Flemish scientist seeks dark matter at Cern
In Geneva, the biggest machine on Earth is ready for a new start, as scientists prepare to explore unknown territory in particle physics. Petra Van Mulders, who leads a team of 80 physicists, is eager to begin this new adventure
In search of quarks
Since then, the LHC, which sits in a 27-kilometre tunnel underneath Geneva, has had a serious upgrade: Its superconducting cables were improved so they can carry current up to 11,000 amperes, and its magnets were reinforced so they can transform that huge electrical power into a super-strong magnetic field.
While the Higgs particle was discovered in particle collisions with an energy of eight teraelectronvolt (TeV), the LHC’s collision energy has now been increased to 13 TeV. One TeV corresponds to the energy of a mosquito in flight. That’s nothing, of course, until you realise that this energy is crammed into a space that’s a million times smaller than the mosquito.
This energy, by which billions of protons are smashed against each other in the LHC, is the same as that which existed a few moments after the Big Bang. The enlarged “energy window” enables physicists to get closer to the very moment of the birth of the universe – to approximately 0.00000000000001 seconds after the Big Bang.
Unknown territories
But for the several thousands of physicists working at Cern, the institute that oversees the LHC, these huge collision energies mean that it’s more likely to produce exotic particles than ever before.
One of Cern’s regulars is Petra Van Mulders, a Flemish physicist attached to the Free University of Brussels (VUB) and a postdoctoral researcher of the Research Foundation Flanders.
It's like anthropology, where external features are often insufficient to identify a fossil
Last summer, Van Mulders (pictured), who’s only 30, was appointed as the leader of an 80-strong team – making her one of the youngest executives ever at Cern. It’s her mission to optimise the identification of the b quark, a massive elementary particle that was discovered in the 1970s.
“The b quark is well understood, but the problem is that we can’t say for certain that we’re dealing with a b quark in the detector or not,” she explains. “These particles don’t carry nametags, so we have to study their properties to develop better identification methods. You can compare it with anthropology, where external features are often insufficient to identify a human fossil.”
Of course, Van Mulders hopes the LHC will shed light on unknown territories in particle physics. “New physics phenomena can appear in two ways in the LHC,” she says. “Either we discover one or more new particles, or we see deviations from what is expected from the Standard Model of particle physics – the theory that unites all forces of nature, except gravity.”
The precise reconstruction and identification of the b quark, she continues “is crucial for searches for new physics beyond the Standard Model, since the new particles are unstable and will decay to the Standard Model particles”.
More than we can see
Some of these new particles the LHC is aiming for are dark matter particles. Van Mulders: “The Standard Model and its particles only explain 5% of the total amount of matter and energy in the universe. By chasing new particles, we are trying to understand the remaining 95%, which consists of dark matter and dark energy.
“From the movement of stars and galaxies, we know there is much more matter present in the universe than could be explained by atoms or visible matter. So we are certain that other massive particles must exist.”
But if the Standard Model can’t say anything about dark matter, where do these mysterious particles fit in? “The larger energy window will shed light on some of the Standard Model extensions,” she explains. “The most popular theories beyond the Standard Model include another symmetry of nature, so-called supersymmetry. In most supersymmetric theories, a dark matter candidate is present. Therefore, researchers focus on these theories, searching for signs of new physics phenomena.”
While experimental physicists like Van Mulders love the LHC for its colossal particle-creating power, theoretical physicists fear the machine for its merciless debunking of their hypotheses. “It’s hard to tell whether or not we will unravel the true nature of dark matter at the LHC,” says Van Mulders.
However, one thing is certain: “Even in the absence of a discovery, we will learn a lot about what the possible extensions of the Standard Model should look like. Many theories will be either excluded or severely constrained.”
Photo by Jimmy Kets
