According to physicists of the Beijing-based BESIII experiment, their discovery of a new subatomic particle named Zc (4020) hints at a novel class of four-quark objects.

Quarks are the fundamental building blocks of nature. They combine to form protons, neutrons and hundreds of other particles called hadrons.

Until recently hadrons were believed to have only two possible structures. Baryons, including the proton and neutron, are made of three quarks. Mesons, like the pion and kaon, consist of a quark and an antiquark. All mesons and most baryons are highly unstable, living much less than a millionth of a second after production.

In April 2013, the BESIII experiment announced the discovery of a mysterious four-quark particle called Zc (3900).

The properties of the Zc (3900) reveal that it consists primarily of a charm-flavored quark and its antiquark bound together similarly to the long-known charmonium states. The particle is different in that it carries an electric charge, signaling that a light quark-antiquark pair must also be lurking inside.

“While quarks have long been known to bind together in groups of twos or threes, these new results seem to be quickly opening the door to a previously elusive type of four-quark matter,” explained Prof Frederick Harris, a spokesman for the BESIII experiment.

“The unique data sample collected by the BESIII collaboration has continued to yield a stream of clues about the nature of multi-quark objects.”

The recent breakthroughs by the BESIII experiment have come about through a dedicated study of the byproducts of the anomalous Y (4260) particle.

The physicists tuned the energy at which electrons and positrons annihilate matter to 4,260 MeV, which corresponds to the mass of the Y(4260) particle. They used this method to directly produce and collect large samples of the particle’s byproducts.

This method allowed them to first observe the Zc (3900) and then the Zc (4020).

Also recently spotted in the decays is the electrically neutral X (3872), a particle that has been experimentally established for more than 10 years, and has long been suspected to be a four-quark object.

“The year 2013 has so far been an exciting one for the BESIII experiment,” Prof Harris said.

“Using decays of the Y (4260), a family of four-quark objects has begun to appear.”

The new results appear in four papers submitted to the journal Physical Review Letters (paper #1, paper #2, paper #3 and paper #4).

“While the theoretical picture remains to be finalized, more and more clues are suggesting that we are witnessing new forms of matter. And while a new ‘zoo’ of mysterious particles is emerging, it seems a new classification system may soon be at hand to understand it.”

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Bibliographic information: Ablikim M et al. 2013. Observation of a charged charmoniumlike structure in e+e−→(D∗D¯∗)±π∓ at s√=4.26\,GeV. Physical Review Letters, submitted for publication; arXiv: 1308.2760

Ablikim M et al. 2013. Observation of a charged charmoniumlike structure Z_c(4020) and search for the Z_c(3900) in e+e- to pi+pi-h_c. Physical Review Letters, submitted for publication; arXiv: 1309.1896

Ablikim M et al. 2013. Observation of a charged (DD*bar)- mass peak in e+e- –> pi+ (DD*bar)- at Ecm=4.26 GeV. Physical Review Letters, submitted for publication; arXiv: 1310.1163

Ablikim M et al. 2013. Observation of e+e−→γX(3872) at BESIII. Physical Review Letters, submitted for publication; arXiv: 1310.4101