This article was first published by ANU Research School of Physics.

A random conversation between physicists from different areas has led to a new ultrasensitive device to search for dark matter.

The device is inspired by an instrument designed for mitigating noise in gravitational wave detectors, and will be sensitive to extremely light dark matter particles, around 10-21 times the mass of an electron.

“It’s a very different idea from conventional dark matter detectors – doesn’t require significant advances in technology to enhance the sensitivity in this mass range, but relies on things we can do already,” said Dr Lilli Sun, the lead author of the paper detailing the design in Physical Review D. 

“For the type of tiny particles we aim to look for, our sensor can test an interaction strength about 1,000 times weaker than has already been tested.” 

The possible mass of dark matter candidates spans 90 orders of magnitude, from huge black holes down to tiny particles, way lighter than any currently known particle. At the light end of this range the dark matter particles would behave as low frequency waves.

The new device would be sensitive to this ultra-light, wave-like dark matter, and would explore a frequency range between 0.01 Hz and 10 Hz.

“It began with a random conversation, and then we did some brainstorming,” Dr Sun said. 

“The novel design arose because of the interdisciplinary nature of the discussion – it’s a great way to create new ideas.”

To detect the faint signal from dark matter, the noise in the experiments needs to be as perfectly controlled as possible, and it was here that astrophysicist Dr Sun’s random conversation with gravitational wave experimentalists, Associate Professor Bram Slagmolen and Dr Jiayi Qin, hit gold.

The LIGO gravitational wave detector has some of the most exacting standards of noise reduction today, and Associate Professor Slagmolen was putting the finishing touches to a new sensor. The sensor, called Torsion Pendulum Dual Oscillator (TorPeDO), leverages the incredible sensitivity of torsional pendulums to measure low frequency noise in gravitational fields to mitigate noise in LIGO.

To do this, TorPeDO employs two torsional pendulums, that are dumbbell shaped and around 60cm length. They are suspended from the same point, in a cross formation. The end of each pendulum has mirrors at 45 degrees, which form laser cavities between the ends of the two pendulums, and are used to reveal any tiny changes in the local gravity field.

To make TorPeDO’s design sensitive to dark matter instead of gravitational anomalies requires the use of a combination of elements. This is done with 5 kilogram masses at each end of the pendulums, one end aluminium and the other beryllium.

This combination of metals is sensitive to dark matter because of a property of the hypothetical dark matter particles, in which their interaction with normal matter is proportional to the neutron numbers carried by the matter. In this case the beryllium test masses have more neutrons than the aluminium ones, which leads to a differential torque.

The design has specific advantages, Associate Professor Slagmolen said.

“The sensor rejects the common motion, and only measures the tiny differential rotation.”

Another advantage of the design is that it leverages existing technology, Dr Qin said.

“It takes advantage of the existing precision measurement techniques in the gravitational wave instrumentation and optical metrology, such as the Pound-Drever-Hall cavity locking method.” 

The team are now seeking funding to build a prototype of their design.