Top Quark Physics and Search for Higgs Bosons at Hadron Colliders

We particle physicists are trying to answer a very old question - posed e.g. by Goethe's Faust almost 200 years ago: 'So that I may perceive whatever holds the world together in its inmost folds'. In our current understanding matter consists of molecules, which are formed by atoms, which in turn consist of a cloud of electrons orbiting a nucleus. The latter is made of protons and neutrons. While there is no hint of a substructure of electrons, protons and neutrons are made of two types of quarks. No substructure has yet been found for quarks. So we call electrons and quarks elementary particles. However, today we know a lot more elementary particles, e.g. six different types of quarks. How could we find all that out? The principle is rather easy: we shoot particles onto each other and analyse the debris. This tells us what kind of particles exist and what kind of forces govern their interaction. The faster the initial particles are, the deeper we can look into matter and the heavier particles we can find. So we build larger and larger colliders to accelerate the initial particles to higher and higher energies to get a better and better knowledge about the structure of matter. Then we must build huge detectors to reconstruct the debris particles. For instance, their direction can be 'seen' due to the fact that they leave a track in the detector like a plane leaves a condensation trail in the sky. This helps us to identify them. The Large Hadron Collider (LHC) currently under construction at CERN, near Geneva, will be the highest energy collider ever built. In a ring 27 km around it will accelerate two beams of protons to nearly the speed of light before they will be collided head-on. Several detectors will record these collisions. However, many more collisions will happen than can be recorded, up to one billion per second. In fact only 0.0005% of all beam crossings can be kept. A sophisticated system of purpose-built electronics and software algorithms decides which collision events are interesting and should thus be saved. I intend to work on one of the LHC detectors, called ATLAS and help construct this system and ensure its efficient operation. What questions do I hope to answer with the data thus collected? First of all, I am interested in the heaviest known elementary particle, called the top quark. It was discovered only ten years ago at the Tevatron collider at Fermilab near Chicago. Now, of course, one wants to measure it in detail and e.g. find out if it behaves similarly to its five colleagues. At present I am measuring its properties, e.g. its mass, using the D0 detector at the Tevatron. I will continue this work and extend it to measurements at the LHC, where millions of top quarks will be produced. One main question about the top quark is: why is it so heavy, around 40 times heavier than any of the other quarks? And how do elementary particles acquire their mass at all? It is postulated that this happens by interaction with a new field, named after its inventor as the Higgs field. This theory also postulates the existence of a heavy particle, the Higgs particle. I intend to search for it at the LHC. At the LHC one will also search for many other 'new' particles. I am especially interested in particles which have a certain symmetry relation to the Higgs particle - one could even say, I will search for relatives of the Higgs particle. The possible discovery of these and other new particles will extend not only our knowledge of the sub-atomic world, but will even help our understanding of the composition of the whole universe. Astronomical observations show that there must be a much larger amount of matter present in the universe than can be seen - so-called dark matter. The composition of this dark matter, however, remains a mystery. If indeed new particles are discovered at the LHC, they could be candidates for this dark matter.