A team of German scientists has proposed a new way to detect dark matter and its influence on the properties of the thorium-229 nucleus, which is being used in the nuclear clock project by researchers.

Theoretical calculations made by the team of scientists suggest that the new measurements could detect dark matter’s influence even if it were 100 million times weaker than gravity.

 According to one of the research team members, a thorium-229-based nuclear clock would be the ultimate detector for dark matter.

Nuclear clocks versus atomic clocks

One of the major differences between a nuclear and atomic clock is that while the former utilizes the nucleus of an atom, the latter uses the outer electron shells. The nuclear clock, once built, will therefore be more precise, as the nucleus is more stable and less prone to being disturbed by external factors.

Atomic clocks have already been made and are used in various applications such as precise timekeeping, telecommunications, satellite navigation, and research. Today, they are the most accurate timekeeping devices available.

A nuclear clock could replace them and usher in a new era of technology and research.

Scientists around the world, from the United States to Germany, are carrying out research based on the thorium-229 isotope to make one.

The development has been challenging as thorium-229 is rare, radioactive, and extremely costly to acquire in the substantial quantities required for this purpose.

A team of researchers in the US recently found that physical vapour deposition (PVD) of thorium-229 could reduce the amount of this expensive and radioactive isotope needed to make a nuclear clock.

Searching for dark matter through thorium-229

Meanwhile, while several quests to make a nuclear clock are underway, the team led by Prof. Gilad Perez’s theoretical physics group at the Weizmann Institute of Science made the claim that they recognized a new opportunity to advance the search for dark matter.

The team says that they can search for dark matter even before a functional nuclear clock becomes a reality.

Dr. Wolfram Ratzinger, Prof. Gilad Perez, Dr. Fiona Kirk, and Chaitanya Paranjape.
Weizmann

“We still need even greater precision to develop a nuclear clock,” Perez said, “but we’ve already identified an opportunity to study dark matter.”

“In a universe made up only of visible matter, the physical conditions and the absorption spectrum of any material would remain constant. But because dark matter surrounds us, its wave-like nature can subtly change the mass of atomic nuclei and cause temporary shifts in their absorption spectrum,” he added.

The researchers hypothesize that the ability “to detect minute deviations in the absorption spectrum of thorium-229 with great precision could reveal dark matter’s influence” and help them study its properties.

Calculations made by the team suggest that the new measurement could detect dark matter’s influence even if it were 100 million times weaker than gravity.

“Our calculations show that it’s not enough to search for shifts in the resonance frequency alone. We need to identify changes across the entire absorption spectrum to detect dark matter’s effect. Although we haven’t found those changes yet, we’ve laid the groundwork to understand them when they do appear,” said Dr. Wolfram Ratzinger from Perez’s group.

He mentions that although the changes have not been identified yet, they have the groundwork to understand them when they do appear.

“Once we detect a deviation, we’ll be able to use its intensity and the frequency at which it appears to calculate the mass of the dark matter particle responsible,” Ratzinger added.

The researchers also calculated how models would affect thorium-229’s absorption spectrum to determine what dark matter is made of.

“When it comes to dark matter,” Perez said, “a thorium-229-based nuclear clock would be the ultimate detector. Right now, electrical interference limits our ability to use atomic clocks in the search.”

“A nuclear clock would let us detect incredibly slight deviations in its ticking – that is, tiny shifts in resonance frequency, which could reveal dark matter’s influence,” he concluded.

The study was first published in Physical Review X.

Study abstract

The recent laser excitation of the low-lying 229Th isomer transition has started a revolution in ultralight dark matter searches. The enhanced sensitivity of this transition to the large class of dark matter models dominantly coupling to quarks and gluons will ultimately allow us to probe coupling strengths 8 orders of magnitude smaller than the current bounds from optical atomic clocks, which are mainly sensitive to dark matter couplings to electrons and photons. We argue that, with increasing precision, observations of the 229Th excitation spectrum will soon give the world-leading constraints. Using data from the pioneering laser excitation of 229Th by Tiedau et al. [Phys. Rev. Lett. 132, 182501 (2024)], we present a first dark matter search in the excitation spectrum. While the exclusion limits of our detailed study of the lineshape are still below the sensitivity of currently operating clock experiments, we project the measurement of Zhang et al. [Nature (London) 633, 63 (2024)] to surpass it.