Wits researchers pioneer new way to search for dark matter

Dark matter is estimated to make up about a quarter of the universe, yet it does not interact significantly with ordinary matter and its existence is a longstanding puzzle.

The existence of dark matter has been confirmed by a series of astrophysical and cosmological observations, including in stunning recent pictures from the James Webb Space Telescope. However, to date, no experimental observation of dark matter has been reported and is a question that physicists have been investigating for decades.

Prof Deepak Kar of the School of Physics at Wits University said: “The Large Hadron Collider at Cern (the European Organisation for Nuclear Research) is the largest experiment ever built, and particle collisions creating Big Bang-like condition can be exploited to look for hints of dark matter.”

Working at the Atlas experiment at Cern, Kar and his former PhD student, Sukanya Sinha (now a postdoctoral researcher at the University of Manchester), pioneered a new way of searching for dark matter. Their research has been published in the journal Physics Letters B.

“The plethora of collider searches for dark matter over the past few decades so far have focused on weakly interacting massive particles, termed Wimps (weakly interacting massive particles),” said Kar in a statement. “Wimps is one class of particles that are hypothesised to explain dark matter as they do not absorb or emit light and don’t interact strongly with other particles. However, as no evidence of Wimps has been found so far, we realised that the search for dark matter needed a paradigm shift.

“What we were wondering was whether dark matter particles are actually produced inside a jet of standard model particles,” said Kar. This led to the exploration of a new detector signature known as semi-visible jets, which scientists had never looked at before.

Jets

High-energy collisions of protons often result in the production of a collimated spray of particles, collected in what is termed as jets, from decay of ordinary quarks or gluons. Semi-visible jets would arise when hypothetical dark quarks decay partially to standard-model quarks (known particles) and partially to stable dark hadrons (the “invisible fraction”). Since they are produced in pairs, typically along with additional standard-model jets, the imbalance of energy or the missing energy in the detector arises when all the jets are not fully balanced. The direction of the missing energy is often aligned with one of the semi-visible jets.

This makes searches for semi-visible jets challenging, as this event signature can also arise due to mismeasured jets in the detector. Kar and Sinha’s new way of looking for dark matter opens up new directions into the search for the existence of dark matter.

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“Even though my PhD thesis does not contain a discovery of dark matter, it sets the first and rather stringent upper bounds on this production mode, and is already inspiring further studies,” said Sinha.  – © 2023 NewsCentral Media

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