Exotic particle packs a foursome of quarks

The rediscovery of an exotic particle provides the best evidence yet that quartets of quarks exist in a universe dominated by two- and three-quark matter.

By validating the particle’s existence, says lead author Tomasz Skwarnicki, a physicist at Syracuse University in New York, “we are automatically proving that four-quark states exist.”

Quarks, one of the fundamental constituents of matter, never exist on their own. Held together by particles called gluons, quarks and their antimatter counterparts, antiquarks, cluster in threes to form baryons (including protons and neutrons) and in pairs to produce mesons (including pions and kaons).

But in 2003, physicists at the Belle particle collider in Japan discovered a bizarre particle called X(3872) that didn’t seem to fit in either category. Based on its mass (3,872 million electron volts) and the particles it decayed into, X(3872) appeared to consist of a charm quark, an anticharm and at least two other quarks, though its composition has been very difficult to confirm.

Since then, scientists trying to explain the mysterious nature of the particle have inadvertently discovered other odd particles. One controversial example is Z(4430). Its discovery at Belle in 2008 made a splash because unlike X(3872), it has an electric charge. Because the charges of the charm and anticharm quarks cancel each other out, and because researchers had ruled out that Z(4430) was a three-quark particle, many physicists concluded Z(4430) must contain four quarks. However, a competing team disputed the particle’s existence in 2009.

Now a third experiment has chimed in with confirmation of Z(4430)’s existence. Analyzing the subatomic shrapnel that led to the discovery of the Higgs boson (SN: 7/28/12, p. 5), physicists at the Large Hadron Collider beauty experiment sifted through more than 25,000 meson decays and found overwhelming evidence for a negatively charged particle with a mass of 4,430 million electron volts. The results were posted April 7 at arXiv.org. “LHCb definitively confirmed Belle’s claim that this particle exists,” says Indiana University Bloomington physicist Matthew Shepherd.

The unmistakable detection gives LHCb researchers confidence that Z(4430) is a single particle made up of four quarks – most likely a charm, anticharm, down and anti-up. A few other four-quark candidates have emerged within the last year, but those particles came with more question marks. Some theorists say that Z_c(3900), announced last June (SN: 7/27/13, p. 9), is not a single particle. Instead, they think it is a pair of interacting particles. The discovery of Z_c(4020), described in December in Physical Review Letters, awaits confirmation by another experiment. And LHCb has found preliminary evidence for another member of the four-quark club called Z(4240).

The next step, Skwarnicki says, is to figure out the internal composition of Z(4430) and its (possible) four-quark relatives. Skwarnicki is confident that each is a single particle, but physicists want to determine whether the individual particles are unions of two mesons or true tetraquarks – four quarks bound together by gluons. “There’s so much activity going on right now,” Shepherd says. “The prospects for resolving this soon are good.”

Skwarnicki adds that even though these particles don’t exist freely in nature, they may have played a role in the very early universe, when a hot, dense soup of quarks and gluons cooled and condensed into nature’s first multi-quark matter.