"Even so, it should be remembered that the Standard Model is not a final theory of all phenomena, and is therefore inherently incomplete," said Professor Dmitry Budker of the University of California at Berkeley.

Professor Budker and his colleagues conducted the most rigourous trials yet of a fundamental assumption about how particles behave at the atomic scale.

"We tested one of the major theoretical pillars of quantum field theory, the spin-statistics theorem," said lead investigator Dr Damon English of the University of California at Berkeley.

"Essentially we were asking, are photons really perfect bosons?"

The spin-statistics theorem dictates that all fundamental particles must be classified into one of two types, fermions or bosons.

No two electrons can be in the same quantum state -- but any number of bosons can occupy the same quantum state.

The way to tell them apart is by their spin -– not the classical spin of a whirling top but intrinsic angular momentum, a quantum concept.

"There's a mathematical proof of the spin-statistics theorem, but it's so abstruse you have to be a professional quantum field theorist to understand it," said Professor Budker.

"Every attempt to find a simple explanation has failed, even by scientists as distinguished as Richard Feynman."

"The proof itself is based on assumptions, some explicit, some subtle."

"That's why experimental tests are essential."

The researchers set out to test the theorem by using laser beams to excite the electrons in barium atoms.

For experimenters, barium atoms have particularly convenient two-photon transitions, in which two photons are absorbed simultaneously and together contribute to lifting an atom's electrons to a higher energy state.

"Two-photon transitions aren't rare, but what makes them different from single-photon transitions is that there can be two possible paths to the final excited state -– two paths that differ by the order in which the photons are absorbed during the transition," Dr English said.

"These paths can interfere, destructively or constructively."

"One of the factors that determines whether the interference is constructive or destructive is whether photons are bosons or fermions."

Professor Budker described the studay as 'a true table-top experiment, able to make significant discoveries in particle physics without spending billions of dollars.'

"We keep looking, because experimental tests at ever increasing sensitivity are motivated by the fundamental importance of quantum statistics," he said.

"The spin-statistics connection is one of the most basic assumptions in our understanding of the fundamental laws of nature."