We present a measurement of multiple Coulomb scattering of 1 to 6 GeV/c electrons in thin (50-140 µm) silicon targets. The data were obtained with the EUDET telescope Aconite at DESY and are compared to parametrisations as used in the Geant4 software package. We find good agreement between data and simulation in the scattering distribution width but large deviations in the shape of the distribution. In order to achieve a better description of the shape, a new scattering model based on a Student's t distribution is developed and compared to the data.

The Mu3e experiment searches for the lepton flavour violating decay µ + → e + e − e + , aiming for a branching fraction sensitivity of 10 −16 . This requires an excellent momentum resolution for low energy electrons, high rate capability and a large acceptance. In order to minimize multiple scattering, the amount of material has to be as small as possible. These challenges can be met with a tracker built from high-voltage monolithic active pixel sensors (HV-MAPS), which can be thinned to 50 µm and which incorporate the complete read-out electronics on the sensor chip. To further minimise material, the sensors are supported by a mechanical structure built from 25 µm thick Kapton foil and cooled with gaseous helium.

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay µ → eee. In order to reduce background by up to 16 orders of magnitude, decay vertex position, decay time and particle momenta have to be measured precisely. A pixel tracker based on 50 µm thin high voltage monolithic active pixel sensors (HV-MAPS) in a magnetic field will deliver precise vertex and momentum information. Test beam results like an excellent efficiency of > 99.5% and a time resolution of better than 16.6 ns obtained with the MuPix HV-MAPS chip developed for the Mu3e pixel tracker are presented.

A double modulation technique was used to achieve higher sensitivity in absorption measure ments with a pulsed supersonic jet. The wavelength of a lead-salt diode laser was modulated at a frequency of 10 kHz and detected at twice the frequency by a lock-in amplifier, reducing significantly the influence of the low frequency excess laser noise. The output of this lock-in amplifier was measured by a boxcar integrator, driven at 80 Hz, the frequency of the pulsed jet. In case the duration of the jet pulse is long enough, a second lock-in amplifier can also be used instead of the boxcar integrator. The optical path of the diode laser radiation through the jet was increased 16 times by using White type multireflection optics. The achieved value for the minimum detectable absorption of A r-C O complexes in the jet was 2 x 10-5 in relative absorption, limited by the excess noise of the diode laser.