LHC scientists seek to understand dark matter

Through experiments using CERN’s particle superaccelerator, researchers are trying to detect dark matter and understand the origin of the Universe.

By André Julião  |  Agência FAPESP – The Large Hadron Collider (LHC), which enabled physicists to prove the existence of the Higgs boson, the elementary particle that emerged shortly after the Big Bang and endows ordinary matter with mass, may now help them solve one of the most compelling mysteries they face – the nature of dark matter.

The LHC is the most powerful particle accelerator built to date and is operated by the European Organization for Nuclear Research (CERN) on the Franco-Swiss border near Geneva. The physicists responsible are currently using it to try to detect dark matter, thought to account for 25% of the Universe. Only indirect evidence of the existence of dark matter has been found so far.

“The work we’re doing at CERN with the LHC aims at a thorough understanding of the origin of the Universe. This is evidently connected to many other aspects of physics on which we’re also working. We believe 25% of the Universe consists of a kind of matter we call dark matter, although we don’t really know what it is,” said Oliver Buchmueller, Professor of Physics at Imperial College London and a speaker at FAPESP Week London, which took place on February 11-12, 2019.

Ordinary matter, the kind we are familiar with, accounts for only 5% of the Universe, he added. Scientists believe that dark energy and dark matter correspond to 70% and 25% of the Universe, respectively.

Dark energy is thought to drive the accelerating expansion of the Universe. Dark matter must be pulling it back, but not enough to prevent dark energy from intensifying the expansion caused by the Big Bang.

“We’re interested in the unknown 25% that presumably consists of dark matter,” Buchmueller said. “One of the most challenging questions in physics today is what dark matter is actually made up of. Many experimental activities are in progress to look for answers. We aim to get the LHC to produce these hypothetical particles of dark matter directly, so that we can measure them and then say, ‘Yes, they’ve been produced, we’ve seen them, and this really is what dark matter consists of in our Universe’.”

Because dark matter has not yet been directly observed, physicists are developing other general models besides supersymmetry, which states that for every known particle, there should be a supersymmetric partner particle.

One of these physicists is Jonathan Costa, a Brazilian PhD student whose research Buchmueller supervises at Imperial College London. Costa holds an undergraduate degree and a master’s degree from the University of Ponta Grossa (UEPG) in the state of Paraná and has been in the UK since 2015.

At present, he is working with the MasterCode collaboration, which uses computer code to try to fit different versions of the Standard Model to real-world data from the CMS and Atlas detectors at the LHC, among other experiments. His research was one of the topics discussed by Buchmueller at the event.

“My work focuses on phenomenology, an intermediate field between experiment and theory. We take a theoretical model and compare it with experimental data. On that basis, we try to make predictions about certain parameters. This helps determine what we want to look for, because the volume of data is very large and we have to choose a direction,” Costa told Agência FAPESP.

Buchmueller reiterated that the detection of the Higgs boson in 2012 was a scientific milestone but helps to understand only 5% of the Universe, the part that corresponds to ordinary matter. The other 95% remains a mystery.

“Given that dark matter accounts for only 25% of the Universe, it can’t prevent the acceleration of its expansion driven by dark energy. It isn’t strong enough. So we have two competing factors. The search is only just beginning. We need much more time, but we have to start somewhere,” he said.

News and videos about FAPESP Week London are available at: www.fapesp.br/week2019/london.