Can supersymmetry explain dark matter?

Sara Strandberg has always had a leaning towards philosophy. She has been drawn to the unknown and driven by a desire to understand the world. Now she is looking for something that might not even exist: the supersymmetric partner of the top quark. If she finds it, she will be able to explain dark matter, and solve several other mysteries of the universe.

Sara Strandberg

Associate Professor in Particle Physics

Wallenberg Academy Fellow/Wallenberg Scholar

Stockholm University

Research field:
Experimental Particle Physics

“I began studying to be a vet, but while my fellow students were talking about intestines I sat wondering what would happen if I fell into a black hole. So I changed course and started studying physics instead,” Sara explains.

She found there was a place in modern physics for the big issues that have always interested her: why does Earth look like it does? How was the universe created? And how do we fit into the global scheme of things?

“Even as an undergraduate I was drawn to particle physics, since it is the physics that deals with the totally unknown. It is a scientific field in which it is still possible to make discoveries that nobody has ever thought of before.”

Particle physics involves the study of the very smallest constituents of matter, known as elementary particles, and the forces acting between them. As a PhD student Sara studied one of the elementary particles, known as the “top quark”, at the Fermilab particle accelerator, outside Chicago in the U.S.

“Before the CERN Large Hadron Collider (LHC) was built in Switzerland the Fermilab particle accelerator was the best in the world. And it was there that the top quark was discovered in 1995. My task was to measure the properties of the top quark more accurately, since at that time it was the least studied particle.”

The Standard Model

Elementary particles and their interactions in the form of electromagnetic, strong and weak nuclear interactions are described as the “Standard Model”. This model has so far been highly successful as “a theory of almost everything”, but it has become increasingly obvious that it is not a comprehensive model. For instance, it does not incorporate the theory of gravitation, nor does it describe the constituents of dark matter.

All members of the large research teams at the CERN research laboratory are currently trying to work out how to expand the Standard Model to make it more complete.

“Quite simply, we need to expand this theory in order to explain many things we do not understand, such as why dark matter exists, and what it is made of.”

Sara currently has a central role in the ATLAS experiment at CERN. As a Wallenberg Academy Fellow, she wants to help establish how the Standard Model can be extended so as to better describe our world. To this end, she is researching into supersymmetry.

“The grant means a great deal. For instance, I have now been able to employ two post-doctoral researchers. Expanding the team is important, because we can now pursue our own goals. We can also work on several levels. This will enable us to be key players in the hunt for new particles, and that feels great.”

What, then, is supersymmetry? Well, elementary particles can be divided into fermions and bosons. Fermions comprise quarks, electrons and neutrinos, i.e. particles of which matter is made. Some bosons, such as the photon, and the W and Z bosons, are mediators of the fundamental forces. The theory of supersymmetry is a potential extension of the Standard Model, whereby each elementary particle has a “supersymmetric partner”.

“The supersymmetry hypothesis introduces a symmetry between fermions and bosons. It postulates that for each fermion there should be a boson, and for each boson there should be a fermion. This thus means that the number of particles is doubled in the theory, which might seem completely crazy, but is really just a simple extension of the mathematical formalism. All that is added is a single symmetry, and a lot of things are then explained.”

Dark matter candidate

There are a number of indications that the supersymmetry theory is right. First of all, supersymmetry can provide a particle that is a dark matter candidate. Supersymmetry can also help to unify the fundamental forces of nature.

“I am definitely not one hundred percent convinced that supersymmetry exists – we do not know. All we are doing is trying to find deviations from the Standard Model, something that is not as the Standard Model predicts, and supersymmetry is a very good way of looking.”

More specifically, Sara’s task is to use data from the ATLAS experiment to look for the supersymmetric partner of the top quark, i.e. a boson.

“If we find it, we will have proved the supersymmetry hypothesis, and if we find any of the supersymmetric particles at all, we will also in all probability have explained dark matter.”

Important research

Sara describes herself as incredibly curious and enthusiastic. She wonders about everything, and questions most things. She thinks the research she is doing is important.

“Particle physics almost borders on art, since it is a kind of interpretation of the world, and it is important because we have a need to understand. And given what society gets back from basic research, it pays for itself many times over. Cutting-edge technology develops in highly advanced major international projects of this kind. Society then benefits in the form of numerous spin-off projects, such as the World Wide Web and Magnetic Resonance Tomography (MRT).”

Text Anders Esselin
Translation Maxwell Arding
Photo Magnus Bergström


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