Thesis

Electron Bremsstrahlung Studies and Track Based Alignment of the ATLAS Detector

Fig. 1: Photos of sections of the LHC tunnel and the ATLAS detector (Photo source CERN)

The ATLAS Experiment is one of the major experiments located at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. In 2010, ATLAS is expected to start collecting data from proton-proton collisions at a 7 GeV centre-of-mass energy, increasing to the design energy of 14 TeV after 2012.

The LHC and ATLAS have ambitious physics goals which require detectors that are both accurate and efficient. In the centre of the detector, the reconstruction of charged particle tracks is performed by silicon and drift tube based detectors. To attain the physics goals of ATLAS the resolutions of the measured track parameters must not be degraded by more than 20% due to detector misalignments. Thus, the relative positions of the silicon detector elements have to be known to an accuracy of better than 10 micrometers.

This requirement can be achieved by track based alignment algorithm techniques. A global \(\chi^2\) track-based alignment method has been developed and implemented into the ATLAS software.

1st harmonic — Fig. 2: Some of the \( \chi^2 \) invariant modes of a barrel like detector

3rd harmonic — Fig. 2: Some of the \( \chi^2 \) invariant modes of a barrel like detector

The difficulties of aligning a system with over thirty thousand degrees of freedom have been successfully overcome. Simulation studies have shown that the algorithm will be able to align the full detector with collision data to the required accuracy.

In addition to detector misalignments, bremsstrahlung of electrons prior to the calorimeters will be detrimental to electron track reconstruction, and hence the physics discovery capabilities of ATLAS. A method of accounting for the affects of bremsstrahlung using calorimeter cluster position information has been implemented and tested using Monte Carlo simulation.

Original track fitters momentum resolution — Fig. 3: The momentum resolution for 50 GeV electrons for then standard track fit (left) and the method I developed that uses the position of calorimeter cluster as an input into the track fit (right).

Track fitter that uses calorimeter information momentum resolution — Fig. 3: The momentum resolution for 50 GeV electrons for then standard track fit (left) and the method I developed that uses the position of calorimeter cluster as an input into the track fit (right).

A copy of my complete thesis can be found here