Experiments to measure the mass of neutrinos

Physics is largely built on theories, principles, and standard models that encompass laws of nature. Neutrinos, particles so tiny that they are thought to be massless by many, have shaped the entire universe and are fundamental components of physics, but much remains unknown about them. A breakthrough in understanding them therefore may shift the paradigms of current standard models of physics. Dr. Hamish Robertson, Boeing Distinguished Professor of Physics at University of Washington, is working to measure the mass of neutrinos -- a fundamental mystery in particle physics for decades. The fact that neutrinos have mass is the only known contradiction to the standard model of particle physics. Knowing the mass will help in building a new theory of matter. Moreover, neutrinos are the only particles that are able to escape the sun’s core into space and take only eight minutes to get to Earth traveling at the speed of light while the sun’s energy takes thousands of years to reach our surface and warm us. Therefore, neutrinos from the sun can tell us about energy production in the sun’s core, and potentially provide insight into the sun’s variability and lead to practical applications in climate change.

The fact that neutrinos can escape the sun so easily means that they are also hard to catch. In order to measure the mass of the neutrino, Dr. Robertson designs very sensitive experiments that catch the electrons that are made at the same time as the neutrinos. In 1988 when many scientists still believed that neutrinos were massless, Dr. Robertson joined a project called the Sudbury Neutrino Observatory (SNO) in Canada and used that detector to show that neutrinos "oscillate", which indicates that they must have mass. To expand on this idea, Dr. Robertson currently engages in the KATRIN project initiated in Germany in 2000 and a newer experiment called Project 8 that will test a novel way of measuring the neutrino mass. The KATRIN experiment is expected to start taking data in 2017, and will run for five years. Project 8, now in early research and development, will build up to a final project in 2020. Running the most sensitive and sophisticated experiments on neutrino mass, Dr. Robertson thus continues to make great strides in particle physics, pioneering the field and expanding our understanding of the world.

Current projects include:

  • The KATRIN Experiment: To measure the mass of the neutrino, Dr. Robertson and his team catch the electrons to see if the electron can take all of the energy available in the decay of tritium, a radioactive isotope of hydrogen, or if some small amount is reserved by the neutrino for its rest mass. This involves finding ways to measure electron energy very precisely, because the neutrino mass is tiny, less than 1/200000 that of the electron. In Seattle, Dr. Robertson works with a student on a way to detect a single electron entering the huge spectrometer, so they can measure the time it takes to go through. If this works, it will cut the time to run the experiment and also improve its performance.
  • Project 8: An experiment just starting, Project 8 involves measuring cyclotron radiation from electrons as a way to determine their energy, as the frequency of radiation is a good measure of the energy. Dr. Robertson and students have been able to prove that this works, and is working to turn it into a neutrino mass experiment. Still in an early R&D phase, Project 8 has four phases planned, with the first small-scale use of tritium beginning early 2016. The final, big experiment would begin construction in about 2020.
  • The MAJORANA Project: Aside from measuring the mass of neutrinos, Dr. Robertson also hopes to look at whether the neutrino is really a different particle from the anti-neutrino, or if it is the same. Only neutrinos among particles could be the same as their antiparticles, and a breakthrough in understanding this property would help make a correct new standard model of particle physics. The MAJORANA Project is an international effort to search for neutrinoless double-beta decay in germanium, and addresses one of the most important questions in fundamental particle physics of today. For this project, Dr. Robertson works to measure the neutrino rate, but unfortunately isotopically enriched materials like germanium that are needed to see the process are highly expensive. Adequate funding will therefore help propel the experiment forward.

Dr. Hamish Robertson is Boeing Distinguished Professor in the Department of Physics at the University of Washington, and Director of Center for Experimental Nuclear Physics and Astrophysics (CENPA). Dr. Robertson has been interested in science from a very young age. When he was 10 years old, his father bought him a crystal radio set. He was so fascinated that he could take the kit, put it together, and hear radio signals coming in. After that, he was very interested in electronics, which turned into an interest in science. He found that by knowing electronics he could do physics in a way that was unusual. When he was studying nuclear physics as a faculty member at Michigan State University, he became very interested in neutrinos and that it was a basic science question that so many people wanted to know the answer to.

He has been working on neutrinos, ghostly and elusive particles, since 1980. In that year his colleague, Tom Bowles, and he sat down together at a conference to map out a plan to measure the mass of the neutrino, which was thought to be zero by most. The problem really appealed to him because it was important in science, and technically challenging to do. Shortly after they started, an announcement came out of Russia that the mass had been measured, but not everyone believed it.

Hamish moved from Michigan State to Los Alamos to work on a new kind of measurement, using gaseous tritium as a source of neutrinos and electrons. He and his colleague were eventually able to show that the Russians were wrong, and that the mass of one kind of neutrino (the electron type) was too small to close the universe gravitationally. But they could not say anything about the two other types. Then in 1988, they joined a new and completely different project, the Sudbury Neutrino Observatory (SNO) in Canada, and used that detector to show that neutrinos "oscillate" which means that they must have mass, and it also relates the masses of all three types. Now Hamish is back with tritium, conducting ever more sensitive experiments to measure the mass. Oscillations provide a lower limit and therefore a scalable range to look in, if researchers can just make the experiment sensitive enough. To this end, Dr. Robertson has two experiments, one in Germany called KATRIN, and one in Seattle called Project 8.

Outside of research, Dr. Robertson loves to ski and hike, when not reading or listening to classical music.

For more information, visit http://www.npl.washington.edu/
KATRIN: http://www.ikp.kit.edu/english/index.php
Project 8: http://www.project8.org/

Polanyi Prize awarded to SNO Collaboration, 2006

National Academy of Sciences, 2004

Fellow, American Academy of Arts and Sciences, 2003

American Physical Society Tom W. Bonner Prize, 1997

Alfred P. Sloan Foundation Fellowship, 1976