This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. XYZ states don't fit cleanly into the prevailing model of particles made of quarks, the elementary building blocks of nature, and, for the first time, researchers at Jefferson Lab 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." 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 an 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.
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