There is excitement among particle physicists following evidence of the existence of a four-quark particle which has been seen in two experiments — the Belle particle detector in Japan and in BESIII detector in China. In two recently published papers in Physical Review Letters, M. Ablikim et al and Z.Q. Liu et al., of BESIII and Belle, respectively, describe the detection of a “new charged charmoniumlike” state.

The particle, named Z(3900), is a composite of two quarks and two antiquarks. This arrangement differs from what is known about strongly interacting particles. Strongly interacting particles called hadrons are believed to exist in two varieties, three-quark states, named the “baryons” (protons, neutrons, etc) and quark-antiquark bound states called the “mesons” (pions, kaons, etc). These have been the only strongly interacting particles observed and considered by theorists for a long time.

Atoms and molecules arise from two kinds of electric charges: positive and negative. But quarks, which are bound together by strong interactions, have three kinds of charges which are characterised by a quantum number called colour (as three kinds of primary colours exist). The theory describing the strong interaction of quarks is called quantum chromodynamics or QCD.

Coloured quarks like the North or South Pole of a magnet can never be isolated, and all particles have to be colour-neutral. The simplest way to build up particles with these is to consider quark-antiquark pairs or states made of all three quarks. Now this new particle that has been discovered, which consists of two quarks and two antiquarks, is again a perfectly valid state, being colour-neutral.

“The observation of unexpected resonances is not completely new, and some resonances have been seen in several of the experiments including ones at LHC,” says Dr Rahul Sinha, a member of the Belle collaboration, in Japan, and a theoretical physicist at the Institute of Mathematical Sciences, Chennai.

However, the discovery of the new quark particle at low energies shows that there is a lot to be understood about bound-state formation in QCD and the nature of hadrons.

There is a debate about whether the particle is a “molecule” made up of two mesons or a true “tetraquark.” But Dr Sinha favours the tetraquark interpretation because the state is more stable than it would be had it been a loosely bound molecule.

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