Physics activities in Mainz
Search for the Higgs Boson
Of all particles of the Standard Model, the Higgs boson is the only one that has not yet been observed experimentally. Yet it is acrucial part of the underlying theory of the Standard Model as it provides a way to describe particle masses. If the Standard Model Higgs boson exists, direct exclusion limits and indirect constraints indicate that it may be possible to observe it at the Tevatron experiments CDF and D0.
The mass of the Higgs boson is not known, and both the production cross section and the decay branching fractions depend on the mass.
For Higgs masses above roughly 135GeV, the decay channel H->WW becomes dominant. The Mainz group works on this decay channel, in particular the case involving leptons in the final state which is easy to trigger and has small backgrounds.
Advanced Analysis Algorithms
While the reconstruction of collider data often relies on complicated algorithms, e.g. for track or jet reconstruction, the analysis of the reconstructed events is often based on much simpler techniques even in cases where advanced statistical methods may yield a significant advantage. An example is the measurement of the top quark mass at the Tevatron experiments CDF and D0: The socalled matrix element method, developed by the D0 experiment for the analysis of data taken in the 1990s, allowed for a reduction of the limiting statistical uncertainty by an amount corresponding to more than twice the data, analysed with standard techniques. The matrix element method has since been very successfully employed in a range of measurements by both CDF and D0.
The Mainz group works on the generalization of the method to additional physics processes and its application to the different experimental situation at the ATLAS experiment at the LHC collider.
Search for Supersymmetry
Many theories that go beyond the Standard Model of particle physics postulate a symmetry between fermions and bosons called supersymmetry. This symmetry cannot be exact since supersymmetric partners to Standard Model fermions and bosons have not yet been observed. Nevertheless, it is possible that the symmetry is broken and that these supersymmetric partners have masses of order 100 GeV and above. If this is the case, production of supersymmetric particles could be observable at the Tevatron experiments CDF and D0.
The Mainz group actively pursues the search for pair-production of charginos and neutralinos that decay via cascades into final states containing three charged leptons that can be measured and two supersymmetric particles that escape direct detection. This topology of three charged leptons and missing transverse energy from the escaping supersymmetric particles yields a striking signature with very small Standard Model background.
One of the mysteries of the universe is the existence of matter and the absence of significant amounts of antimatter. It seems that there is not a complete symmetry between matter and antimatter in nature. The properties of the weak interaction allow for such an asymmetry. It is related to the fact that periodic oscillations between a particle and its associated antiparticle are possible for some special cases like K or B mesons.
While for Bd mesons, this oscillation has been observed in the 1980s, the oscillation of Bs mesons is much faster and has only recently been discovered at the Tevatron experiments.
The Mainz group has worked on the full reconstruction of hadronic Bs decays. This method has the great advantage that it allows an unambiguous determination of the decay time of the Bs meson.