Tetraquark Or Two Meson Molecule Discovered

A new particle, called Z_c(3900) is either a tetraquark (an elusive hadron made up of four quarks in a single composite particle) or a molecule made up of two mesons (which are composite particles made of two quarks each).  Either possibility would be the first confirmed sighting of a particle of that type.  This particle is one of the decay products of another unknown particle called Y(4260).

The numbers refer to the estimated particle mass in MeV/c^2 (mega-electron-volts/speed of light squared; the large letter code for general properties displayed by the particle and the c subscript denotes the presence of a charm quark in the particle.

All particles made of quarks found to date (and under the Standard Model) are "confined" to composite particles which are strong force color neutral (i.e. they have "red", "green" and "blue" flavored quarks, or have a quark that is, for example "red" and "antired").  So far, all known composite particles made of quarks come in two quark varieties called mesons, and three quark varieties called baryons.  Prior to this report and some previously unpublished conference reports, no tetraquarks, pentaquarks or larger number of quarks have been observed so far.

But, the rules of quantum chromodynamics (QCD), that governs the strong force interactions of quarks, does not forbid composite particles made up of more than three quarks so long as their are strong force color neutral, which is theoretically possible for any number of quarks two or greater if the structure can be small enough to allow the short range strong nuclear force to operate between all of the quarks involved.

Likewise while baryons (made up of three quarks, such as protons and neutrons) have been observed to form atomic nuclei which in turn form molecules bound electromagnetically, no composite structures made of multiple mesons have previously been observed even though they aren't prohibited by the rules of QCD either.

The Belle and BESIII teams were both studying an odd particle called  when they realized that it decayed to make another interesting particle, Z_c(3900). Its mass, says Poling, who is part of the BESIII team, suggests that it is an electrically neutral meson made up of two quarks with opposite charges, called charm and anticharm. But surprisingly, both teams found that Z_c(3900) has an electrical charge.

In fact, Poling says no two-quark or three-quark combinations can explain Z_c(3900)’s charge and mass. That is leading physicists to the more exotic and exciting conclusion that the particle consists of four quarks: a charm and an anticharm along with an up and an antidown, which are extremely light and create a net positive charge. “The particle’s charge makes it a smoking gun for a four-quark state,” says Tomasz Skwarnicki, a physicist at Syracuse University in New York.

Assuming the evidence for a four-quark arrangement holds up, the big question will be how those quarks are arranged. Z_c(3900) could be a single entity of four quarks, Skwarnicki says, but it could also be a coupling of two mesons, analogous to two atoms linking up to form a molecule.

Previous conference report papers on possible tetraquark sightings have tended to favor the two meson molecule explanation, which is the less revolutionary and dramatic of the two possibilities.

Of course, Z_c(3900), like all composite particles made of quarks other than protons and neutrons, is a highly unstable particle with a mean lifetime measured in fractions of a second that almost immediately decays to simpler particles. 

So, while its discovery is relevant to clarifying the fine details of what is possible in QCD, and perhaps to the dynamics of the period immediately following the Big Bang, the discovery has essentially no practical technological applications and it will not explain any important phenomena observed outside of intense man made particle accelerator conditions.

If it is a meson molecule, rather than a true tetraquark, the discovery might actually tend to support the theory that there is some unarticulated and unknown law of physics pertinent to QCD that forbids the creation of unitary composite quark structures with more than three quarks, even though none of the current rules of QCD clearly impose this limitation.  This would be as much of a major breakthrough as a finding that unitary composite quark structures with more than three quarks are possible.

A meson molecule model would imply that something similar to the strong force interactions that in diluted form bind atomic nuclei together applies to mesons as well.  The experimentally measured lifetime of Z_c(3900) as an indirect measure of this indirect version of the strong force, could help to better pin down experimentally the speed of strong force interactions generally.  These would also be valuable discoveries.

Source

Andrew Grant, "First four-quark particle may have been spotted: Finding might shed light on how nucleus is held together", Science News (June 21, 2013) citing M. Ablikim et al., "Observation of a charged charmoniumlike structure in e+e−→π+π−J/ψ at √s=4.26 GeV.", Physical Review Letters (June 17, 2013) and Z. Q. Liu et al., "Study of e+e−→π+π−J/ψ and observation of a charged charmoniumlike state at Belle.", Physical Review Letters  (June 17, 2013).