Sterile neutrino research ramps up

An international collaboration of researchers from Lawrence Livermore National Laboratory (LLNL) and other scientific institutions is ramping up its studies into “sterile neutrinos” to discover and better understand dark matter.

Sterile neutrinos are theoretically predicted new particles that offer an intriguing possibility in the quest for understanding the dark matter in our universe.

Unlike the known “active” neutrinos in the Standard Model (SM) of particle physics, these sterile neutrinos do not interact with normal matter as they move through space, making them very difficult to detect.

Scientists from four countries, led by LLNL and the Colorado School of Mines, have demonstrated the power of using nuclear decay in high-rate quantum sensors in the search for sterile neutrinos. Their findings, initially made in 2021, are the first measurements of this kind.

The primary aim of the team’s research is to seek out one of the most promising candidates for dark matter, the strange unidentified material that permeates the universe and accounts for 85% of its total mass.

In early April, 25 physicists and material scientists from the collaboration — known as “BeEST,” for Beryllium Electron-capture with Superconducting Tunnel junctions — met for two days at the University of California Livermore Collaboration Center.

Among those participating were researchers from LLNL, the Colorado School of Mines, Vancouver-based TRIUMF, Canada’s particle accelerator center; the University of Strasbourg (France), the NOVA University of Lisbon (Portugal) and two companies — Santa Fe, N.M.-based STAR Cryoelectronics and Oakland-based XIA.

“The workshop was fantastic,” said LLNL physicist Stephan Friedrich, who leads the LLNL portion of the sterile neutrino collaboration. “It forms a real team when you can meet in-person. What made this special is that this is the first time we’ve ever been able to meet in-person because of COVID. We had lots of lively discussions.”

Friedrich’s team at LLNL includes physicists In Wook Kim and Geon-Bo Kim, who analyze experimental data, researchers Amit Samanta and Vince Lordi, who perform the material science simulations and Connor Bray, a physics graduate student from the Colorado School of Mines, who will do his thesis project on this experiment.

The team’s LLNL experiment involves implanting radioactive beryllium-7 atoms into superconducting sensors developed at Livermore. When the beryllium-7 decays by electron capture into lithium-7 and a neutrino, the neutrino escapes from the sensor, but the recoil energy of the lithium-7 provides a measure of the neutrino mass. If a heavy sterile neutrino were to be generated in a fraction of the decays, the lithium-7 recoil energy would be reduced and produce a measurable signal, even though the elusive neutrino itself is not directly detected.     

Friedrich describes the sterile neutrino search problem in this manner: how do you find something that doesn’t emit light, doesn’t absorb light and doesn’t interact with material except through its mass? He likens the sterile neutrino search to firing a gun.

“If you know how much gun powder [or energy] you have and you can measure the recoil, then you can infer the properties of the bullet. The energy of the recoil is a measure of the energy and mass of the projectile,” Friedrich said, “just like the energy of the lithium-7 recoil is a measure of the energy and the mass of the neutrino.” 

Friedrich helped develop a set of radiation detectors known as superconducting tunnel junctions (STJs) while he was a Ph.D. student at Yale University in the 1990s. These detectors consist of two metal films separated by a thin insulator, and they are cooled to ultra-low temperatures close to absolute zero, or -459 degrees Fahrenheit, so that they become superconducting and can act as high-precision detectors.

He initially developed the detectors for X-ray astronomy for NASA and later for experiments at Lawrence Berkeley’s Advanced Light Source to understand how plants produce oxygen. In 2018, Friedrich met Kyle Leach, a nuclear physicist at the Colorado School of Mines, who suggested that the STJ detectors would be perfect for detecting sterile neutrinos.

The LLNL sterile neutrino research started in 2019 and received three-and-a-half years of Laboratory Directed Research and Development funding before bridging over to funding from the Department of Energy’s Office of Nuclear Physics.

In their initial one-month experiment with a single STJ detector, the data excluded the existence of sterile neutrinos in the mass range of 100 to 850 kiloelectron volts down to a 0.01% level of mixing with the active neutrinos — better than all previous decay experiment in this range.

“Since that time, we’ve been working to make our experiments bigger and better and therefore more sensitive,” Friedrich said.

In their recent experiments, the team has run 20 sensors for two months with a higher dose of beryllium-7, taking some 160 terabytes of high-quality data that is now undergoing analysis, according to Friedrich. Future experiments are expected to run 100 sensors for three months and will be among the world’s most sensitive searches for a suite of potential new “dark” particles.