Particle physicists seem to have a pretty good handle on the fundamental particles of the universe, but there are some glaring holes in this understanding. Quarks are a good example of this. We know that all nuclear matter is made up of quarks, and we have a pretty good understanding of how two quarks interact at close range. But our quark theory cannot tell us which quark combinations will result in a bound particle or a stable nuclei. All we can go on is experience, and experience has shown that particles with four quarks do not exist. But the situation may have changed with the possible discovery of a new particle containing at least four quarks. Two separate groups, both reporting in Physical Review Letters, have seen evidence for this strange particle, called Zc(3900). Although the data is open to other interpretations, it’s clear that our understanding of quarks has a long way to go.
The evidence for Zc(3900) comes from two independent groups: the BESIII Collaboration at the Beijing Electron Positron Collider, China, and the Belle Collaboration at the High Energy Accelerator Research Organization in Tsukuba, Japan. It is the business of both labs to accelerate electrons and positrons to nearly the speed of light, smashing them into each other and carefully analyzing the resulting debris. Taken together, the two collaborations have uncovered 466 events that appear to have a Zc(3900) in their debris.
We study the process e+e-→π+π-J/ψ at a center-of-mass energy of 4.260 GeV using a 525 pb-1 data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be (62.9±1.9±3.7) pb, consistent with the production of the Y(4260). We observe a structure at around 3.9 GeV/c2 in the π±J/ψ mass spectrum, which we refer to as the Zc(3900). If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the π±J/ψ invariant mass spectrum, neglecting interference, results in a mass of (3899.0±3.6±4.9) MeV/c2 and a width of (46±10±20) MeV. Its production ratio is measured to be R=(σ(e+e-→π±Zc(3900)∓→π+π-J/ψ)/σ(e+e-→π+π-J/ψ))=(21.5±3.3±7.5)%. In all measurements the first errors are statistical and the second are systematic
SOURCES – Physics Review Letters
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