The existence of stable exotic baryons, such as tetraquarks (composed of four quarks) or pentaquarks (composed of five quarks), is an area of active research in particle physics. While the Standard Model of particle physics does not predict the existence of stable tetraquarks or pentaquarks, experimental observations in recent years have provided evidence for their existence.
Tetraquarks and pentaquarks were initially proposed as theoretical constructs in the 1960s and 1970s, but experimental confirmation remained elusive for several decades. However, in 2003, the Belle experiment in Japan observed a particle known as the X(3872), which was a candidate for a tetraquark state. Since then, additional experimental evidence for tetraquarks and pentaquarks has been reported by various particle physics experiments, including the LHCb experiment at CERN.
It's important to note that while some of these exotic baryons have been observed, their stability can vary. Some tetraquark and pentaquark states have relatively short lifetimes and are not considered stable in the same sense as protons and neutrons. However, stable or long-lived exotic baryons are an active area of investigation, and researchers are studying their properties and attempting to understand their role in the structure of matter.
The existence and stability of exotic baryons are still subjects of ongoing research, and further experimental data and theoretical developments are necessary to fully understand their nature and properties.