Project 8 takes on the mystery of neutrino mass

Humble neutrinos — electrically neutral particles that glide through the universe, unaffected by the forces of nature — have helped to shape the cosmos. They play a role in nuclear fusion, radioactive decay, and the dispersal of heavy elements around the universe when stars go supernova.

Yet they’re elusive, like a field of dandelion fluff on a breezy spring day. Grabbing one to get a closer look is like grabbing a handful of nothing.

Since 1998, when scientists proved that neutrinos could oscillate and therefore must have mass, physicists around the world have devised elaborate experiments aimed at getting an accurate neutrino mass measurement.

One of those experiments, Project 8, a long-term collaboration of international scientists including Yale researchers, just proved the viability of a new method to measure neutrino mass. Physicists from Yale’s Wright Laboratory involved in Project 8 include Karsten Heeger, the Eugene Higgins Professor of Physics and chair of the Department of Physics and a principal investigator for Project 8; graduate students Talia Weiss and Arina Telles; postdoctoral associate Pranava Teja Surukuchi; associate research scientist Penny Slocum; research scientist James Nikkel; and Luis Saldana, who defended his Ph.D. thesis on Project 8 in 2021.

In a new study published in Physical Review Letters, Project 8 researchers report that they can reliably track and record a relatively infrequent natural occurrence called beta decay. Each beta decay event emits a tiny amount of energy when a rare radioactive variant of hydrogen — called tritium — breaks apart, creating three new subatomic particles: a helium ion, an electron, and a neutrino.

Rather than try to directly detect the neutrino — which effortlessly passes through most detector technology — the Project 8 detector captures the microwave radiation emitted from newborn electrons as they spiral around in a magnetic field. These electrons carry away most — but not all — of the energy released during a beta decay event. It’s that missing energy that can reveal the neutrino mass.

Although this subtraction strategy has been used previously by other research teams, the Project 8 team’s Cyclotron Radiation Emission Spectroscopy (CRES) detector is able to measure electron energy with unprecedented precision.

Project 8 is supported by multiple investments from the U.S. Department of Energy Office of Science, including the Office of Nuclear Physics and its Early Career Research Program, as well as the Pacific Northwest National Laboratory (PNNL) and Lawrence Livermore National Laboratory (LLNL). The research has also been supported by the National Science Foundation and internal investments by Yale, PNNL, LLNL, Massachusetts Institute of Technology, the University of Washington, and the Karlsruhe Institute of Technology Center Elementary Particle and Astroparticle Physics at the Karlsruhe Institute of Technology in Germany.

Heeger, a member of Yale’s Faculty of Arts and Sciences, and Weiss spoke with Yale News about the experiment and its significance in the search for neutrino mass.

Why is it important to know the mass of a neutrino?

Talia Weiss: Among all the known fundamental particles — that is, all the basic building blocks of the universe — the neutrino is the only one whose mass remains a mystery. So, the neutrino mass is a crucial missing piece in the puzzle of particle physics.

Neutrinos are major players in the universe’s evolution from a sea of hot, dense matter to the landscape of galaxies we see today. Once we know the neutrino mass, that number will enter into calculations that describe the universe’s history and predict its future.

A final, key reason why the neutrino mass matters: It may help us understand the very question of what mass is. This thing we call mass seems to emerge when a particle — say, an electron — interacts with a different particle called the Higgs boson. But neutrinos may be the odd particles out. They are unusually light, weighing at least a million times less than all other massive particles. This could point to a special mechanism that supplies the neutrino with mass. But to study that mechanism, physicists want to know just how heavy the neutrino is.

How precise a measurement will the CRES detector be able to attain?

Weiss: Project 8 has already developed a highly precise CRES detector. We can tell what energy an electron has to a precision of about one electron-volt. That’s tiny. If you eat a single Cheez-It, your body will gain 100,000,000,000,000,000,000 electron-volts of energy (a.k.a. five calories).

In the long run, Project 8 aims to build a CRES detector with a precision 10 -to -20 times better than that. This will require a very uniform magnetic field, which will make electrons spiral around in a predictable fashion. If Project 8 combines that precise detector with enough electrons, we should be able to weigh neutrinos as light as 10 million times lighter than the electron.

Why is this work important to Yale?

Karsten Heeger: Understanding the nature of neutrinos and their mass is a fundamental question in neutrino physics and a central goal of my research effort. Since the discovery of neutrino oscillations in 1998 we have known that neutrinos have mass, but we don’t know what it is. Measuring this fundamental property may help us understand why neutrino mass is so small and what role neutrinos play in the evolution of the universe, the formation of galaxies, and exploding stars. Understanding neutrino mass is a key parameter in particle physics and astrophysics alike.

What is Yale’s role in the latest finding?

Weiss: Yale played an important role at several stages, from operating the CRES apparatus to analyzing data. The Yale group specializes in simulating the whole lifecycle of an electron and the light it emits. Those simulations revealed what Project 8’s data should look like. So, once Project 8 had real data in-hand, the simulations helped us pinpoint which signals were actually caused by electrons, and which were just noise.

Yale also played a leading role at the very last stage of the process: placing an upper limit on how heavy the neutrino can be. At Yale, we performed one of two main Project 8 analyses. (The two analyses took different approaches to statistics. Project 8 has published a paper on the approach employed at Yale.)

Looking to the future, the Yale group harnessed Project 8’s recent data to test new methods — based on artificial intelligence [AI] — that identify the special signature of light from a CRES electron. AI methods will likely be essential as Project 8 works to achieve higher precision.

What has been the most intriguing facet of the project for you?

Weiss: The CRES method was only conceived a little over a decade ago, so Project 8 has had the opportunity to meaningfully explore and characterize this method for the first time. For our recent finding, I devised models describing Project 8’s data, which was exciting and challenging, because some features of the data surprised us. By working to understand those features, we substantially advanced our knowledge of CRES. It’s fascinating to work on a project that is pioneering a new technique with such great potential to impact particle physics.

What are the next steps for Project 8?

Weiss: There are two next steps: Scaling up the experiment and using atoms of tritium (instead of molecules) as our source of electrons. Both steps will require creative thinking.

To measure the neutrino mass, Project 8 needs more electrons, which means a larger apparatus. Right now, we are experimenting with designs for a large-scale device that can detect a CRES signal.

We face another challenge, too: When molecules are produced by radioactive decay, they vibrate and rotate, which adds uncertainty to our data. To solve this problem, Project 8 plans to switch from tritium molecules to tritium atoms. The tricky part: When those atoms touch a surface, they’ll pair up into molecules, again. We plan to magnetically suspend the atoms of tritium — restraining them from hitting the walls.

What is Yale’s role in these next steps?

Heeger: Now that we have demonstrated that CRES can be used to measure neutrino mass the goal is to develop an experiment sensitive enough to reach beyond the sensitivity of current laboratory experiments at a fraction of an electron volt. This will require scaling up the CRES technique to larger volumes, demonstrating that we can detect single electrons at unprecedented low levels of power, and developing a robust way for event identification and reconstruction. This work will require innovative technical, computing, and analysis solutions, and we are excited to build the first large-volume demonstrator here at Wright Lab in the coming years.