Pauli exclusion principle test in an underground laboratory

The Pauli Exclusion Principle (PEP) is one of the pillars of quantum physics and the foundation of modern physics. It is at the basis of our understanding of nature and has consequences for the world of elementary particles up to objects in the universe - but it has no simple explanation. We know that the Pauli Principle is very well fulfilled leaving still the question open about the limit of validity. A possible (tiny) violation of this principle would point to new physics possibly showing up at the Planck scale but might be present at lower energies. A method to test the PEP experimentally was developed by Ramberg-Snow. PEP is tested for electrons, i.e. elementary particles having no interaction with the studied system thus circumventing the Messiah-Greenberg super-selection rule. These new electrons are provided by a strong electric current which flowing through a piece of solid metal. Pauli-forbidden transitions in this metal exhibit an energy shift in the transition energy resolvable by x- ray spectroscopy. A search for x-ray transition events can be performed with high sensitivity but requires substantial background discrimination. In our previous VIP experiment at the underground laboratory LNGS (Gran Sasso) we used an improved Ramberg- Snow experimental setup exploiting charge coupled devices as x-ray detectors. In this experiment we could deduce an upper limit for the Pauli exclusion principle violation in the order of 10-29. The proposed VIP2 experiment will be performed in the underground laboratory LNGS in Gran Sasso taking again advantage of the excellent shielding against cosmic rays. A strongly improved compact setup with passive and active shielding will be used. Silicon drift detectors will serve as x-ray detectors providing a timing signal used in anticoincidence with scintillators (veto counters) to suppress actively background events. With the VIP2 experiment we want to improve the limit for PEP violation by 2 orders of magnitude reaching the range of 10-31. In the framework of this proposed project all steps from the setup, test and running of the experiment at LNGS up to data analysis and studies of systematics are foreseen. The VIP2 project is accepted by LNGS according to the Scientific Committee` conclusions.

Wolfgang Pauli discovered a fundamental rule of nature named after him the so-called Pauli Exclusion Principle, which could explain the periodic system for the first time. In 1940 Pauli got the Nobel Prize for Physics. Pauli himself tried to explain this rule with a theoretical approach but a simple explanation could not be found. Today we know that the Pauli Exclusion Principle is a consequence of the spin-statistics theorem. The world is divided in half-integer and integer spin systems. According to our observations the Exclusion Principle is extremely well fulfilled and is the basis for the stability of matter up to the stability of neutron stars. Nevertheless there is still the question about the possibility of a tiny violation of this rule. This FWF project VIP2 is aiming at an experimental test of the Pauli Exclusion Principle for electrons with unprecedented ultra-high sensitivity. The basic method is based on the search for Pauli-forbidden atomic transitions in copper atoms. The forbidden transitions to the fully occupied ground state are shifted by about 300 electronvolt and enable a direct test by modern X-ray detectors. One compares the X-ray spectra in the region of interest (about 8 kilo-electronvolt) with and without current flowing through the copper conductor. In the case of current flowing one can test if electrons are performing Pauli-forbidden transitions in the copper conductor (detectable by energetically shifted X-rays). The conduction electrons can be seen as test-fermions and enable a sound test at the highest sensitivity level (corresponding to a probability upper limit <10-30 relative to allowed transitions). These tests in the lepton sector are very important also in view of speculations about spin-statics violations in the neutrino (also a lepton) sector, which would point to new physics. The VIP2 experiment is running in the underground laboratory Gran Sasso/Italy. Here the background related to cosmic muons is suppressed by a factor 1/1.000.000 therefore an ideal environment for detecting very rare processes. The high sensitivity of the experiment requires high X-ray detection efficiency and energy resolution, which can be ensured with the used silicon-drift detectors (SDDs) as X-ray detectors. Furthermore, SDDs have timing capability, which can be used for active shielding with scintillation detectors. In the FWF project running time the VIP2 experiment reached the world-best result for the upper limit of the validity of the Pauli Exclusion Principle for electrons using the method of new electrons. The VIP2 results were published and discussed in many presentations. The continuation of VIP2 with optimized setup, detectors and shielding is planned in the framework of the approved new FWF project.