Case Western Reserve University was responsible for the design of the electron gun, central to calibrating the KATRIN apparatus.

With tritium data acquisition now underway, U.S. institutions are focused on analyzing these data to further improve our understanding of neutrino mass. These efforts may also reveal the existence of sterile neutrinos, a possible candidate for the dark matter that, though accounting for 85% of the matter in the universe, remains undetected.

"KATRIN is not only a shining beacon of fundamental research and an outstandingly reliable high-tech instrument, but also a motor of international cooperation, which provides first-class training of young researchers," said KATRIN co-spokespersons Guido Drexlin of the Karlsruhe Institute of Technology and Christian Weinheimer of the University of Münster in a statement.

Now that KATRIN scientists have set a new upper limit for the mass of the neutrino, project scientists are working to narrow the range even further.

"It's really exciting to be able to measure a cosmological building block in the laboratory," Parno said.

The U.S. Department of Energy's Office of Nuclear Physics has funded the U.S. participation in the KATRIN experiment since 2007.