GlueX Experiment Finds Two New XYZ Particle Candidates
The GlueX Collaboration at Jefferson Lab has discovered evidence for two new structures, Y(2240) and X(1830), while searching for the exotic particle
Physicists at the Thomas Jefferson National Accelerator Facility have identified evidence for two unexpected structures that may belong to the exotic family of subatomic particles known as XYZ states. The discovery was made by the Gluonic Excitations (GlueX) Collaboration while searching for a different particle, Y(2175), using a high-energy photon beam.
Researchers detected the signals when photons from the Continuous Electron Beam Accelerator Facility (CEBAF) struck protons in a liquid hydrogen target. The experiment was designed to investigate hybrid mesons, where excited gluons contribute to particle structure. Instead of finding the sought-after Y(2175), the team observed two new structures at nearby masses.
The Search for Strangeonium
Many XYZ states have been found in the charmonium and strangeonium sectors, which involve charm or strange quarks paired with their antimatter counterparts. The Y(2175) particle, first reported in 2006 by the BaBar experiment at SLAC, is a candidate strangeonium state with a mass of approximately 2.16 billion electron volts (2.16 GeV). It had only been observed through electron-positron annihilation until the GlueX team attempted to find it via photoproduction.
Klaus Goetzen, a GSI physicist working at Jefferson Lab, noted that 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. He said that states close in mass might or might not be the same entity.
An Unexpected Discovery
The GlueX experiment uses a unique high-intensity photon beam. Malte Albrecht, a Jefferson Lab staff scientist, said no other experiment has a facility with a photon beam of this intensity at the energy they have available. The setup generates vast amounts of data, filling a typical laptop hard drive within minutes.
While sifting through this data, researchers found no sign of Y(2175). They did, however, find two new signals. One structure appeared at roughly 2.24 GeV and was named Y(2240). The second appeared at approximately 1.82 GeV and was designated X(1830).
Albrecht said they went searching for a confirmed XYZ candidate with a photon beam but instead found two other structures. He called it new information.
Statistical Significance of the Signals
The strength of the two new signals differs significantly. The GlueX collaboration measured their statistical certainty.
| Particle | Mass (GeV) | Significance | Confidence Level |
|---|---|---|---|
| Y(2240) | ~2.24 | 5σ | ~99.9994% |
| X(1830) | ~1.82 | 3σ | ~99.7% |
The Y(2240) signal reached the gold standard in particle physics of five sigma, meaning the probability it is a false result is less than one in a million. The X(1830) signal was weaker but still notable at three sigma.
The Path Forward for Exotic States
The discovery adds to the growing zoo of exotic hadrons that defy the traditional quark model. A researcher said now they are facing a zoo of so-called exotic states.
Theorists must now work to interpret the new findings. The next step is to figure out which exotic quark configurations nature might have realized here, said Nerling. The structures could be hybrid states with excited gluons, tetraquarks, or molecule-like combinations of particles.
Justin Stevens, a William & Mary professor and GlueX spokesperson, explained the goal is to understand if there is a gluonic contribution to the particle structure. With the measurements established, future experiments can be designed to pin down the true nature of Y(2240) and X(1830).
