X rays test the Pauli exclusion principle
  Since the publication of the models describing a small violation of the Pauli exclusion principle (PEP) there has been an explosion of word-wide interest in PEP tests and related theories.   PEP forbids an atom to have more than 2 electrons in the K-shell. If PEP is slightly violated, a third electron can occasionally join in. This would result in an anomalous X-ray emission. A high-sensitivity experiment places an upper limit of the order of 10^{-26} on the PEP violating parameter.   I will outline the main theoretical and experimental ideas in this new exciting area. 
A.Yu.Ignatiev