Experimental search for the violation of Pauli exclusion principle

Abstract

The VIolation of Pauli exclusion principle -2 experiment, or VIP-2 experiment, at the Laboratori Nazionali del Gran Sasso searches for X-rays from copper atomic transitions that are prohibited by the Pauli exclusion principle. Candidate direct violation events come from the transition of a 2p electron to the ground state that is already occupied by two electrons. From the first data taking campaign in 2016 of VIP-2 experiment, we determined a best upper limit of for the probability that such a violation exists. Significant improvement in the control of the experimental systematics was also achieved, although not explicitly reflected in the improved upper limit. By introducing a simultaneous spectral fit of the signal and background data in the analysis, we succeeded in taking into account systematic errors that could not be evaluated previously in this type of measurements.

High Precision Test of the Pauli Exclusion Principle for Electrons

“…For the current on spectrum an additional Gaussian component was introduced representing the eventual PEP violating K α transition line, the centre of the line was set at 7746.73 eV (see Refs. [9]). In Figure 4b bottom the residuals from the second fit are shown for the two spectra.…”

“…In Equation (2) N new = (1/e) ∆t I(t)dt is the number of current electrons injected in the copper target over the acquisition time period (with current) ∆t, the factor 1/10 accounts for the capture probability (per electron-atom scattering) into the 2p state (see Ref. [12]), N int = D/µ is the minimum number of electron-atom scatterings, where D is the effective length of the copper strip and µ the scattering length for conduction electrons in the copper strip, to conclude = 1.8% is the detection efficiency factor, obtained by means of a Monte Carlo (MC) simulation (as described in [9]). By substituting µ = 3.9 × 10 −6 cm, e = 1.602 × 10 −19 C, I = 100 A, and the effective length of the copper strip D = 7.1 cm (the same used in the MC simulation), using the three sigma upper bound of 3 · ∆N X = 201 to give a 99.7% C.L., the following upper limit is obtained for the PEP violation probability:…”

“…More details on the VIP-2 experimental apparatus, the trigger logic, data acquisition and slow control can be found in Ref. [9].…”

“…In what follows the upgraded VIP-2 experimental apparatus (see Refs. [9,10]) will be presented, and the analysis of a first set of data (collected in 2016) will be described. As will be shown VIP-2 already improved the upper limit imposed by VIP on β 2 /2 (after three years of data taking), which represents the best limit ever on the PEP violation probability for electrons.…”

On the Importance of Electron Diffusion in a Bulk-Matter Test of the Pauli Exclusion Principle

“…We expect the number of non-Paulian transitions, and correspondingly the number of emitted X-rays, to be very low and to find a value of β very close to zero. In the real world, experiments operate with non-negligible environmental X-ray backgrounds, and by comparing the number of measured X-rays with and without current in the target one finds an upper bound for β 2 /2, both in the RS experiment [1] and in improved versions of the same experimental setup, such as VIolation of Pauli experiment (VIP) [3] and VIP-2 [4,5]. A first and rather obvious remark is that the minimum number of scatterings L/µ is far from being a realistic estimate of the actual number of scatterings, which must be much larger because electrons diffuse through the metal and perform complex random walks.…”