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Minnesota physicists contribute to discovery of apparent four-quark particle

APS/Alan Stonebraker

Physicists in the School of Physics and Astronomy are members of one of the two groups that recently reported the discovery of an intriguing new particle state named Zc(3900). This exceedingly short-lived particle has physicists excited because it presents an unexpected twist in the Standard Model of particle physics, which is the playbook for how the building blocks of matter fit together.

Studying states like the Zc(3900) in accelerators is key to understanding the universe. "The more complete our picture of the elementary particles and their interactions," says Professor Ron Poling, a member of the BESIII collaboration, "the better we’ll understand where we started out and how we got to where we are."
Quarks are believed to be the smallest constituents of nuclear matter, composing protons and 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. We understand the observed hadrons as combinations of six "flavors" of quarks. 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 Zc(3900) is different, however, in that it carries an electric charge, signaling that a light quark-antiquark pair must also be lurking inside. This discovery is momentous because no four-quark particle has ever been conclusively demonstrated before now.

Dr. Jianming Bian, a research associate in the School of Physics and Astronomy, used BESIII data to confirm the Zc(3900) observation and is now leading studies to help ascertain its nature. "It has been an exciting time," says Bian. "Once we knew that our observation was not a statistical fluke, we looked to theoretical physicists to help us figure out what’s inside the Zc(3900)." One of the leading theorists in this effort is Professor Mikhail Voloshin of the W.I. Fine Theoretical Physics Institute in the School, whose predictions of the detailed properties corresponding to different internal quark arrangements ("meson molecules," "hadro-charmonium" and "tetraquarks") are guiding the experimentalists’ efforts. Additional results are expected very soon.

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