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Data Analysis

The dream of modern physics - the theory of everything - is not yet realised. Although the modern theory of elementary particles and fields, the Standard Model, describes the results of all particle scattering experiments with high precision, many questions remain open, such as

• How are all fundamental interactions unified?
• Why are there three generations of quarks and leptons?
• Why are there six quark flavours and their mixing?
• What is dark matter?
• ...

Various extensions of the Standard Model have been proposed to deal with these issues. Some of these theories would bring a new revolution in physics, if they are confirmed by experiments. Analysing data collected with the ATLAS detector at the LHC, we are looking for the signatures predicted in these theories. Currently we are interested in the supersymmetry between bosons and fermions, and in models with extra dimensions. But before any discovery can be claimed, one has to ensure that no detector effects fake the signals of new physics. Therefore we also perform extensive detector studies. We focus on calibrations of the calorimeters and on understanding of hadronic jets.

Supersymmetry (SUSY)

The theory of Supersymmetry (SUSY) assumes a new symmetry of nature. SUSY predicts for each fermion (matter particle with spin 1/2) a bosonic partner (force particle with integer spin) and vice versa. It is one of the best motivated extensions of the Standard Model and could solve many of its current problems. For instance, SUSY could explain the Higgs mass (hierarchy problem) and delivers a good candidate for the dark matter in our universe. If SUSY is realized in nature, there is a multitude of new particles that we can discover with the LHC. SUSY particles are produced in the strong interaction at the LHC which leads to large expected event yields. As a consequence, there is a good chance that SUSY will be one of the first new-physics signals at the LHC.

Extra Dimensions

Our world can have more than three space dimensions. Additional, so far unseen, dimensions can appear at very small distances, if they are "compactified". In this case, the well-known Newton's gravity law would change at these small distances: instead of 1/r² the dependence of the gravity force on the distance r would be 1/r²⁺ⁿ for the number of additional dimensions n. Then the gravity would rise much steeper towards smaller distances and eventually could reach the strength of the other fundamental interactions even at the TeV scale, thus making a unification of all four interactions possible. A strong gravity would create a number of interesting signals at the LHC, such real graviton production or virtual graviton exchange. A real graviton would escape into the extra dimensions, while we would observe just a single hadronic jet and a huge missing energy. Another spectacular effect would be the production of microscopic black holes. They would decay instantly via the Hawking radiation, and we would observe events with many particles of very high energies in the ATLAS detector.

Jet Studies

Many signals of new physics at the LHC involve hadronic jets. A jet is created from a scattered quark or gluon due to the confinement. It is reconstructed in the calorimeter system. The goal of the jet reconstruction is to obtain the initial parton energy out of the measured jet energy. In this procedure one has to take into account both the detector effects (non-compensating calorimeter, dead regions, passive material, calorimeter noise etc.) and the physics effects (final state radiation, multiple interactions, jet algorithm features etc.). Complicated Monte-Carlo based methods of jet reconstruction and of energy calibration are worked out in ATLAS. However, the final energy scale has to be checked in the data. Our group works on such methods of "in-situ" jet calibration.