Effective field theories
Research in particle physics often goes as follows: We have a theory, we predict an observable, we measure it and draw conclusions about our theory. An effective theory is an approximation of a full theory. It allows us to make precise predictions, without having to worry about all the aspects of our theory that don't matter for the observable we are interested in.
In our group, we use effective field theories to connect observables at different experiments. If, for example, we predict that new heavy particles leave their trace in top-quark production at the LHC, we can calculate how the same particles modify meson decays at a flavor experiment like LHCb or Belle II. In an effective theory, this connection works even if we don't know anything about these new particles! By combining observables at different energy scales, we can test theories at very high energies and resolve their features much better than using data from one experiment alone.
Most of the particles we know of don't play a role in our everyday lives. If they happen to be produced, they decay almost instantly into lighter, stable particles: electrons and bound states of quarks, which make up the matter we are made of. But there are exceptions like the muon, which can travel a distance before it decays.
The lifetime of a particle is often related to determined by how strongly it interacts
with the particles it decays into. It might well be that new particles exist that interact only very little with other particles. So far, many of these particles have escaped our attention, because most experiments search for instantly decaying particles. In our group, we try to think out of the box and propose new search strategies
for long-lived particles. The huge potential to discover hidden new particles drives our community to re-purpose existing experiments, and even to propose an entire new detector suite like the Forward Physics Facility at the LHC.
There are many good reasons to search for new particles, may they be heavy or barely interacting or hidden from observation in any other way. My primary motivation for these searches is dark matter. About 85 percent of all matter in the universe is dark
, that is, we don't see it in telescopes. But we do observe its effects on visible matter in galaxies, in the cosmic microwave background, or in our theory of structure formation in the early universe.
As a particle physicist, I find the option that dark matter could be a new kind of particles
extremely exciting. In our group, we build models for particle dark matter and test them at laboratory experiments. The goal is to find good candidates for dark matter that fit into the cosmic history and at the same time leave signatures at colliders
, in dedicated direct detection experiments
like Xenon-nT, or in any other testable way. Since we still know little about the fundamental properties of dark matter, our search is mostly driven by the experiments we can build. Discovering a dark matter particle would be such a ground-breaking event that we won't leave any stones unturned.