Physicists have resurrected a particle that may have existed in the first hot moments after the Big Bang. Arcanely called Zc(3900), it is the first confirmed particle made of four quarks, the building blocks of much of the Universe’s matter.
Until now, observed particles made of quarks have contained only three quarks (such as protons and neutrons) or two quarks (such as the pions and kaons found in cosmic rays). Although no law of physics precludes larger congregations, finding a quartet expands the ways in which quarks can be snapped together to make exotic forms of matter.
“The particle came as a surprise,” says Zhiqing Liu, a particle physicist at the Institute of High Energy Physics in Beijing and a member of the Belle collaboration, one of two teams claiming the discovery in papers published this week in Physical Review Letters1, 2.
Housed at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, the Belle detector monitors collisions between intense beams of electrons and their antimatter counterparts, positrons. These crashes have one-thousandth the energy of those at the world’s most powerful accelerator, the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland, but they are still energetic enough to mimic conditions in the early Universe. Collision rates at KEK are more than twice those at the LHC, and they occasionally give birth to rare particles not found in nature today — ephemeral creatures that wink into existence for an instant and then fall to pieces.
Some of that subatomic shrapnel matches what would be expected from the breakdown of a particle containing four quarks bound together: two especially heavy ‘charm’ quarks and two lighter quarks that give the particle a charge. With 159 of these Zc(3900) particles in hand, the Belle team reports that the chance that its result is a statistical fluke is less than 1 in 3.5 million1. “They have clear evidence of a particle with four quarks,” says Riccardo Faccini, a particle physicist at the Sapienza University of Rome.
The new particle has also been vouched for by a second experiment, the Beijing Spectrometer III (BESIII) at the Beijing Electron Positron Collider. BESIII found 307 Zc(3900) particles, sifted from 10 trillion trillion electron–positron collisions2.
“This gives credence to all of the other particles that Belle has seen,” says Fred Harris, a particle physicist at the University of Hawaii in Manoa and a spokesman for BESIII. In 2008, Belle found another four-quark candidate3, and in 2011, it saw two other particles that may have been made of four ‘bottom’ quarks4 — but no other particle colliders have confirmed those sightings.
No one questions the number of quarks in the latest particle. More controversial is their arrangement, which could have implications for quantum chromodynamics, the theory describing the strong force that connects quarks. Theorists fall primarily into two camps.
One side proposes that the particle is actually a union of two ordinary particles called mesons, which contain one quark and one antiquark. Zc(3900) particles could be made up of two mesons joined by a loose connection to form a molecule-like structure (see ‘Quark soup’).
“They have clear evidence of a particle with four quarks.”
Other theorists have tentatively labelled the new particle a true tetraquark — four quarks stuck together tightly to form a compact ball. Within the ball, two quarks are bound together, as are two antiquarks. Such pairings do not occur in any known particle and would thus introduce new building blocks of matter — with the potential to guide computer simulations aimed at working out all the structures that quarks can form.
Proponents of the tetraquark theory point out that a ‘molecule’ made of mesons should split easily into two halves, and that such a breakdown has not appeared in the data. “The signature of a molecule is not seen, which favours the tetraquark picture,” says Ahmed Ali, a particle physicist at DESY, Germany’s high-energy physics laboratory in Hamburg. But the experiments’ margin of error is still too great to rule out the possibility of molecular mesons breaking down. Another way to test the two theories would be to look for other particles that each predicts should exist.
Hoping to end the debate, researchers at BESIII are continuing to dig through data collected since their first experimental run in December and January. Depending on what they find, the unmasking of Zc(3900) may have to wait for the new, more powerful version of the Belle detector planned to come online in 2015.
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