The Pauli Exclusion Principle (PEP) plays a fundamental role in our understanding of many phenomena in chemistry and physics, e.g. periodic table of the elements, electric conductivity in metals, degeneracy pressure (responsible for the stability of white dwarfs and neutron stars). PEP can be explained as a consequence of spin statistics. Although it has been spectacularly confirmed by the number and accuracy of its predictions, the foundation of PEP lies deep in the structure of quantum field theory and no simple proof can be given up to now.

In autumn 2008 an international Workshop on the theoretical and experimental aspects of the spin-statistics connection and related symmetries took place in Trieste. Our institute was co-organizing this event which exhibited the huge interest in PEP and in the connection to spin-statistics. A lively debate on its limits is going on and many experiments to search for tiny violations of PEP are under way. The most precise study of PEP for electrons is represented by VIP in which our institute is participating.

Based on an experimental procedure performed by Ramberg and Snow, the VIP experiment aims for a substantial improvement of the upper limit for PEP for electrons (improvement by 2-4 orders of magnitude) by using a high sensitivity apparatus in the low background environment of the underground laboratory of Laboratori Nazionali di Gran Sasso (LNGS).
The experimental method is based on introducing new electrons into a copper strip and to look for X-rays resulting from the 2p → 1s anomalous (spin-statistics forbidden) X-rays emitted if one of the new electrons would be captured by a Cu atom and cascades down to the 1s state already filled with two electrons of opposite spin. The energy of this transition would differ from the normal Ka-transition by about 400 eV (7.64 keV instead of 8.04 keV), providing an unambiguous signal of the PEP violation. For the X-ray detection we employ the CCD X-ray detector system used for the DEAR (DAFNE Exotic Atom Research) experiment, which has successfully completed its program at the DAFNE collider at LNF-INFN.

The measurement alternates periods with no current in the Cu strip, in order to evaluate the X-ray background in conditions where no PEP violating transitions are expected to occur, with periods with current through the Cu strips, thus providing “fresh” electrons, which might possibly violate PEP.

The rather straightforward analysis consists of the evaluation of the statistical significance of the normalized subtraction of the two spectra in the region of interest.

The VIP apparatus is installed and is taking data in the Gran Sasso underground laboratory. A substantially improved upper limit of 6×10^-29 for PEP violation for electrons was obtained thus improving the limit by a factor of about 300 compared to the result of Ramberg and Snow. Today this is the best value ever reached on the probability of PEP violation for electrons.

Outlook

The data taking will continue in 2009. In parallel a further improved experiment (VIP2) employing new X-ray detectors (SDDs) and an active shielding is in discussion in order to improve the limit further down to the region 10^–30–10^–31. Feasibility studies will take place during 2009 in the laboratory as well as in LNGS.