Extremely short-lived particles, such as mesons and baryons, are known to consist of quarks based on experimental observations and the theoretical framework of the Standard Model of particle physics. Here are a few reasons why it is likely that these particles are composed of quarks:
Evidence from Particle Collisions: When high-energy particle collisions occur, such as those in particle accelerators, the resulting debris can be detected and studied. These experiments have revealed the existence of a wide range of particles that decay rapidly. By analyzing their decay patterns, researchers have been able to identify the presence of quarks within these particles.
Fractional Electric Charges: Quarks carry fractional electric charges. For example, up quarks have a charge of +2/3 and down quarks have a charge of -1/3. By combining different quarks with fractional charges, composite particles can be formed that exhibit whole-number electric charges. This observation is consistent with the quark model, where mesons are composed of a quark and an antiquark, while baryons consist of three quarks.
Deep Inelastic Scattering: Deep inelastic scattering experiments involve firing high-energy particles at a target and measuring the scattered particles. These experiments have provided evidence for the existence of point-like constituents within protons and neutrons, indicating that these composite particles are composed of smaller particles. Quarks are the fundamental constituents that best explain the observed scattering patterns.
Quark Confinement: Quarks are subject to a phenomenon known as quark confinement, which states that individual quarks are never observed in isolation. They are always found in bound states within larger particles due to the strong nuclear force. This confinement explains why quarks are not directly observed as free particles but can only be detected indirectly through the study of their composite particles.
These various lines of experimental evidence, combined with the successful predictions and explanations provided by the quark model within the framework of the Standard Model, strongly support the idea that extremely short-lived particles consist of quarks.