Meet The Tetraquarks – A Whole New Family Of Sub Atomic Particles

This blue spike in the graph is what the discovery of a new sub atomic particle looks like to the physicists studying the data collected by their experiment.

This blue spike in the graph is what the discovery of a new sub atomic particle looks like to the physicists studying the data collected by their experiment. The spike corresponds to the energy of the charmed Z (4020) particle.

A whole new family of sub-atomic particles is being uncovered by an international team of high-energy physicists working together in China in a collaborative experiment known as BESIII.

They say the discovery of a strange new  electrically charged subatomic particle called Zc(4020) is a sign that they have begun to unveil an entirely new and previously unknown group of particles which are composed of four quarks.

Quarks, the smallest known building blocks of matter, are the constituent parts of larger particles such as the protons and neutrons that make up ordinary atoms in the everyday world around us. And, until these new discoveries, quarks had only been seen together in groups of two or groups of three.

“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 the University of Hawaii in 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.”

Beijing Spectrometer Detector. Image courtesy: the BESIII Collaboration

The Beijing Spectrometer Detector – the instrument that studies the decay products of the anomalous Y(4260). Image courtesy: the BESIII Collaboration

The scientists from 11 countries collaborating on the Beijing Spectrometer (BESIII) experiment at the Beijing Electron Positron Collider in China, set out to study a new particle known as the anomalous Y(4260) particle – so called because it has a mass equivalent to an energy of 4260 MeV (mega electron volts) . As a striking and unexpected first observation from these new studies, the collaboration reported in April 2013 that the Y(4260) particle in fact decays to another new, and perhaps even more mysterious, particle that they named the Zc(3900) which is thought to consist of four quarks.

Quarks come in a number of different types, or flavours, one of which is known as “charm” and it has an anti-matter equivalent known as “anti-charm”. Harris explained that: “The new observations challenge what was thought to be a well-understood system of possible configurations of charmed- and anti-charmed quarks, those that were considered to be among the simplest and most easily understood subatomic particles.”

Recent discoveries of several new particles, including the Y(4260) and now the Zc(3900), suggest that more complex structures have to be considered such as so called tetraquark states – or four-quark particles.

New particles are created by smashing matter and anti-matter together

The recent breakthroughs by the BESIII collaboration have come about through a dedicated study of the byproducts from the decomposition of  the anomalous Y(4260) particle. Using the Beijing Electron Positron Collider (BEPCII) in China, scientists tuned the energy at which electrons and their anti-matter counterparts, positrons, crash together and annihilate themselves within the machine. They tuned the experiment 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 numbers of Y(4260) particles so that they could analyse many examples of these particles decomposing into other particles in the hope of seeing examples of the new four quark objects.

This experimental method allowed the BESIII collaboration 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 that 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 finalised, 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 Hawaii 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 charmonium like 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

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

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This post is based on materials supplied by the University of Hawaii in Manoa: here and here.

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