Testing Pauli Exclusion Principle for electrons at the LNGS underground laboratory: The VIP-2 experiment

The VIP-2 experiment tests the Pauli Exclusion Principle (PEP) for electrons at the Gran Sasso underground National Laboratory (LNGS) of INFN in Italy, looking for a possible violation. The LNGS provide an extremely low background environment, ideal for performing high precision X-ray spectroscopy measurements on electrons atomic transitions. The core of the VIP-2 experimental apparatus is based on a copper target circulated by a Direct Current (DC) and surrounded by silicon drift detectors (SDDs), which offer excellent performance in X-ray spectroscopy in the energy range experimentally observed by VIP-2. The aim of VIP-2 is to look for eventual PEP-forbidden K_α transitions (2p → 1s) in copper atoms, when the 1s level would be already occupied by two electrons, in contradiction with PEP. The energy of the K_α forbidden transitions is about 300 eV less than the nominal energy of the K_α PEP-allowed transition. This energy shift is due to the screening effect produced by the extra electron in fundamental level, and is detectable by means of a high precision X-ray spectroscopy measurement. About ten years ago, the VIP experiment set the best upper limit on the PEP violation probability β^2/2 < 4.7 × 10^−29 for electrons. The goal of the VIP-2 experiment is to improve this limit by two orders of magnitude. This paper presents a new preliminary result, obtained by analysing two sets of data collected with a partial configuration of the VIP-2 apparatus.