Electron spectroscopy with PERC

This proposal together with six other proposals in priority area B addresses a number of questions which are at the forefront of particle physics, with main emphasis on the search for new physics beyond the Standard Model of particles physics, and in particular, on the question of unification of all forces shortly after the Big Bang. This grand unification is not part of the Standard Model, and new symmetry concepts are needed like left-right symmetry, fundamental fermion compositeness, new particles, leptoquarks, supersymmetry, and many more. Precise symmetry tests of various kinds are coming within reach with the proposed facility PERC, where count rates of neutron decay products are increased by a factor 100 compared to best experiments. Our goal is ß spectroscopy, where the spectra and angular distributions of the emerging decay particles will be distortion-free on the level of 10-4 .

The Standard Model (SM) of elementary particles and fields is a theory that describes all interactions of subatomic particles, except those due to gravity. Although the SM explains a wide variety of experimental results, it falls short of being a complete theory of fundamental interactions. Extensions of the SM are needed to explain its deficiencies, such as the origin of mass, neutrino oscillations, and the nature of dark matter and dark energy. Precision measurements of the parameters describing the beta decay of free neutrons serve to probe theories that lie beyond the SM, such as supersymmetry (SUSY) or Grand Unified Theories (GUT). Complementary to direct searches for new particles in high energy physics with colliders, we indirectly probe the existence of such particles and provide constraints for new physics beyond the SM with high-precision and high-statistics experiments. To this end, we develop the new instrument PERC (Proton and Electron Radiation Channel). At its exit, PERC delivers neutron decay products under well-defined and precisely variable conditions. Depending on the parameters studied, the analysis of the extracted particles is performed with different and specialized detectors. In this way, we can measure energy spectra and angular correlations of the decay products with unprecedented precision. The main goal of this project was to design and construct the joint instrument PERC, in collaboration with the University of Heidelberg, Germany. After completion of design of the superconducting magnet coil system, a European call for tender for the manufacturing of the magnet system was launched by the German Research Foundation. PERC has been ordered at BNG, Würzburg, and is going to be built within the next 15 months. The main component of PERC is the more than 11 m long magnet system that produces a strong longitudinal magnetic field of 0.5 T up to 6 T. In order not to disturb other experiments in the vicinity of PERC, we have designed a magnetic field return yoke for the magnet system. The return yoke suppresses the stray magnetic field to the cardiac pacemaker limit, without disturbing the internal magnetic field and its homogeneity of up to 10-4. Another goal was to design and construct a system for electron momentum spectroscopy. To this end, we propose a novel type of momentum spectrometer where the particles momenta are separated according to their drift in a circularly curved magnetic field. Advantages of the spectrometer are that all particles are adiabatically transported and that particles with very small momenta (dispersion smaller than aperture width) can be measured. In addition, we have set-up one detection system of the electron spectrometer PERKEO III at the Atominstitut, in order to test and improve the performance of a scintillation detector, with photomultiplier read-out, for the high count rates projected at PERC.