Physicists Hunted a Known Particle at Jefferson Lab, Found Two Unexpected Structures Instead

Researchers at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility set out to confirm a known but puzzling particle and instead found evidence for two unexpected new structures, results that could help clarify a confusing class of particles called XYZ states that don't fit the conventional quark-based picture of matter. The findings, from the Gluonic Excitations (GlueX) Collaboration in Jefferson Lab's Experimental Hall D, are reported in Physical Review Letters.

"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." Since the 1950s, high-energy collisions have revealed hadrons, composite particles built from quarks bound by the strong nuclear force, organized since 1964 by the quark model that underpins the Standard Model. Since the 2000s, though, physicists have found many hadrons with properties that don't fit that model, calling them "XYZ states." "We are in a new era here, similar to 70-odd years ago," said Frank Nerling of Germany's GSI Helmholtz Centre. "First, a zoo of hadrons was discovered. Now, we're facing a zoo of so-called exotic states."

The GlueX team was hunting for a state called Y(2175), first reported in 2006 by the BaBar experiment at SLAC at a mass of about 2.16 billion electron volts and later confirmed by China's BES and Japan's Belle experiments, all using electron-positron collisions. Its properties don't fit a conventional quark-antiquark pair; it may be a hybrid of quarks and gluons, a four-quark "tetraquark," or a molecule-like arrangement. Until GlueX's search, it had never been observed through any other process.

GlueX instead used photoproduction, firing high-energy photons at a stationary proton target. The expected Y(2175) signal did not appear; instead, the researchers found two different structures in the same general mass region. "The challenge is that you have many measurements around the world in very different experiments that have to find consensus about what they are seeing," said Klaus Goetzen, a GSI physicist working at Jefferson Lab. "It's more complicated than it sounds, because there are states that are close by in mass and might or might not be the same thing."