High-energy physicists predict new family of four-quark objects

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, which includes scientists from UH Mānoa, previously announced the discovery of a mysterious four-quark particle called Z_c(3900) in April 2013.

“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 UH 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.”

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

About the BESIII Experiment:

The Beijing Spectrometer (BESIII) experiment at the Beijing Electron Positron Collider is composed of about 350 physicists from 50 institutions in 11 countries. U.S. groups include Carnegie Mellon University, Indiana University, The University of Minnesota, The University of Rochester, as well as physicists in the High Energy Physics Group, in the Department of Physics and Astronomy at the University of Hawai‘i at Mānoa.

The scientists have reported their findings to the scientific journal Physical Review Letters, including:

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

For more information, visit: http://www.phys.hawaii.edu/newsEvents/docs/bes-pr-11-2013.pdf