Despite science's best efforts to classify the vast menagerie of subatomic particles discovered over the past few decades, some exotic varieties defy explanation. Now, nuclear physicists at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility have found evidence of two unexpected structures that could help better sort the zoo of exotic particles.
These structures may provide new insights into a perplexing family of objects known as XYZ states. The XYZ don't fit cleanly into the prevailing model of particles made of quarks, the elementary building blocks of nature, and for the first time Jefferson Lab researchers observed two such signals produced by a beam of high-energy photons interacting with a proton target.
The results, reported by the Gluonic Excitations (GlueX) Collaboration in Experimental Hall D at Jefferson Lab, were recently published in the journal Physical Review Letters and could go a long way in unraveling how one of the universe's fundamental forces plays a role in 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."
The Particle Zoo
In the 1950s, physicists began discovering a slew of subatomic particles, known collectively as hadrons, in high-energy collisions. Hadrons are composite systems made up of two or more quarks bound together by the strong nuclear force. Protons and neutrons, each with three quarks, are well-known examples of hadrons (though they had already been identified decades earlier).
This new batch of hadrons included a subset of short-lived particles called mesons, which typically contain a quark bound to its antimatter counterpart, the antiquark. To classify these bound states, physicists developed a theoretical framework called the quark model in 1964. This original model included three "flavors" of quarks: up, down and strange. Up and down quarks, for instance, come together to make up protons and neutrons. The up, down and strange quarks are the lightest quarks.
A revolution in particle physics followed with the 1974 discovery of the charm quark, an even heavier quark, which eventually led to expansion of the quark model to include six quark flavors. This discovery helped establish a framework for the Standard Model, the overarching theory of fundamental forces and elementary particles, and led to the formation of a spectrum of hadronic structures.
In the next few decades, particle accelerators became increasingly powerful and ever more sensitive to subtle physics processes. With those advances, after the turn of the century, researchers began picking up signals of many new hadrons with exotic quantum properties outside the original quark model.
Physicists were discovering these particles faster than they could define names for them, so they came to be known as XYZ states for lack of a better label.
"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."
Strange Neighbors
On the hadronic spectrum, particles that include a charm quark and their antimatter partner, an anti-charm quark, form a region of similar masses called charmonium. Likewise, hadrons that include strange and anti-strange quarks occupy the strangeonium region. Many XYZ states reside in those sectors.
In 2006, scientists with the BaBar experiment at the DOE's SLAC National Accelerator Laboratory reported a possible strangeonium state at a mass of about 2.16 billion electron volts (2.16 GeV). As an XYZ candidate, it was denoted as Y(2175). The BaBar experiment produced Y(2175) by colliding negatively charged electrons (e-) and their positively charged antimatter partners, positrons (e+), in a process known as e+e- annihilation.
Y(2175) displayed quantum properties that may not be consistent with a quark-antiquark pair. A possible interpretation of its structure is a hybrid state of two strange quarks dominated by excited gluons, the carriers of the strong force. Other potential explanations include a four-quark state, known as a tetraquark, or a molecule-like combination of other composite particles.
Other electron-positron (e-e+) collider experiments, including the Beijing Spectrometer (BES) in China and Belle in Japan, later confirmed the existence of Y(2175). However, Y(2175) had yet to be seen outside of 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 went looking for Y(2175) using photoproduction, where a photon beam interacts with protons inside a fixed target. Instead of finding Y(2175) via this mechanism, the experiment revealed something else nearby.
A First for GlueX
The GlueX Experiment at Jefferson Lab is designed to search for hybrid mesons, where excited gluons directly contribute to the particles' structure. Quantum chromodynamics (QCD), the theory describing the strong force, predicts the existence of these exotic states.
"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."
The GlueX program relies on the Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility that enables the research of more than 1,700 physicists worldwide. Using an ultrathin wafer of diamond, GlueX converts CEBAF's electrons into a beam of high-energy photons with parallel spins. Every second, millions of these photons impinge on protons inside a liquid hydrogen target, with the resulting spray of particles picked up by a large-acceptance spectrometer.
"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 apparatus produces vast amounts of data, enough to fill up an average laptop's hard drive every few minutes. The GlueX team went sifting through that data for signs of Y(2175), which had never been confirmed via the photoproduction mechanism.
During their search, researchers found a pair of new structures with nearby masses, meaning they could be similarly strange. One structure checked in at a mass of about 2.24 GeV and was named Y(2240). The other structure, labeled X(1830), corresponds to a mass of about 1.82 GeV.
"One of the interesting things about this result is that we 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."
What's Next
GlueX observed Y(2240) with a high degree of certainty, at a confidence level of about 99.9994%. In statistics, this is expressed as five sigma (5σ) significance and means the chances of an invalid result are less than one in a million. The X(1830) measurement had 3σ significance, translating to a confidence of about 99.7%.
With the results validated at such a high significance, it will now be up to theoretical physicists to come up with new predictions and proposals for future measurements.
"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."
The results also place an upper limit on the probability of Y(2175) being produced in photoproduction, which will help guide future experiments and set the stage for a new era for GlueX.
"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."