Carnegie Mellon University
August 10, 2017

CMU Physicists Join International Collaboration to Weigh the Lightest Particle in the Universe

By Amy Pavlak Laird

Physicists from Carnegie Mellon University are part of an international collaboration that’s aiming to weigh the universe’s most elusive particle—the neutrino.

Postulated in 1930 and detected in 1956, the neutrino has captivated scientists for decades. Neutrinos are the most abundant particle in the universe—and the lightest, weighing at least 250,000 times less than an electron. Because neutrinos are so lightweight, attempts to determine their mass have failed, so far. A new experiment, called KATRIN (Karlsruhe Tritium Neutrino), hopes to change that.

“With KATRIN, everything is bigger, everything is better,” said Assistant Research Professor of Physics and alumna (S 2006, 2011) Diana Parno, who is an analysis co-coordinator for the KATRIN experiment, which is based in Karlsruhe, Germany.

Compared to previous, similar experiments done in the 1980s and 1990s, KATRIN is 10-times more sensitive, a feat that required some engineering marvels. From the massive 23-meter-long, 10-meter-wide spectrometer to the 10-centimeter state-of-the-art detector, the experiment’s components push the boundaries of technology.

Neutrinos themselves are extremely difficult to detect—they are very lightweight and have a neutral charge so they pass through material without a trace. So, instead of directly weighing neutrinos, KATRIN scientists will deduce their mass by analyzing the radioactive decay of gaseous tritium, a highly radioactive isotope of hydrogen. When tritium undergoes beta decay, a neutrino is released along with an electron. KATRIN is designed to use the electrons released during beta decay, steering them toward and then through the spectrometer to the detector, where KATRIN scientists can very precisely measure their energy. They’ll use that to determine the mass of the neutrino.

KATRIN may be the last hope of finding the mass of the neutrino using this method.

“I am an optimist,” Parno said. “I am hopeful that we will make a measurement, that the neutrino mass will end up to be within our sensitivity range. I think that would be a lot of fun.”

Pinning down the mass of the neutrino has implications for furthering many areas of physics. Neutrinos affected the way that structures formed in the early universe, leaving their mark on the Cosmic Microwave Background. Theoretical physicists have long sought to explain the tremendous size gap between the electron and the neutrino, so knowing the neutrino mass will give them another piece of the puzzle, and perhaps throw a wrench in the Standard Model.

“The neutrino is a wonderful, surprising particle. I think that its mass is actually going to be outside the classical Standard Model,” Parno said.

Parno and her group are involved with maintenance, operation and characterization of the main KATRIN detector system, data-quality assurance for the entire experiment, and background studies. In her role as analysis co-coordinator, she will be hosting a KATRIN analysis workshop August 7-11 on Carnegie Mellon’s Pittsburgh campus, bringing a few dozen members of the analysis coordination team (pictured at left) to Pittsburgh to discuss where they stand at this stage of the experiment. Although KATRIN is set to begin taking data in 2018, the team is busy analyzing the preliminary data being generated now as they test the equipment. 

“We are asking ourselves: Are we taking good data? How do we get from here to a physics result?” Parno explained.

In addition to Parno, Carnegie Mellon Physics Professor Gregg Franklin and Physics graduate students Larisa Thorne and Ana Paula Vizcaya Hernández are also part of the KATRIN collaboration.