The Pauli exclusion principle (PEP) represents one of the basic principles of modern physics and, even if there are no compelling reasons to doubt its validity, it still spurs a lively debate, because an intuitive, elementary explanation is still missing, and because of its unique stand among the basic symmetries of physics. A new limit on the probability that PEP is violated by electrons was estabilished by the VIP (Violation of the Pauli exclusion principle) Collaboration, using the method of searching for PEP forbidden atomic transitions in copper. The preliminary value, ½ β^2 < 4.5 × 10^−28, represents an improvement of about two orders of magnitude of the previous limit. The goal of VIP is to push this limit at the level of 10^−30.
