Using a new instrument, researchers in the College of Arts and Sciences at Indiana University Bloomington are closer than ever to finding the mass of the so-called ‘ghost particle,’ which has been studied for more than 75 years.

In the wake of this discovery it was hard to tell who was buzzing more: Walter Pettus, assistant professor in the College’s Department of Physics, or the neutrinos he was studying. Professor Pettus was days away from co-publishing a paper that provided a proof-of-concept for an instrument that could measure a neutrino’s mass more precisely than ever before.

That ‘can’t’ is on the brink of becoming a ‘couldn’t’, thanks to a new technique co-developed by Pettus and his nearly 50 colleagues in the research collaboration Project 8. Their instrument measures radiation emitted by an electron (the neutrino’s counterpart) to indirectly gauge the neutrino’s energy and thus its mass. On September 6th, the group published their results in Physical Review Letters.

So, the team devised an entirely new form of detection called Cyclotron Radiation Emission Spectroscopy. The powerhouse behind this technique is tritium, a rare, radioactive type of hydrogen that emits electrons and neutrinos as it decays. “Dirty secret: we never measure the neutrino,” said Pettus. Instead, the team measured the radiation created by a neutrino’s charged counterpart, the electron, and then solved for the neutrino’s mass using a series of equations.

As it turns out, when to measure was almost as important as what to measure. According to Einstein’s theory of special relativity, E=mc2, the faster a particle moves, the heavier it is. When the electrons and neutrinos are emitted in tritium decay, they’re typically traveling close to the speed of light. By shifting their focus to the endpoint of the decay, where the electron travels the fastest and the neutrino travels the slowest, the researchers observed the subtle impact of the neutrino mass. Those slow neutrinos had the least relativistic effect, and thus their mass was closest to their ‘real’ value.

Their technique was a great success. The team’s form of spectroscopy was able to tune out background ‘noise’ almost entirely, leading to high-resolution energy data that has excited the neutrino community. “A re-think like what Project 8 has demonstrated is really a once-in-a-lifetime kind of event,” said Mark Messier, chair of the Department of Physics at IU who was not involved in the study.

The current version of the instrument is located at the University of Washington in Seattle, where Pettus did his postdoctoral studies and initially became involved with Project 8. Just as Pettus himself transitioned from the West Coast to Bloomington, he hopes that the instrument will do the same.

“The fact that there are so many cool neutrino experiments going on here is one of the reasons I came to IU,” said Pettus. “We’ve got one of the best neutrino programs in the country,” he said, one more than 75 years in the making.

IU has been recognized as a hub for neutrino research since 1946, when professors Emil Konopinski and Lawrence Marvin Langer returned to Indiana after their work with J. Robert Oppenheimer on the Manhattan Project. The pair studied neutrinos at IU, even helping to develop the theoretical and experimental techniques that instruments like KATRIN used to measure neutrino mass.

Today, a total of three labs led by six professors at IU focus their research on neutrinos, each with a different angle. “On any given day you might be able to find all these world experts on the second floor of Swain West - it’s an incredible place to do neutrino research,” said Department Chair Messier.

Conveniently, the Bloomington campus has not only the specialists to support Project 8’s apparatus, but also the space.

From 1976 to 2010, the Multidisciplinary Engineering and Sciences Hall housed the Indiana University Cyclotron Facility, a major nuclear research space that Langer helped to design. “As we think about, what does this apparatus look like 1010 larger, it actually fits pretty well in the cyclotron facility,” said Pettus. His group plans to evaluate the dimensions and conditions of the space to see if the apparatus could find a new home in Bloomington in the coming years.

“It’s really great doing this work at a place like Indiana with deep historical connections to the field,” said Pettus. “It feels ‘right’ to bring it full circle, and hopefully this chapter can actually complete the circle and get a definite answer.”