Quirky Four-Quark Quantum Monster Discovered

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While looking for a strange state of matter in two particle accelerators, another, totally unexpected particle has been discovered. Say hello to the Zc(3900) particle, a particle that physicists had no clue could exist.

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The hunt for the Higgs boson is one of the most fundamental quests in physics history. Entire particle accelerators have been built, at least in part, to detect the elusive particle that mediates mass in the quantum world, thereby shoring up the last missing piece of the Standard Model. But don’t be fooled into thinking that just because the Higgs has been found — or, at least, something that looks like the Higgs has been found — there’s nothing left in the subatomic realm to be discovered. On the contrary.

Enter the Zc(3900) — an odd particle containing four quarks. This is the first time a four-quark particle has ever been discovered in the wild. Usually, quarks combine in groups of twos and threes creating common particles known as hadrons. Protons and neutrons are ‘everyday’ examples of three-quark hadrons (baryons); pions and kaons are ‘not-so-everyday’ examples of two-quark hadrons (a.k.a. mesons).

But the Zc(3900) seems to be a trendsetter. It appears it has gathered together four quarks — one “charm” and one “anti-charm quark,” plus one “down” and one “anti-down quark” — to create a jumbo hadron. Technically, this particle is a ‘tetraquark’ — a four-quark meson — that has until now has just been a hypothetical form of matter.

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The discovery was made independently by two particle accelerators; the Belle experiment in Japan and the BESIII experiment in China. However, neither were looking for, or even predicted, the Zc(3900).

Of interest was another odd quantum creature called the Y(4260). During experiments at both institutions, they were making attempts at studying the Y(4260) that was first discovered in 2005. It is a subatomic oddball that seems to contain a “charmed” quark, an “anti-charmed” quark and an extra gluon. Gluons are bosons that mediate the strong force, which binds atomic nuclei together. Therefore, any hadron will contain a gluon, binding component quarks. But Y(4260) is carrying some extra baggage (i.e. the extra gluon).

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During the Belle and BESIII particle collisions, spawning the creation of Y(4260) particles, physicists noticed an “excess” or “spike” in the debris of the collision at an energy level of 3.9 gigaelectronvolts (GeV). As it turns out, this energy is approximately four-times the mass of a proton and suggestive of a new, exotic particle containing four quarks.

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Although physicists from both teams are aware that the signal at 3.9 GeV may be some other quantum effect (it could be two different particles containing two quarks bonding in a novel way for a short period of time, for example), the 460 detections of Zc(3900) that have been measured so far strongly suggests a new type of matter has been discovered.

The teams have published their work in two papers in the journal Physical Review Letters.

No doubt other international particle accelerators will now try making their own Zc(3900)’s in the hope of further characterizing this new variety of matter, proving that just because the Higgs may have the all the celebrity attention right now, the quantum world still has a universe of possibilities just waiting to be tapped into.

Image: Artist’s impression of a particle collision. Credit: Getty