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 Letters.
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 million. “They have clear evidence of a particle with four quarks,” says Riccardo Faccini, a particle physicist at the Sapienza University of Rome. More here Particle Containing 4 Quarks Is Confirmed for First Time