Search for physics beyond the standard electroweak model with the WITCH experiment.

A measurement of the beta-neutrino angular correlation coefficient a yieldsinformation on possible exotic couplings in the weak interaction. To this end theenergy distribution of the recoiling daughter nucleus after decay, which dependson a, is measured precisely. Any deviation of the measured distribution with theone expected from the Standard Model can reveal new physics. If no deviationis found stringent limits can be set on the possible presence of different types ofnew physics beyond the Standard Model. The WITCH experiment, located atISOLDE, CERN aims to determine a with a final precision below 1%.Ion bunches are created with REXTRAP and injected in the WITCH setup. Theenergy of these ion bunches is pulsed down in the Pulsed Drift Tube section, priorto the capture of the ions in the first of two Penning traps. The motion of theradioactive ions is cooled before the transfer to a second Penning trap, the decaytrap, which acts as the scattering-free source of the experiment. The ions can decayin this trap while being stored for a couple of seconds. When an ion undergoes decay, it can escape the trap and the energy of the recoiling daughter nucleusis measured with a retardation spectrometer and an MCP detector. By varyingthe electric potential barrier in the retardation spectrometer, the recoil energyspectrum is obtained.The expected recoil energy spectrum is reconstructed with an ion trackingsimulation program, called SimWITCH, and a Penning trap simulation program,called Simbuca. This versatile simulation package was developed to investigate thebehavior of multiple ions in a Penning trap. Due to its novel approach of usinga graphics card (GPU) instead of a conventional CPU, the Coulomb interactionbetween the ions can be calculated much faster. Furthermore Simbuca incorporatesthree realistic buffer gas models, the possibility of importing realistic electric andmagnetic field maps and different order integrators with adaptive step size anderror control.The simulation package was used to analyze the first acquired retardation spectrumwith WITCH, taken in an experiment in June 2011. Even though the statisticsgathered is low, a first determination of a with the WITCH experiment waspossible. The analysis method for a typical WITCH experiment will be explainedand it will be shown that a statistical precision below 1% can indeed be reachedwith theWITCH experiment. An outlook is given about the follow up experimentsin October and November 2011, in which enough statistics were obtained to reacha statistical precision of around 4%. The WITCH experiment is now in a phasewhere systematic effects will have to be looked into and a preliminary result canbe obtained even before a final dataset for a high precision result can be collected.The development of Simbuca made it possible to look at the behavior of multiple(106 to 107) ions in a Penning trap, a regime that is currently being reached bythe setup. Due to the mutual Coulomb interaction between the ions, they willcluster together to form an ion plasma which rotates around its own center ofmass. The influence of this behavior on the resonant quadrupole eigenfrequencywill be discussed. An estimate for the range of the plasma parameters is extractedfrom simulations as well. Finally, an outlook is given on how to influence and/ordetermine these parameters. This will analyze the already available as well asfuture high statistics datasets of WITCH.nrpages: 218status: publishe