Mesons are particles that contain a quark and an antiquark held together by the strong nuclear force. There are six different "flavours" of quark -- up, down, strange, charm, bottom and top -- so it is possible to create a large number of different mesons. The new meson found by SELEX (Segmented Large X baryon Spectrometer) contains a strange quark, which is a light quark, and a charm antiquark, which is much heavier.

The SELEX physicists analysed data collected in proton-antiproton collisions at the Tevatron collider at Fermilab. To their surprise, they found that their new meson -- which has a mass of 2635 MeV -- had a lifetime that was three times longer than that of lighter mesons. Normally, the half life of a meson gets shorter as the mass increases. Moreover, the new meson decayed into another meson known as the eta meson six times more often than predicted by theory.

"This new particle is showing a possible deviation from the expected path that most mesons take," said Christopher Hill, a theorist at Fermilab. "It suggests that some intriguing new dynamical aspect of the strong force is at work, and it opens the door for many future explorations, at Fermilab and around the world."

The new meson is the latest in a list of recent surprising discoveries in particle physics. These include several particles called pentaquarks that contain five quarks, a particle called the X(3872) that appears to be made of four quarks, and another meson called the Ds(2317) that does not behave as predicted.