Scientists find new clue in dark matter puzzle

A new paper by University of Melbourne researchers explores an important clue as to the nature and origin of dark matter.

The research, led by PhD student Alexander Ritter and supervised by Professor Ray Volkas, explores the puzzle of why ordinary matter and dark matter mass densities are observed to be of similar magnitudes.

The theoretical physicists delved into a well-established cosmological fact - that the present-day mass density of dark matter is about five times that of ordinary matter.

As the factor of five is surprisingly close to one in cosmological theory terms, this suggests the nature of dark matter and its cosmological origin may be related to the nature of ordinary matter and its cosmological origin. (The alternative is that the factor of five is a mere coincidence.)

This hypothesis motivates that the dark matter particle should have a mass related to that of the proton, the latter of which is set by the fundamental energy scale of the strong interactions that bind quarks into protons. The concentration of ordinary matter is set by the matter-antimatter asymmetry of the universe. The ordinary matter mass density is equal to the proton mass multiplied by the concentration.

Continuing to draw the parallel, this suggests that there is both dark matter and dark antimatter, and that the present-day dark matter concentration is set by a cosmological dark matter/dark antimatter asymmetry related to the ordinary-matter asymmetry. In this way, dark matter would parallel ordinary matter, a scenario known as “asymmetric dark matter”.

The new work analysed how the dark matter mass can be connected to the proton mass. Adopting an analogue of the ordinary strong interactions in the dark sector, Mr Ritter and Professor Volkas found circumstances under which dark proton-like or dark neutron-like states would compellingly have a mass of similar magnitude to the ordinary proton.

According to Professor Volkas, the research can help narrow the range where direct detection experiments can search for dark matter in their experiments. 

“We have used existing theories to try to explore a relatively new idea about the density of dark matter,” Professor Volkas said.

“These observations might offer us a clue as to what dark matter is and how it arose in the history of the universe. In particular, it motivates that the dark matter mass is similar to the proton mass. Were a positive dark matter direct detection signal to indicate such a mass scale, it would be time to get very excited!” Professor Volkas said.