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Particles and interactions

Particle physicists discover new meson

18 May 2005 Belle Dumé

Physicists may have discovered the first "hybrid meson" at the KEK laboratory in Japan. The meson, first predicted over 25 years ago, appears to contain a gluon in addition to the quark and antiquark that are usually found in mesons (S-K Choi et al. 2005 Phys. Rev. Lett. 94 182002). It is currently known as the Y(3940) because it has a mass of 3940 MeV/c2.

The new meson was observed in electron-positron collisions by the international Belle collaboration at KEK and quickly decays into two well-known particles called the Omega and J/psi. The properties of the decay have led the Belle team to believe that it is not a standard quark-antiquark particle but may be a hybrid meson containing a charm quark, a charm antiquark and a gluon. The existence of such hybrid charm-anticharm-gluon particles was first predicted theoretically in 1978.

Although many of the properties of Y(3940) match those expected for a hybrid meson, its mass — which is about the same as that of a single helium atom — is much lower than theory predicts. The Belle collaboration now hopes to solve this enigma by further analysing its data.

The new meson is the latest in a list of recent surprising discoveries in particle physics. These include several particles called pentaquarks (which may or may not exist) 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.

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