# New particle turns up in Japan

14 Nov 2003 Belle Dumé

The Belle collaboration at the KEK laboratory in Japan has discovered a new sub-atomic particle which it is calling the “X(3872)”. The particle does not fit into any known particle scheme and theorists are speculating that it might be a hitherto unseen type of meson that contains four quarks (arxiv.org/abs/hep-ex/0309032; Phys. Rev. Lett. to be published). The discovery has been confirmed by the CDF collaboration at Fermilab in the US, where the new particle is being called the “mystery meson”.

Mesons are particles that contain a quark and an antiquark that are held together by the strong nuclear force. Since there are six different “flavours” of quark – up, down, strange, charm, bottom and top – it is possible to form a large number of different mesons.

The Belle team measured the decay of B-mesons – mesons that contain a bottom quark – produced in electron-positron collisions at the KEK B-factory in Japan. The team plotted the number of candidate events for B mesons against mass and observed a significant spike in the distribution at 0.775 GeV. This corresponds to a mass of nearly 3872 MeV. The particle decayed almost immediately into other, longer lived particles.

The KEK team says that the mass of this new meson is higher than theoretical predictions. Moreover, the way in which it decays also differs from theory. One possibility is that current models of the strong force need to be modified. Alternatively it could be that X(3872) is the first example of a “molecular state” meson that contains two quarks and two antiquarks.

Until recently particle physicists had only ever detected particles that contain two or three quarks. However, in the past year evidence has emerged for another four-quark particle known as the Ds(2317) and a five-quark particle known as the pentaquark.