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A section of a cryogenic CRES cell, where electrons are produced in radioactive decay and magnetically trapped. The cell waveguide has a diameter of 10.03 mm and a length of 132 mm (the distance between the RF windows). The cyclotron radiation travels axially up the waveguide (left in the circular view), toward the loudspeakers and readout electronics. credit: Physical review letters (2023). doi: 10.1103/PhysRevLett.131.102502

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A section of a cryogenic CRES cell, where electrons are produced in radioactive decay and magnetically trapped. The cell waveguide has a diameter of 10.03 mm and a length of 132 mm (the distance between the RF windows). The cyclotron radiation travels axially up the waveguide (left in the circular view), toward the loudspeakers and readout electronics. credit: Physical review letters (2023). doi: 10.1103/PhysRevLett.131.102502

The humble neutrino, an elusive subatomic particle that easily passes through ordinary matter, plays a huge role among the particles that make up our universe. In order to fully explain how the universe arose, we need to know its mass. But, like many of us, he avoids being weighed down.

Now, an international team of researchers from the United States and Germany leading an ambitious endeavor called Project 8 reports that their signature strategy is a realistic competitor to be the first to measure the mass of neutrinos. Once fully scaled up, Project 8 could help reveal how neutrinos influenced the early evolution of the universe as we know it.

In 2022, the Katrin research team has set an upper limit on how heavy a neutrino can be. This achievement was a monumental feat that took decades to complete. But these results simply narrow your search window. Katrin will soon reach its target detection limits, and maybe one day exceed them, but the featherweight neutrino may be even lighter, which begs the question: “What’s next?”

In their latest study, Project 8 submitted a report Physical review letters that they can use a completely new technology to reliably track and record a natural event called beta decay. Each event releases a tiny amount of energy when a rare radioactive form of hydrogen – called tritium – decays into three subatomic particles: a helium ion, an electron and a neutrino.

The ultimate success of Project 8 depends on an ambitious plan. Rather than trying to detect a neutrino — which easily passes through most detector technology — the research team instead followed a simple measurement strategy that can be summarized as follows:

We know that the total mass of a tritium atom is equal to the energy of its parts, thanks to Einstein. When we measure a free electron from beta decay, and know the total mass, the “missing” energy is the neutrino’s mass and motion.

“In principle, as technology develops and expands, we have a realistic opportunity to get to the scale needed to determine neutrino mass,” said Brent Vandefender, a Project 8 principal investigator at the Department of Energy’s Pacific Northwest National Center. Lab.

Why Project 8?

These researchers chose to pursue an ambitious strategy because they considered the pros and cons and concluded that it could work.

Talia Weiss is a graduate student in nuclear physics at Yale University. She and her project colleagues spent 8 years figuring out how to accurately extract electron signals from electronic background noise. Christine Claessens is a postdoctoral fellow at the University of Washington and earned her Ph.D. In Project 8 at the University of Mainz, Germany. Weiss and Claessens performed the two final analyzes that set limits on the neutrino mass derived from the new technique for the first time.

Credit: Pacific Northwest National Laboratory

“The neutrino is incredibly light,” Weiss said. “It is more than 500,000 times lighter than an electron. Therefore, when neutrinos and electrons are created at the same time, the mass of the neutrino has only a small effect on the motion of the electron. We want to see this small effect. Therefore, we need a super-accurate way to measure how fast electrons move.”

Project 8 builds on just such a technique, one pioneered more than a decade ago by physicists Joe Formaggio and Ben Monreal, then working at MIT. An international team came together around the idea and project format 8 to turn the vision into a practical tool. The resulting method is called cyclotron radiation emission spectroscopy (CRES).

It picks up the microwave radiation emitted by newborn electrons as they rotate in a magnetic field. These electrons carry most, but not all, of the energy released during a beta decay event. It’s that missing energy that can reveal the neutrino’s mass. This is the first time that tritium beta decay, and an upper limit on neutrino mass, has been measured using the CRES technique.

How do scientists weigh neutrinos? Credit: Sarah Levine/Pacific Northwest National Laboratory

The team is only interested in tracking these electrons because their energy is key to revealing neutrino mass. While this strategy has been used previously, the CRES detector measures crucial electron energy with the potential to extend it far beyond any existing technology. It is this scalability that sets Project 8 apart. Elise Nowitzki is an assistant professor at the University of Washington and has led many aspects of the newly published work.

“Nobody does this,” Nowitzki said. “We’re not taking an existing technology and trying to tweak it a little bit. We’re kind of living in the Wild West.”

In their latest experiment, conducted at the University of Washington in Seattle, the team tracked 3,770 tritium beta decay events over the course of an 82-day experimental period in a single pea-sized sample cell. The sample cell is cryogenically cooled and placed in a magnetic field that traps the emerging electrons long enough for the system’s recording antennas to register the microwave signal.

More importantly, the team did not record any false signals or background events that could be mistaken for the real thing. This is important because even a very small background can mask the neutrino mass signal, making interpretation of the useful signal more difficult.

From chirps to signals

A subgroup of Project 8 researchers, led by experimental physicist Noah Oblat of PNNL, but with the participation of dozens of others from multiple institutions, has developed a suite of specialized programs — each named after different insects — to take raw data and turn it into signals that can be analyzed. And the project engineers put on their tinkering hats to invent the different parts that make Project 8 come together.

Credit: Pacific Northwest National Laboratory

“We have engineers who play a critical role in this effort,” Nowitzki said. “It’s kind of out there from an engineer’s point of view. Experimental physics is kind of on the border of physics and engineering. You have to get enterprising engineers and practical-minded physicists to collaborate, and make these things come into existence because these things aren’t in the textbooks.”

Get to the finish line

Now that the team has demonstrated its experimental design and system powered by tritium particles, they have another pressing task. A subset of the full team is now working on the next step: a system that produces, cools and traps individual tritium atoms. This step is difficult because tritium, like its more abundant cousin, hydrogen, prefers to form molecules. These particles would make Project Team 8’s ultimate goals unattainable. The researchers, led by physicists at the University of Mainz, are developing a testbed to create and trap atomic tritium using complex arrays of magnets that will prevent it from even touching sample cell walls – where it is almost certain to return to the molecular level. Form.

This technological advance, and the scaling up of the entire device, will be the critical steps to reaching and ultimately exceeding the sensitivity achieved by Kathryn’s team.

Currently, the research team, which includes contributing members from ten research institutions, is testing designs to scale the experiment from a pea-sized sample chamber to a chamber 1,000 times larger. The idea is to capture more beta decay events with a larger listening device, from the size of a pea to the size of a beach ball.

Credit: Pacific Northwest National Laboratory

“Project 8 is not just a bigger and better CRES experiment, it is the first CRES experiment and was the first to ever use this detection technology,” Oblath said. “This hasn’t been done before. Most of the experiments have a history of 50 or 100 years, at least for the detection technology they’re using, whereas this is quite new.”

more information:
a. Ashtari Esfahani et al., Beta-tritium spectrometry and neutrino mass determination from cyclotron radiation emission spectroscopy, Physical review letters (2023). doi: 10.1103/PhysRevLett.131.102502

Journal information:
Physical review letters

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