8 project is an important milestone in measuring the mass of neutrinos. Neutrinos are elusive subatomic particles that effortlessly pass through ordinary matter and play an important role in the particles that make up our universe. To fully explain how our universe formed, we need to know its mass. But, like many of us, it's finding ways to avoid being weighed.
In 2022, the KATRIN research team determined an upper limit on how massive a neutrino could be. This landmark achievement was the result of decades of hard work. But these results only narrow the search window. KATRIN will soon reach and even one day exceed its target detection limit, but featherweight neutrinos may be even lighter, which begs the question: "What's next? What's next?"
TA GPH12The cyclotron radiative emission spectroscopy (CRES) seen here is the key to a completely new approach aimed at determining the mass of the elusive neutrino. Source: Alec Lindemann, Project 8 Team
Tracking ghost particles
In the latest study , the Project 8 team reports in the journal Physical Review Letters that they can use a completely new technique to reliably track and record a natural phenomenon called beta decay. When a rare radioactive variant of hydrogen - tritium - decays into three subatomic particles: helium ions, electrons and neutrinos, each decay releases a tiny amount of energy.
The ultimate success of Project 8 depends on an ambitious plan. Rather than trying to detect neutrinos directly - which can pass through most detector technologies without difficulty - the team used a simple measurement strategy that can be summarized as follows:
Einstein told us that the total mass of a tritium atom is equal to the energy of its parts. When we measure the free electrons produced by beta decay, we know the total mass, and the "missing" energy is the neutrino's mass and motion.
Brent VanDevender, one of the principal researchers of Project 8 at the Department of Energy’s Pacific Northwest National Laboratory, said: " In principle, as technology develops and scales up, it may be possible to reach the range required to determine the mass of neutrinos "
Why Project 8?
These researchers chose to pursue this ambitious strategy because they had examined the pros and cons and concluded that it was feasible.
Talia Weiss is a graduate student in nuclear physics at Yale University. She and her Project 8 colleagues have spent years studying how to accurately distinguish electronic signals from electronic background noise. Christine Claessens is a postdoc at the University of Washington. She received her PhD from Project 8 at the University of Mainz in Germany. Weiss and Claesens conducted two final analyses, placing the first constraints on the neutrino masses derived from the new technique.
Weiss said: "The neutrino is incredibly light. It is more than 500,000 times lighter than the electron. So when a neutrino and an electron are produced at the same time, the effect of the neutrino mass on the electron's motion is minimal. We want to see this tiny effect. So we need a super-accurate way to measure how fast the electron is moving." Project
8 relies on just such a technology, conceived more than a decade ago by physicists Joe Formaggio and Ben Monreal, then both at MIT. An international team rallied around the idea and formed Project 8 to turn the idea into a practical tool. The resulting method is called cyclotron radiative emission spectroscopy (CRES). It captures the microwave radiation emitted by newborn electrons as they spin in a magnetic field. These electrons carry away most, but not all, of the energy released during beta decay. It's this missing energy that reveals the neutrino's mass. This is the first time that CRES technology has been used to measure the beta decay of tritium and set an upper limit on the mass of neutrinos.
How do scientists weigh neutrinos? Image source: Animation produced by Sara Levine at Pacific Northwest National Laboratory
Innovative methods and challenges
The research team is only interested in tracking these electrons because their energy is key to revealing the mass of neutrinos. While this strategy has been used before, the electron energy measured by the CRES detector is so critical that its scalability potential exceeds any existing technology. And this scalability is what sets Project 8 apart. Elise Novitski is an assistant professor at the University of Washington and led many aspects of the newly published work.
Nowitzki said: "No one is doing this. We're not taking existing technology and trying to tweak it a little bit. We're kind of in the Wild West."TAGPH2 0
In the latest experiment at the University of Washington in Seattle, the team tracked 3,770 tritium beta decay events in a pea-sized sample cell over an 82-day experimental window. The sample cell is cryogenically cooled and placed in a magnetic field that captures emerging electrons for a long time, allowing the system's recording antenna to record the microwave signal.
Best of all, the team recorded zero false signals or background events that could not be mistaken for real signals. This is important because even a very small background can mask the neutrino mass signal, making interpretation of the useful signal more difficult.
Researchers part of Project 8, led by PNNL experimental physicist Noah Oblath, also developed a suite of specialized software - each named after various insects - to take raw data and convert it into signals that can be analyzed. Project engineers also put on their tinkering hats and invented various parts to make Project 8 a success.
Nowitzki said: "Our engineers are critical to this work. From an engineer's perspective, this is a layman's approach. Experimental physics is at the interface of physics and engineering. You have to have particularly adventurous engineers and practical-minded physicists to collaborate, Make these things happen, because these things are not in the textbooks. "
Getting to the finish line
Now that the research team has proven that their design and experimental system can work with tritium molecules, they have an urgent task ahead of them. Part of the team is working on the next step: building a system that can generate, cool and capture individual tritium atoms. This step is tricky because tritium, like its more abundant cousin hydrogen, prefers to form molecules. These molecules will make the Project 8 team's ultimate goal impossible to achieve. Led by physicists from the University of Mainz, the researchers are developing a test bed to create and capture atomic tritium using a complex array of magnets. This prevents the atomic tritium from coming into contact with the sample cell walls - as it will almost certainly revert to its molecular form at the cell wall.
Advances in this technology, along with upgrades to the entire instrument, will be critical steps in reaching and ultimately exceeding the sensitivity achieved by the KATRIN team.
Currently, a research team composed of members from ten research institutions is testing designs to scale experiments from a pea-sized sample chamber to one a thousand times larger. The idea is to use larger listening devices to capture more beta decay events - from the size of a pea to the size of a beach ball.
"Project 8 is not only a bigger and better CRES experiment, it is the first CRES experiment and the first to use this detection technology," Oblath said. "This has never been done before. Most of the experiments are 50 or 100 years old, at least the detection technology they use, and this is really new."