Hunt for an Exotic Particle at Jefferson Lab Turns Up Two Unexpected Structures

Physicists at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) set out to detect a known but puzzling particle called Y(2175) using a beam of high-energy photons fired at a proton target. Instead, the GlueX Collaboration, working in Experimental Hall D, found two unexpected structures at nearby masses that had never been seen before. The results were published in Physical Review Letters.

Y(2175) belongs to a group of particles called XYZ states — hadrons, or particles built from quarks, that do not fit the standard picture in which particles are made of either three quarks (like the proton) or a quark paired with its antimatter opposite. Y(2175) was first reported in 2006 by the BaBar experiment at the DOE's SLAC National Accelerator Laboratory, made by colliding electrons with positrons, and was later confirmed by China's Beijing Spectrometer and Japan's Belle experiment. Until this study, it had never been produced through any process other than that electron-positron collision.

The GlueX experiment was built to study hybrid mesons, particles in which the "glue" that binds quarks together — carried by particles called gluons — is thought to directly shape the particle's structure, as predicted by quantum chromodynamics, the theory of the strong nuclear force. "We went searching for a confirmed XYZ candidate with a photon beam but instead found two other structures," said Malte Albrecht, a Jefferson Lab staff scientist. "It's new information."

Frank Nerling, a Jefferson Lab collaborator from Germany's GSI Helmholtz Centre for Heavy Ion Research and Goethe University Frankfurt, compared the moment to the mid-20th century, when high-energy collisions first revealed a large "zoo" of subatomic particles. "We are in a new era here, similar to 70-odd years ago," he said. "First, a zoo of hadrons was discovered. Now, we're facing a zoo of so-called exotic states."