A search for one exotic particle uncovered two strange new structures

Physicists have spent decades organizing the growing collection of subatomic particles, yet some discoveries continue to resist easy classification. Researchers at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility have now identified evidence for two unexpected structures that may help clarify part of this increasingly complicated particle landscape. The signals could shed light on a puzzling group of objects known as XYZ states. These states do not fit neatly into the conventional picture of particles built from quarks, the fundamental building blocks of matter. For the first time, Jefferson Lab researchers detected two such signals created when a beam of high-energy photons interacted with a proton target. The findings come from the Gluonic Excitations (GlueX) Collaboration in Experimental Hall D at Jefferson Lab and were recently published in Physical Review Letters. The results could help scientists better understand how one of nature's fundamental forces contributes to the formation of matter. "We went searching for a confirmed XYZ candidate with a photon beam but instead found two other structures," said Malte Albrecht, a staff scientist at Jefferson Lab. "It's new information." How the Particle Zoo Took Shape Beginning in the 1950s, high-energy collision experiments started revealing large numbers of subatomic particles collectively known as hadrons. Hadrons are composite particles containing two or more quarks held together by the strong nuclear force. Familiar examples include protons and neutrons, which each contain three quarks (though they had already been identified decades earlier). Among the newly discovered hadrons were short-lived particles called mesons. These typically consist of a quark paired with its antimatter counterpart, the antiquark. In 1964, physicists introduced the quark model to organize these bound states. The earliest version contained three quark "flavors": up, down and strange. Up and down quarks, for example, are the ingredients that form protons and neutrons. Up, down and strange are also the three lightest types of quarks. Particle physics changed dramatically again in 1974 with the discovery of the heavier charm quark. The quark model was eventually expanded to include six flavors. The charm quark discovery helped build the framework that became the Standard Model, the broad theory describing elementary particles and fundamental forces, while also expanding the known spectrum of possible hadronic structures. As particle accelerators became more powerful and detectors grew increasingly sensitive, researchers gained access to subtler physical processes. After the start of the 21st century, experiments began revealing many hadrons with unusual quantum properties that did not fit comfortably within the original quark model. The discoveries accumulated so quickly that physicists adopted the general label XYZ states for many of these poorly understood particles. "We are in a new era here, similar to 70-odd years ago," said Frank Nerling, a Jefferson Lab collaborator from Germany's GSI Helmholtz Centre for Heavy Ion Research and Goethe University Frankfurt. "First, a zoo of hadrons was discovered. Now, we're facing a zoo of so-called exotic states." Searching the Strange Quark Sector Hadrons containing a charm quark and its antimatter partner, an anti-charm quark, occupy a region of the hadron spectrum with similar masses known as charmonium. In the same way, particles containing strange and anti-strange quarks populate the strangeonium region. Many XYZ states have been detected in these two sectors. In 2006, researchers working on the BaBar experiment at the DOE's SLAC National Accelerator Laboratory reported a possible strangeonium state with a mass of approximately 2.16 billion electron volts (2.16 GeV). Because it was considered an XYZ candidate, the particle was designated Y(2175). BaBar created Y(2175) by colliding negatively charged electrons (e-) with their positively charged antimatter counterparts, positrons (e+), through a process known as e+e- annihilation. Y(2175) displayed quantum behavior that may be difficult to explain as a conventional quark-antiquark pair. One possibility is that it represents a hybrid state involving two strange quarks and excited gluons, the particles that carry the strong force. Scientists have also proposed that it could be a four-quark configuration called a tetraquark or a molecule-like combination of other composite particles. Later electron-positron (e-e+) collider experiments, including the Beijing Spectrometer (BES) in China and Belle in Japan, confirmed the existence of Y(2175). Until now, however, Y(2175) had not been observed through a process other than e-e+ 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, another GSI physicist conducting research 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." The GlueX Collaboration set out to search for Y(2175) using photoproduction. In this process, a photon beam strikes protons held inside a fixed target. Y(2175) did not appear through this production mechanism. Instead, researchers detected something unexpected at nearby masses. GlueX Finds an Unexpected Pair The GlueX Experiment at Jefferson Lab 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 that describes the strong nuclear force, predicts that 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 the spokesperson for GlueX. "That's one of the investigations, to try to understand whether there is a gluonic contribution to the structure we see." GlueX uses the Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility that supports research by more than 1,700 physicists around the world. An ultrathin diamond wafer converts CEBAF's electrons into a beam of high-energy photons with parallel spins. Millions of these photons strike protons inside a liquid hydrogen target every second. A large-acceptance spectrometer then records the spray of particles produced in the interactions. "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." The experiment generates enormous quantities of information, enough to fill the hard drive of an average laptop within minutes. Researchers searched through those data for evidence of Y(2175), which had never previously been confirmed through photoproduction. Instead, they identified two structures with nearby masses, suggesting that the objects could have similarly unusual origins. One appeared at roughly 2.24 GeV and was designated Y(2240). The second, called X(1830), appeared at approximately 1.82 GeV. "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." How Strong Are the New Signals? GlueX detected Y(2240) with a very high level of statistical certainty, corresponding to a confidence level of about 99.9994%. Physicists describe this threshold as five sigma (5σ) significance, meaning the probability that the signal is invalid is less than one in a million. The signal for X(1830) was weaker but still notable. It reached 3σ significance, corresponding to a confidence level of approximately 99.7%. With the measurements now established at these levels of significance, theorists can begin developing new predictions about what the structures might represent and what additional experiments could distinguish among the possibilities. "The next step is to figure out which exotic quark configurations nature might have realized here," Nerling said. "Theorists may come to further conclusions and identify measurements that could help pin down the real nature of these particular states." A New Phase for Exotic Particle Searches The study also establishes an upper limit on how likely Y(2175) is to be produced through photoproduction. That constraint can help physicists design and interpret future experiments. For GlueX, the two unexpected signals may mark the beginning of a much broader exploration of exotic hadrons using high-energy photon beams. "It really opens the door for a whole new set of hadron spectroscopy measurements we can make with GlueX," Stevens said. "We've got much more data to sort through, so this is just the beginning of the story."