The Pauli Exclusion Principle for electrons tested by VIP-2 at the LNGS laboratories

The VIP-2 experiment tests the Pauli Exclusion Principle (PEP) in a copper target circulated by a Direct Current, hunting for atomic K alpha transitions (2p -> 1s) that violate the PEP. Silicon Drift Detectors (SDDs) are installed around the target to measure the atomic transitions with high-precision x-ray detectors. A PEP-violating K alpha transition would occur if an electron arrived on the 1s orbital which is already occupied by two electrons. The energy of the K alpha forbidden transition in copper is expected to be shifted down by about 300 eV with respect to the nominal energy of the K alpha PEP-allowed transition (similar to 8 keV). The SDD resolution is 190 eV (FWHM) at 8 keV and allows measuring and disentangling possible PEP-violating K alpha transitions in copper. The VIP experiment sets an upper limit on the Pauli exclusion principle violation probability equal to beta(2)/2 < 4.7x10(-29) for electrons in copper. The VIP-2 experiment, presently in data taken at the National Laboratories of Gran Sasso (LNGS) in Italy, aims at improving the VIP limit by two orders of magnitude. This paper will present a new preliminary upper limit obtained by analyzing two sets of data collected in a preliminary run with the VIP-2 apparatus.