Test of Lorentz symmetry with a 3 He,129 Xe clock-comparison experiment

In case of violation of CPT- and Lorentz Symmetry, the minimal Standard Model Extension (SME) of Kostelecky and coworkers predicts sidereal modulations of atomic transition frequencies as the Earth rotates relative to a Lorentz-violating background field. One method to search for these modulations is the so-called clock-comparison experiment, where the frequencies of co-located clocks are compared as they rotate with respect to the fixed stars. In this work an experiment is presented where polarized 3He and 129Xe gas samples in a glass cell serve as clocks, whose nuclear spin precession frequencies are detected with the help of highly sensitive SQUID sensors inside a magnetically shielded room. The unique feature of this experiment is the fact that the spins are precessing freely, with transverse relaxation times of up to 4.4 h for 129Xe and 14.1 h for 3He. To be sensitive to Lorentz-violating effects, the influence of external magnetic fields is canceled via the weighted difference of the 3He and 129Xe frequencies or phases. The Lorentz-violating SME parameters for the neutron are determined out of a fit on the phase difference data of 7 spin precession measurements of 12 to 16 hours length. The result of the fit gives an upper limit for the equatorial component of the neutron parameter b_n of 3.7×10^(−32) GeV at the 95% confidence level. This value is not limited by the signal-to-noise ratio, but by the strong correlations between the fit parameters. To reduce the correlations and therewith improve the sensitivity of future experiments, it will be necessary to change the time structure of the weighted phase difference, which can be realized by increasing the 129Xe relaxation time.

Production of Z bosons in proton-proton collisions at root out s = 10 TeV: Expectations for early measurements at the ATLAS experiment

The production of the Z boson in proton-proton collisions at the LHC serves as a standard candle at the ATLAS experiment during early data-taking. The decay of the Z into an electron-positron pair gives a clean signature in the detector that allows for calibration and performance studies. The cross-section of ~ 1 nb allows first LHC measurements of parton density functions. In this thesis, simulations of 10 TeV collisions at the ATLAS detector are studied. The challenges for an experimental measurement of the cross-section with an integrated luminositiy of 100 pb−1 are discussed. In preparation for the cross-section determination, the single-electron efficiencies are determined via a simulation based method and in a test of a data-driven ansatz. The two methods show a very good agreement and differ by ~ 3% at most. The ingredients of an inclusive and a differential Z production cross-section measurement at ATLAS are discussed and their possible contributions to systematic uncertainties are presented. For a combined sample of signal and background the expected uncertainty on the inclusive cross-section for an integrated luminosity of 100 pb−1 is determined to 1.5% (stat) +/- 4.2% (syst) +/- 10% (lumi). The possibilities for single-differential cross-section measurements in rapidity and transverse momentum of the Z boson, which are important quantities because of the impact on parton density functions and the capability to check for non-pertubative effects in pQCD, are outlined. The issues of an efficiency correction based on electron efficiencies as function of the electron’s transverse momentum and pseudorapidity are studied. A possible alternative is demonstrated by expanding the two-dimensional efficiencies with the additional dimension of the invariant mass of the two leptons of the Z decay.