Research Group Prof. CzakonCopyright: © RWTH
The top-quark is the heaviest known particle with a mass similar to that of a gold atom, or equivalently about hundred seventy times the mass of the proton. Contrary to the other quarks, it can almost be treated as a real particle, since it decays faster than it can form a color neutral state. It has a strong coupling to the Higgs boson and, therefore, influences the properties of the latter. In fact, the Higgs boson is currently mostly produced through a virtual fluctuation of a top-quark-anti-top-quark pair from the vacuum. The top-quark itself plays an important role in many extensions of the Standard Model.Copyright: © LHC Top WG
The research of the group in this field is directed at understanding the production and decay mechanisms of the top-quark during collisions at the highest energies reachable at the Large Hadron Collider at CERN near Geneva. We provide high quality predictions for experimental analyses, which might one day lead to the discovery of new states of matter should discrepancies between data and theory appear. Our predictions are also used to understand the structure of the proton, in particular the gluon content of this nucleon. Amongst the many open theoretical questions, we try to understand the meaning of the top quark mass parameter, which turns out to be much less obvious than it would seem.
Infrared properties of gauge theories
Quantum Field Theory is usually formulated within perturbation theory, i.e. as an expansion in a coupling constant assumed to be small. While there exist non-perturbative methods, for example lattice gauge theory involving the use of the largest computer ressources known to man, our greatest hope to uncover the secrets of nature at the tiniest scales lies in the use of perturbative methods. For some problems, however, it is necessary to resum some classes of contibutions to all orders. These cases are characterized by large differences of scales involved in the problem. For example, the production of a heavy-quark-anti-quark pair close to the kinematic threshold involves radiation of nearly massless particles with small energy. The scale difference between the radiation energy and the mass of the heavy quark leads to the breakdown of naive perturbation theory. In our group we try to understand factorization theorems based on the infrared properties of gauge theories. These factorization theorems allow for resummation based on the celebrated renormalization group methods.
Parton showers with quantum effects
Whenever a collision occurs at the Large Hadron Collider and similar machines producing strongly interacting particles, a theoretical description is necessary to understand the meaning of the measured outcome. Theorists must simplify the experimental setup to obtain a tractable model. A standard method is to replace the actual hadrons as seen in the detector by partons, quarks and gluons. This picture requires the inclusions of the multiple splittings of the partons, which reminds of a shower. Hence the name, parton shower. Until now, parton showers are based on a semi-classial approximation, in which partons split as if they were real classical particles. In our group, we work on an improved description, which aims at including quantum interference and coherence effects.