The mutation - in a process known as decay - was predicted under the so-called Standard Model (SM) of physics which describes how the universe works at the most fundamental level, but until now scientists had never seen it.

The discovery, announced at a conference in Japan, will sharpen efforts to find evidence for super symmetry, dubbed SUSY, a theory explaining some cosmic mysteries, and for other "New Physics" ideas beyond the SM's confines, CERN experts said.

The LHC, which went into operation at the research centre near Geneva in early 2010, had confirmed the model as its initial target, but its longer-term goal is to push beyond that into new realms envisaged by theorists.

"The detailed implications of this latest result will take a while to work through, but one thing is easy to state: the Standard Model has survived another test," U.S. physicist and CERN-watcher Matt Strassler said in a blog post.

A decay of the type recorded by CERN's LHCb experiment, in which a Bs meson particle was transformed after a collision in the LHC into a muon and an anti-muon, was foreseen under the model, developed in the second half of the 20th century.

Scientists had been trying to spot the decay - which in the arithmetically-intense world of particle physics had been correctly predicted to happen to one Bs meson in every 300 million - for well over a decade.

Oliver Buchmueller, of the Geneva research centre's CMS experiment, told Reuters that although the nature of the decay narrowed the energy range where SUSY traces might be found, it also left plenty of room for these to turn up later.

"This is another piece in the puzzle and with it the world appears even more SM (Standard Model)-like," he said. "It supports SUSY, because that is the only theory that can include the Standard Model in a wider concept of New Physics."

SUSY, which provides for the existence of unseen heavy "super-partners" to all known particles, is controversial, with some physicists expressing doubts about it as an explanation of cosmic oddities like the strange speed of rotation of galaxies.

The doubters have more than once pronounced the theory dead. But it would find its place in New Physics, which includes science fiction-like constructs including dark matter, dark energy, string theory and extra dimensions.

CERN hopes that some of these, and other concepts yet unspecified, might transmute from theory into fact when in late 2014 it doubles the power of the LHC, which runs 27 kms (16.8 miles) in a circle under the Swiss-French border.

The decay development follows the CERN announcement in July of the sighting in the LHC of a new particle - believed to be, although it remains to be confirmed - the Higgs boson, a key building bloc of nature that gives mass to matter.

The discovery, after three decades of searching in three different colliders, put the last major missing element into the Standard Model, drawn up by scientists on the basis of what was known and seemed likely about the universe.

The decay reported in Japan, at a Kyoto gathering of particle physicists, emerged - like the presumed Higgs - from light-speed collisions in the LHC that recreate the primeval disorder that followed the Big Bang some 13.7 billion years ago.

(Editing by Michael Roddy)

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