Physicists have finally figured out how pentaquarks are built

To make a quark quintet, combine a trio and a duo.

Exotic subatomic particles called pentaquarks contain five smaller particles called quarks and antiquarks. But those particles aren’t a simple clump of five constituents rattling around. Instead, the pentaquarks are molecule-like agglomerations of a pair of smaller particles, each of which consists of either three quarks or a quark and an antiquark, scientists report in the June 7 Physical Review Letters.

First spotted in 2015 at the Large Hadron Collider in Geneva, pentaquarks were unlike anything seen before (SN: 8/8/15, p. 8). All previous known quark-containing particles were either baryons — particles such as protons and neutrons which contain three quarks — or mesons, which consist of one quark and one antiquark. But pentaquarks, with their five component particles, didn’t fit into either of those categories.

“There was no clear picture how these pentaquarks were built,” says particle physicist Tomasz Skwarnicki of Syracuse University in New York, a coauthor of the study.

Some scientists thought that the pentaquarks’ five constituents could mingle on an equal footing. But detailed measurements from the LHCb experiment reveal that the pentaquarks are made from two known particles, a baryon and a meson, stuck together. That finding makes the particles a bit less exotic than had been speculated.

Still, Skwarnicki says, the result reveals new facets of particle physics: Before this result, it wasn’t clear that baryons and mesons could glom onto one another at all.