By Dr David Whitehouse
BBC News Online science editor
Scientists have found a sub-atomic particle they cannot explain using current theories of energy and matter.
Fermilab confirmed the discovery
The discovery was made by researchers based at the High Energy Accelerator Research Organisation in Tsukuba.
Classified as X(3872), the particle was seen fleetingly in an atom smasher and has been dubbed the "mystery meson".
The Japanese team says understanding its existence may require a change to the Standard Model, the accepted theory of the way the Universe is constructed.
X(3872) was found among the decay products of so-called beauty mesons - sub-atomic particles that are produced in large numbers at the Tsukuba "meson factory".
It weighs about the same as a single atom of helium and exists for only about one billionth of a trillionth of a second before it decays into other longer-lived, more familiar particles.
Although this is extremely short-lived by human standards, scientists say that a billionth of a trillionth of a second is nearly an eternity for a sub-atomic particle this heavy.
Particles smaller than the atom are grouped into families depending upon their mass, spin and electric charge.
But X(3872) is peculiar in that it does not fit easily into any known particle scheme and, as a result, has attracted a considerable amount of attention from the world's physics community.
Its discovery was recently confirmed by researchers at the Fermi National Accelerator Laboratory in Illinois, US, home of the Tevatron, the world's largest atom smasher. It was the US outfit that gave X(3872) its mystery tag.
A normal meson is comprised of a quark and an antiquark held together by the "colour" force, also called the "strong" force because it is the most powerful known in nature.
The large variety of meson particles that have been found to date reflect the many different ways that these combinations can be achieved.
However, again, X(3872) does not match theoretical expectations for any conceivable quark-antiquark arrangement.
To explain it, theoretical physicists may have to modify their theory of the colour force; or make X(3872) the first example of a new type of meson, one that is made from four quarks (two quarks and two antiquarks).