In a groundbreaking development, physicists have uncovered intriguing evidence of an exotic atom-like system, a neutral meson bound to an atomic nucleus via the strong force. This discovery, made by two international collaborations, could revolutionize our understanding of hadron masses and the fundamental symmetries of quantum chromodynamics in nuclear matter. The strong force, one of the four fundamental forces, governs the binding of quarks into hadrons and the cohesion of protons and neutrons within atomic nuclei. Neutral mesons, short-lived particles composed of a quark and an antiquark, are also subject to the strong force, which can bind them to atomic nuclei in a manner analogous to the electromagnetic force binding electrons to nuclei. The study of these meson-based nuclear systems is crucial for comprehending the properties of the strong force, and the eta prime meson, η′, stands out as particularly intriguing. Its large mass cannot be explained by a simple quark model, a phenomenon known as the U(1) problem, which was first raised in the 1970s by physicist Steven Weinberg. Modern theories attribute this mass to chiral symmetry breaking in quantum chromodynamics, the fundamental theory of the strong force. These theories predict that the η′ meson's mass should decrease in a nuclear system, and this is precisely what the researchers set out to test. The experiment involved a beam of protons striking a ¹²C atomic nucleus at near-relativistic speeds, removing a neutron to form a deuteron that propagates away, leaving behind a highly energetic ¹¹C nucleus. In rare instances, the η′ meson binds to the ¹¹C nucleus, forming an η′-mesic nuclear system. However, the challenge of identifying these rare events was significant due to the large number of background events, which were 100 to 1000 times higher than the signal events. To address this, the researchers developed a new experiment that efficiently selects signal events by tagging the particles they decay into, allowing them to measure both the forward-traveling deuteron and the decay products of the short-lived η′-mesic nuclear state. The results indicate a 60 MeV reduction in the η′ meson mass in nuclear matter, supporting theoretical scenarios that attribute the origin of the η′ meson mass to chiral symmetry breaking and gluon dynamics. The team, comprising researchers from various collaborations, is now planning follow-up experiments to confirm the observation and increase the significance to the 5σ level, a crucial threshold for establishing new quantum states in particle and nuclear physics. This discovery opens up exciting possibilities for further exploration, shedding light on the strong nuclear force and its intricate role in the subatomic realm.
