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.
A FWF project (P 25529-N20, project leader J. Marton) was approved and started with January 15th, 2013. In 2013 the VIP2 experimental setup was assembled at the Stefan Meyer Institute in Vienna and transported to the Laboratori Nazionali di Frascati/INFN (LNF/INFN) for testing. The final installation of the VIP2 setup in the underground laboratory of LNGS/INFN in the Gran Sasso laboratory – the final destination - will be done after the careful examination and testing of the full VIP2 setup in Frascati. The active shielding consisting of plastic scintillation detectors with readout by solid-state photo-detectors (SiPMs) was developed at SMI and transported to LNF in Frascati.
The lucite housing of the VIP2 setup, which will provide a clean gas surrounding of VIP2, was transported already to LNGS in Gran Sasso.
First tests of the vacuum systems and the target cooling system were performed in the first half of 2013. With August 2013 a postdoc, Dr. Hexi Shi, financed by the project started his work. The SiPM readout of the active shielding scintillation detectors was tested with cosmic rays. The efficiency of the scintillators was measured and was found to be better than 95% as anticipated. The timing resolution was measured using the relative timing of a 3-scintillation detector setup. The timing resolution was also studied at the beam test facility (BTF) at LNF employing tagged 500 MeV/c electrons and positrons. The typical time resolution is 2-3 ns FWHM. With the BTF beam the position sensitivity of the detection efficiency was studied but has found to be not significant.
The x-ray detection with silicon drift detectors (SDDs) was setup and tested. A timing resolution of SDDs <600 ns was found. The target system was tested with current flowing through the copper foils. The stability of the vacuum and cooling system were checked.
Therefore, the testing of the VIP2 apparatus made progress with encouraging results.