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29 May 2019

Changing the way we search for Dark Matter

You may have heard that 95% of the universe is missing. It’s not truly missing but it is currently not directly detectable. This is what is known as dark energy and dark matter. For years scientists have been searching for dark matter and now a team at King’s College London, collaborating with colleagues at the University of Cambridge, have come up with a novel way of thinking about dark matter and how to look for it.

Telescope pointing at night sky
Telescope pointing at night sky

Traditionally, scientists have assumed dark matter particles exist everywhere and have been looking for the effect of dark matter particles interacting with known detectable particles, for example, those that make up the noble gases. These highly inert (unreactive gases) are used because their behaviour is highly predictable. The gases are kept in huge tanks with the expectation that any sudden unexpected change in the energy of one of the particles (and therefore its speed and movement) implies it has been hit with a particle of dark matter (known as an interaction). So far this has not happened.

As a dark matter particle has never been detected, researchers have had to guess what they may be like and what their properties might be. It has conventionally been thought that dark matter must be made up of WIMPs or Weakly Interacting Massive Particles with a relatively large mass of 1GeV to 1TeV. To give an idea, 1 GeV is the mass of a proton, or 10^-19 the mass of a grain of salt. That’s 1/10,000,000,000,000,000,000. 1TeV is 1000 times bigger than 1GeV. Researchers have designed multiple experiments to detect dark matter WIMPs and have slowly worked through the various mass and interaction probability combinations that the particles may have. In other words, their size and the likelihood that two particles will collide. In doing so they have been able to describe what dark matter particles are not.

This has left a large unexplored set of properties of lower mass particles, and this is where the King’s team comes in. James Alvey, Dr Miguel Campos and Professor Malcolm Fairbairn, Department of Physics, and Dr Tevong You, University of Cambridge, posit that dark matter particles could be created and that this happens via inelastic scattering of cosmic rays. Essentially, cosmic rays (highly energetic protons) hit protons in the atmosphere (each made of three quarks), and rather than bouncing off each other, they split into smaller particles made of two quarks each called pions and ɳ-mesons which then decay into dark matter particles and are scattered through the atmosphere. As detailed in the famous equation E=MC², energy can become mass and mass can become energy. It is currently unknown how many dark matter particles with high energies could be created from the proton interactions and so the team have had to make estimations. Using estimations of the flux of dark matter particles (how they are travelling) that should reach Earth, existing experiments can be used to tackle a region previously unexplored: small masses between 1MeV and 1GeV and high interaction probabilities. So far the team, funded by the European Union and the STFC, have excluded a sizeable chunk of the previously unexplored properties. They are now working to explore the next set of unexplored properties on their list.

Where researchers are searching for dark matter particles.
Where researchers are searching for dark matter particles.

But why does this matter? Well, there are a few reasons. Firstly, as dark matter particles are, as yet, undetected, researchers don’t know how they interact with known particles, summarised in what is called the Standard Model of physics. It is important then to make sure all the possible bases have been covered. The detection of dark matter particles would open up a whole new field of physics as researchers try to understand how dark matter interacts with the world around us. Secondly, maybe the current model of physics is wrong and the detection of dark matter will need physicists to rewrite everything we thought we knew. Finally, and the scariest option for researchers: dark matter will never be detected, even though it is there, because it only interacts with other particles at a gravitational level in a way which cannot be measured directly.

You can explore more about dark matter through the Dark Matter exhibition at the Science Gallery London.

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