Pauli Exclusion Principle Holds Strong: Rigorous Test Finds No Violations

The Pauli exclusion principle, a fundamental rule in quantum physics, dictates that two identical fermions (such as electrons) cannot occupy the same quantum state. An international collaboration of physicists has conducted one of the most rigorous experimental tests of this principle to date, finding no evidence of its violation.

Using the VIP-2 experiment at Italy's Gran Sasso National Laboratory, the team focused on electrons within copper atoms. This experiment establishes the strongest limits yet for potential violations involving electrons in atomic systems. These findings significantly constrain speculative theories beyond the Standard Model, including those proposing internal electron structure or 'Quon models.' The research was published in Scientific Reports in November 2025.

Austrian-Swiss physicist Wolfgang Pauli first outlined the exclusion principle in 1925. It explains phenomena such as why electrons fill atomic shells, the rigidity of solids, and the stability of dense objects like white dwarf stars. Catalina Curceanu of the Italian National Institute for Nuclear Physics (INFN) stated:

"Even a minuscule violation of the principle could have significant consequences across physics, from atomic scales to astrophysics."

Testing the Limits: The VIP-2 Experiment

The VIP-2 experiment aimed to detect normally forbidden atomic transitions that would produce X-rays at specific, non-standard energies if the Pauli exclusion principle were violated. Researchers introduced a large number of 'new' electrons into copper targets, a method referred to as an 'open-system configuration.'

Lead author Alessio Porcelli of Jagiellonian University explained that this approach is crucial because quantum mechanics forbids violations from appearing in closed systems, enabling a clear and decisive test of the principle.

The copper target was monitored for several years using low-noise X-ray detectors, installed deep underground to shield them from cosmic radiation. No such X-ray signals were observed. This allowed the team to set the most stringent upper limit so far on possible Pauli exclusion principle violations for electrons in atomic transitions, corresponding to a probability smaller than two parts in 10^43.

Porcelli described this achievement as:

"the strongest experimental constraint ever achieved for electrons in open systems."

Implications for Physics Theories

While the Standard Model of particle physics assumes the exactness of the Pauli exclusion principle, many proposed extensions do not. The VIP-2 results significantly restrict the landscape of viable alternative theories. This particularly impacts the 'Quon model,' which suggests particles could behave almost like fermions but with occasional violations.

Kristian Piscicchia of the Enrico Fermi Research Center stated that the results place very stringent constraints on possible deviations from standard fermionic behavior for electrons, strongly restricting the alternative Quon models.

Furthermore, the findings challenge theories suggesting that electrons possess a hidden internal structure, as such a substructure would likely weaken the Pauli exclusion principle – an effect not observed by VIP-2. Even certain approaches within quantum gravity theories that predict Pauli Exclusion Principle violations at levels now excluded by the experiment are affected.

Curceanu emphasized:

"Any viable extension of quantum theory must reproduce the Pauli exclusion principle with extraordinary precision."

She added that the VIP-2 results significantly narrow the type of new physics that could appear, while also guiding the next generation of even more sensitive experiments. The Foundational Questions Institute (FQxI) partially supported this work. The next-generation VIP-3 experiment is planned to push the sensitivity of such tests even further, continuing the search for possible deviations.