According to BESIII physicists, their discovery of a new particle named Zc (4020) hints at a novel class of four-quark objects.
The exotic subatomic particle could be the first clear indication that more than three quark particles can be produced experimentally. If it is what physicists think it is, the particle could provide clues about the force that holds nuclei together and perhaps about the earliest moments of the universe.
“We have very solid evidence of an unconventional particle,” says Ronald Poling, a physicist at the University of Minnesota in Minneapolis. “But it’s the interpretation — the possibility that it has four quarks — that makes it very exciting.”
Physicists have known since the 1960s that protons and neutrons are made up of quarks, as are hundreds of other particles. All of these particles can be divided into two categories: mesons, which contain two quarks, and baryons (including protons and neutrons), which contain three.
Over the last decade many physicists, including those at the Belle experiment in Japan and the BESIII experiment in China have fruitlessly searched for particles with more than three quarks. Probing a particle’s insides is tough because physicists can’t see quarks directly. Instead they have to measure all the properties they can for a given particle, such as its mass, charge and decay products, looking for unusual characteristics that can be explained only by a peculiar combination of quarks.
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 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.”
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