An international team of high-energy physicists says the discovery of an electrically charged subatomic particle called Z_c(4020) is a sign that they have begun to unveil a whole new family of four-quark objects.  

The Beijing Spectrometer (BESIII) collaboration previously announced the discovery of a four-quark particle called Z_c(3900) in April of this year.  The results have come about through a dedicated study of the byproducts of the anomalous Y(4260) particle. 

Using the Beijing Electron Positron Collider (BEPCII) in China, scientists tuned the energy at which electrons and positrons annihilate matter to 4260 MeV, which corresponds to the mass of the Y(4260) particle. The BESIII Collaboration used this method to directly produce and collect large samples of the particle's byproducts, or decays.

This experimental method allowed the BESIII collaboration to first observe the Z_c(3900) and then the Z_c(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 peak is evidence for Z_c(4020) decaying to π^±h_C., according to new results. Credit: BESIII Collaboration

 "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," said Frederick Harris, a professor of physics and astronomy at University of Hawaii Mānoa, and 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. 

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

Preprints:

Observation of Z_c(4040) in e+e- --> D*D*- pi+ process at 4.26 GeV arXiv:1308.2760

Observation of a charged charmoniumlike structure Z_c(4020) and search for the Z_c(3900) in e+e- to pi+pi-h_c arXiv:1309.1896

Observation of a charged (DD*bar)- mass peak in e+e- --> pi+(DD*bar)-at Ecm=4.26 GeV arXiv:1310.1163

Observation of the X(3872) in e+e- --> gamma pi+pi- J/psi at sqrt(s) around 4.26 GeV arXiv:1310.4101

Upcoming in Physical Review Letters

Source: University of Hawaii at Manoa