An extensive new experiment has reaffirmed one of the fundamental tenets of quantum physics, placing the strongest limits to date on potential violations of the Pauli exclusion principle. The VIP-2 collaboration, based at the Gran Sasso National Laboratory in Italy, reported their findings in Scientific Reports in November 2025, confirming that electrons strictly adhere to this foundational rule.
The Pauli exclusion principle, established by Austrian-Swiss physicist Wolfgang Pauli in 1925, states that identical fermions—such as electrons—cannot occupy the same quantum state simultaneously. This principle underpins the structure of atoms, the rigidity of solids, and the stability of dense astrophysical objects like white dwarf stars.
However, physicists have long sought to test the principle’s absolute validity, probing for potential tiny violations that could hint at new physics beyond the Standard Model. “If the Pauli exclusion principle were violated, even slightly, it could have profound implications across atomic physics and astrophysics,” explained Dr. Catalina Curceanu, spokesperson of the VIP-2 team and physicist at the Italian National Institute for Nuclear Physics (INFN).
Innovative Approach to Detect Forbidden Transitions
The VIP-2 experiment employs ultra-sensitive X-ray detectors deep underground at Gran Sasso, shielding them from cosmic radiation. The team focused on copper atoms, injecting a large number of “new” electrons into the system—a strategy that enables the detection of possible violations. If electrons were to bypass the exclusion principle, they could produce exceptionally rare “forbidden” atomic transitions, emitting X-rays at characteristic energies.
“Introducing electrons that haven’t interacted with the atomic system before allows us to perform a clean, decisive test,” said lead researcher Alessio Porcelli of Jagiellonian University and INFN. Over several years, the detectors monitored the copper targets for the tell-tale signals of such forbidden transitions.
No Evidence of Violation, Stringent New Limits
The experiment detected no such forbidden X-ray signals. This absence enables the team to set the most stringent upper limit yet—improving previous constraints by many orders of magnitude—on potential violations. Specifically, they determined that any violation probability must be less than two parts in 10^43, an unprecedented level of precision.
Implications for New Physics Theories
These results impose severe restrictions on theories extending beyond the Standard Model. Notably, they challenge the validity of the so-called Quon model, which suggests particles could exhibit behavior intermediate between fermions and bosons. “Our findings strongly limit the possibility of electrons behaving anomalously,” emphasized Kristian Piscicchia of the Enrico Fermi Research Center.
Furthermore, the results cast doubt on hypotheses proposing that electrons possess hidden internal structures or subcomponents, as such features would likely cause detectable deviations from the exclusion principle. Even some quantum gravity theories, which predict potential violations, are now constrained by these findings.
Guiding Future Research
“Any new physics must respect the Pauli exclusion principle with extraordinary precision,” remarked Dr. Curceanu. The VIP-2 experiment’s success sets the stage for the next phase—VIP-3—which aims to push sensitivity even further, continuing the quest to uncover or rule out new fundamental phenomena.
Supported by the Foundation for Quantum and Fundamental Investigations (FQxI), this research exemplifies the ongoing effort to test the deepest principles of the quantum world. As Dr. Curceanu concluded, “Our work narrows the landscape of possible new physics, guiding future explorations into the foundations of nature.”
