Why Search for Y(2175)?

In 2006, the BaBar experiment at DOE's SLAC National Accelerator Laboratory reported a possible "strangeonium" state — built from strange and anti-strange quarks — with a mass of approximately 2.16 billion electron volts (2.16 GeV). Designated Y(2175), it was produced by colliding electrons with positrons, their antimatter counterparts, in a process called e+e− annihilation.

Its quantum behavior may be hard to explain as an ordinary quark-antiquark pair. Scientists have proposed it could be a hybrid containing excited gluons — the particles that carry the strong force — a four-quark tetraquark, or a molecule-like combination of other composite particles.

Later experiments, including the Beijing Spectrometer in China and Belle in Japan, confirmed Y(2175) exists. But until now, no one had observed it through any process other than electron-positron annihilation.

"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."

How Strong Are the New Signals?

Y(2175) did not appear in GlueX's photoproduction data. Instead, the collaboration found two structures at nearby masses: Y(2240) at roughly 2.24 GeV and X(1830) at approximately 1.82 GeV.

The Y(2240) signal reached five sigma (5σ) significance — about 99.9994% confidence, meaning the probability the signal is invalid is less than one in a million. The X(1830) signal was weaker but still notable, at 3σ, corresponding to roughly 99.7% confidence.

"One of the interesting things about this result is that we didn't observe Y(2175) at the place we were searching," Albrecht said. "We found something new using a completely different physics process, and that's really intriguing. But now that these have been observed, that doesn't mean we're done."

The study also sets an upper limit on how likely Y(2175) is to be produced through photoproduction, a constraint that can help physicists design and interpret future experiments.

What Comes Next?

GlueX was built specifically to investigate hybrid mesons — exotic particles in which excited gluons may directly contribute to the internal structure. Quantum chromodynamics (QCD), the theory of the strong nuclear force, predicts such states should exist.

"Excited gluonic fields are what could be in these mesons where you have more than just the quark-antiquark pair," said Justin Stevens, a William & Mary physics professor and GlueX spokesperson. "That's one of the investigations, to try to understand whether there is a gluonic contribution to the structure we see."

The experiment relies on the Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility serving more than 1,700 physicists worldwide. An ultrathin diamond wafer converts CEBAF's electrons into high-energy photons with parallel spins; millions strike protons in a liquid hydrogen target every second, and a large-acceptance spectrometer records the resulting particle spray.

"No other experiment has a facility with a photon beam of this intensity at the energy we have available," Albrecht said. "This truly is a unique setup."

With the two signals now established, theorists can develop predictions about what the structures might be and which measurements could distinguish among the possibilities.

"The next step is to figure out which exotic quark configurations nature might have realized here," Nerling said.

"It really opens the door for a whole new set of hadron spectroscopy measurements we can make with GlueX," Stevens added. "We've got much more data to sort through, so this is just the beginning of the story."