What a new jumbo particle reveals about extreme matter

We're used to scientists making ever bigger atoms. Now it seems subatomic particles can be pushed to similar extremes to create new forms of matter. Researchers at two particle detectors reported on Monday the strongest evidence yet for a particle made of more than three quarks, the subatomic building blocks of matter. What does that mean for understanding of matter ? and why does that matter? New Scientist investigates

What is matter made of?
Quarks are the fundamental building blocks and come in six "flavours" ? up, down, strange, charm, bottom and top. Ordinarily, they combine in threes to form particles like neutrons and protons: two ups and one down make a proton, while two downs and one up make a neutron. Neutrons and protons make up atomic nuclei, which combine with electrons to form atoms, which link up to make molecules.

What's the maximum number of quarks that can clump together?
The theory that describes how quarks stick together, quantum chromodynamics is disappointingly mum on this, leaving physicists reliant on observations. While quarks can also pair up to form particles such as kaons and pions, no particle had ever been found to contain more than three quarks.

Eric Swanson of the University of Pittsburgh in Pennsylvania, who was not involved in the new research, describes the hunt to find particles made of ever more quarks as looking for a horse with more than four legs. "There's nothing in nature that says you can't have an eight-legged horse, but we've never seen one," he says. "If someone found an eight-legged horse, you'd say 'Oh, that's interesting!' Then you'd want to fit it into your evolutionary theory."

What had people come up with in the past?
Various experiments have been vying to find the particle equivalent of an eight-legged horse. In 2003, results from the Belle Collaboration at the High Energy Accelerator Research Organization in Tsukuba, Japan, hinted at the existence of a pentaquark, but that was ruled out two years later. After that, focus shifted to the possibility of a tetraquark, with multiple potential candidates for such a structure, most recently in 2010.

What's special about the latest discovery?
On 17 June, Belle and the BESIII Collaboration at the Beijing Electron Positron Collider in China each independently reported evidence for the same particle, unromantically called Z_c(3900), made of four quarks. The fact that two teams saw what looks like the same combination ? as well as the number of instances of the particle seen by each team ? makes this the strongest evidence yet for a structure made of more than three quarks. "This discovery clearly indicates that there is a new particle, which may combine quarks in a way not seen before," says Hisaki Hayashii of the Belle collaboration.

Does any other structure fit the observations?
Rather than being a single particle, Z_c(3900) could also be the subatomic equivalent of a molecule, made of two particles orbiting each other. The new particle could actually be two D-mesons that form a loosely bound state called a hadron molecule.

"That would also be a new form of matter," Swanson says, although it's less exotic than a new kind of proton ? more like discovering a platypus than an eight-legged horse.

In either case, Z_c(3900) should help us to better understand how the strong force binds quarks into larger objects, says Ryan Mitchell of Indiana University.

So when will we know whether it's a platypus or an octo-horse?
So far, the two experiments have seen a combined total of 466 instances of the Z_c(3900) particle. They'll need between 10 and 100 times more sightings to be able to tell enough about it to distinguish between the two options, says Belle team member Leo Piilonen of Virginia Tech in Blacksburg.

Will that lead to new understanding of matter?
It could lead to a better understanding of quantum chromodynamics. That could, in turn, help us understand the exotic states of matter inside neutron stars or in the very early universe, Swanson says. Neutron stars have cores so dense that atomic nuclei dissolve into a neutron-rich soup, but no one knows exactly how matter behaves under these extreme conditions. Finding out could help us understand how stars evolve and how matter gets distributed throughout the universe.

Journal references: Physical Review Letters, DOI: 10.1103/PhysRevLett.110.252001 and 10.1103/PhysRevLett.110.252002

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