Pentaquarks Make Their Debut

An exotic quark cluster reveals a new way particles can bind together to form matter

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A veritable zoo of never-before-seen particles, including the famed Higgs boson, was generated in recent years inside the Large Hadron Collider (LHC) at CERN near Geneva. Hiding amid the data, another new particle has recently made itself known: the pentaquark, a composite of five quarks, the fundamental bits that make up protons and neutrons. The long-awaited discovery—pentaquarks were first predicted more than 50 years ago—provides insight into how matter's building blocks stick together to organize the universe as we know it.

Before the pentaquark observation, the hundreds of quark-constructed particles splintering out of subatomic collisions were known to exist only as trios of quarks called baryons (including protons and neutrons) or quark-antiquark pairs known as mesons. The limited number of arrangements perplexed physicists because the mathematical model that described quark behavior did not forbid them from combining in other ways. Some research groups reported other permutations—including Z_c(3900), a particle composed of two quarks and two antiquarks—but such cases remain contentious, says Eric Swanson, a physicist at the University of Pittsburgh. And claims of spotting pentaquarks about a decade ago amounted to false positive results.

The new signatures, however, seem to be the real deal. “I've been in this game for 30 years now, and I've seen data come and go,” says Swanson, who was not involved in the recent finding. “In this case, the data are clear, and I don't see a compelling alternative explanation.” The LHC researchers describe their work in a paper published in August in Letters.

The data fail to reveal whether all five quarks are bound tightly together or whether a baryon is loosely bound to a meson, like some kind of subatomic molecule (). Future experiments at the LHC, which rebooted at higher energy levels in April, could tease apart the relation or probe the existence of other pentaquark configurations, says Sheldon Stone, a physicist on the collider's quark project. The mere confirmation of pentaquarks, however, shows that the garden variety of particles previously seen in ordinary matter and even high-powered collisions hardly paints a complete picture of the universe's matter. The finding has reignited curiosity about additional exotic particles yet to be uncovered.

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