June 20th, 2013 by Michael Ricciardi
The particle physics community is all stirred up by the potential discovery of an exotic new particle whose properties can best be explained if it were to violate one of the basic conventions of Quantum Chromodynamics (QCD, a key component of the “Standard Model”), that is, the maximum number of quarks that nuclear particle can have.
The particle (or signal data indicating one) is referred to cryptically as the “Zc(3900) observations.”
Two separate teams of physicists conducting independent experiments have found tantalizing evidence for a rather “exotic” new particle call Zc(3900). What makes this particle (if that’s what it is) so exotic? Well, it probably has four quarks.*
* The word ‘quark’ comes from James Joyce’s genius literary work ‘Finnegan Wake’: ‘Three quarks for muster Mark…’ (suggested by Murray Gell-Mann in 1963)
All about quarks
Quarks are fundamental sub-atomic particles which serve as the “building blocks” for bigger nuclear particles like neutrons and protons. Such atomic particles conventionally are comprised of combinations of just three quarks, possessing just six different “flavors” (types), called up, down, top, bottom, charm, and beauty. The quarks are held together by small, interacting particles called (quite aptly) gluons. A combination of three quarks is known generically as a hadron. Bound, three-quark groupings form stable atomic nuclei or any of a great number of independent (non nuclear) particles. With a few known exceptions (usually particles missing just one quark, which form particles called mesons), hadrons have just three quarks, and that was final.
But not any more. Zc(3900) exhibits curious properties and behaviors that compel physicists to describe it as a four-quark particle.
The evidence for this new particle was compiled by two teams: the BESIII Collaboration at the Beijing Electron Positron Collider, China, and the Belle Collaboration at the High Energy Accelerator Research Organization in Tsukuba, Japan. These respective labs continuously conduct particle acceleration experiments with electrons and positrons (the antimatter form of the electron) — smashing them into each other at almost the speed of light.
But in the last batch of such collision experiments, the two team have collectively uncovered 466 collision events whose “debris” shows evidence of the Zc(3900) particle.
The theory of quantum physics that describes the behavior (QCD) of nuclear particles (note: electrons, which are leptons, are described by quantum electrodynamics, or QED) is effective for making predictions at high energies. However, at lower energies, where quarks bind together (via gluons) to form large, stable particles, QCD runs into difficulties. This sticking point makes it difficult for physicists to say with certainty (and nothing is entirely certain in the quantum world, remember, it’s all about probability) how many quarks a particle should, or must, have.
These recent experiments may be changing the rules. Originally, the two teams were trying to tease out the nature of another mysterious new particle — reported previously here on PS — called Y(4260)…which was previously known as Y(4140).
It is now believed that this particle is composed of a charm quark, an anticharm quark, and an extra gluon. But the gluon here is not one of the normal, “shared” gluons found in nuclear hadrons. No, this gluon is a “permanent” member,like the quarks themselves. The idea of a permanent gluon led physicists to postulate a hypothetical particle called a “glueball” composed of, yes, all gluons. This would be a strange particle indeed, like an atom made entirely of photons — particles of “pure” light.
But unlike photons, gluons can interact with each other to form quite odd combination that are not found amongst electromagnetic particles (described by QED).
It was in making this “enigmatic” Y(4260) particle that the BESIII and Belle labs discovered evidence for this other enigmatic particle: Zc(3900), a particle that physicists have never seen before.
So, why four quarks?
Well, the theorizing involves the decaying Zc(3900) into other positively or negatively charged particles called pions (which are mesons consisting of up and down quarks and antiquarks) and a second particle denoted as J/Ψ (which is a neutral meson made of a charm-anticharm quark pair).
What all the complicated quantum mechanical calculating comes down to is that, after adding up all the probabilities, it seems that this decay state of Y(4260) — which in theory should be electrically neutral — has an extra charge (a “nonzero net charge”). This extra charge (depending on which combination of + or – pion is used) means that there must be an extra quark (to confer the charge). So then, this particle must therefore go through an intermediate particle phase — the Zc — which as it turns out, is four times heavier than a proton (note: at 3900 mega-electron-volts, thus the name).
There are several other possible four-quark combinations and physicist continue to debate these – as well as the four quark theory itself. An alternative explanation for the data is that Zc is not a new particle at all; that what is being observed is actually an interaction between two D mesons.
These D mesons are composed of a charm quark with an up or down quark and so, they have basically the same quark content (as shown in Fig. 1(c). Alternative models show that these mesons possess an attraction of sufficient power to explain the data without needing to create a new particle.They note that the difference between the two theorized states (i.e., new particle or two mesons) is very small.
So, which theory is correct? Only more collision experiments will tell. In the meantime, physicist now have a real particle mystery to solve…which is great timing, considering that the recent 2012 confirmation of the Higgs boson (and the Standard Model) left physicists in quantum limbo, concerned for the future of high-energy physics.
Source material (including quark diagram) for this post came from the Phys.aps.org article: ‘Viewpoint: New Particle Hints at Four-Quark Matter’ by Eric Swanson,
Here are the two papers (and journal citations) describing the experiments (you can download each for free):
Observation of a Charged Charmoniumlike Structure in e+e–→π+π–J/ψ at √s=4.26 GeV M. Ablikim et al. (BESIII Collaboration)
Study of e+e–→π+π–J/ψ and Observation of a Charged Charmoniumlike State at Belle Z. Q. Liu et al. (Belle Collaboration)
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