Mesons are subatomic particles made up of a quark and an antiquark pair. These pairs are bound together by the strong nuclear force, mediated by gluons, the exchange particles for this force. Unlike baryons, which contain three quarks, mesons are unique in their quark-antiquark composition, making them bosons rather than fermions. This difference is crucial as it determines their statistical behaviour and how they interact with other particles. Mesons play an essential role in mediating the strong force between baryons in atomic nuclei. The characteristics of a meson depend on the types of quark and antiquark it contains. For instance, a meson composed of an up quark and a down antiquark will have different properties than one made of a strange quark and an anti-up quark. These properties include mass, charge, spin, and lifetime. Mesons are inherently unstable and decay through various processes, often involving the weak nuclear force. Their study is vital in understanding the strong interaction and the behaviour of quarks under high-energy conditions.

The baryon number is a quantum number that represents the difference between the number of quarks and antiquarks in a particle. Each quark is assigned a baryon number of +1/3, while each antiquark has a baryon number of -1/3. The significance of the baryon number lies in its conservation in all known particle interactions. This conservation law states that the total baryon number before and after any reaction must be the same. This principle is crucial in predicting and explaining the outcomes of various particle interactions. For example, in a reaction where a proton (baryon number +1) decays into a neutron (baryon number +1), a positron, and a neutrino, the total baryon number remains conserved. The conservation of baryon number is one of the fundamental symmetries in physics and is key to understanding processes such as nucleosynthesis in stars and the stability of matter in the universe. It also implies that baryons cannot decay into particles with zero baryon number without producing other baryons or antibaryons, which helps maintain the stability of protons and neutrons in atomic nuclei.

Strangeness is a quantum number in particle physics used to describe the presence of strange quarks in particles. A strange quark has a strangeness of -1, while an anti-strange quark has a strangeness of +1. The concept of strangeness is relevant in particle interactions, particularly in those involving the strong and weak nuclear forces. In strong interactions, such as those within atomic nuclei, strangeness is conserved. This means that the total strangeness before and after the interaction remains the same. However, in weak interactions, which are responsible for certain types of particle decay, strangeness is not conserved. The relevance of strangeness is particularly evident in the study of kaons and hyperons, particles that contain strange quarks. These particles often exhibit longer lifetimes than expected because their decay, involving a change in strangeness, can only occur via the weak interaction, which is less efficient than the strong interaction. Understanding strangeness and its conservation or non-conservation in various interactions provides insights into the behaviour of particles under different forces and helps in exploring fundamental aspects of particle physics and the universe.

Hadrons are a class of particles that participate in strong interactions, and they are composed of quarks bound together by the strong nuclear force. The role of quarks in classifying particles as hadrons is central, as hadrons are categorized based on their quark content. There are two main types of hadrons: baryons and mesons. Baryons, including protons and neutrons, are composed of three quarks. For example, a proton is made of two up quarks and one down quark. Mesons, on the other hand, consist of a quark and an antiquark pair. An example is the pion, which can be composed of an up quark and a down antiquark. The specific combination of quarks in a hadron determines its properties, such as mass, charge, and spin. The classification of particles into hadrons based on their quark composition is a fundamental aspect of the Standard Model of particle physics. It helps in understanding the nature of forces that bind quarks together and the interactions that occur at the subatomic level.

Conservation laws involving quarks are pivotal in determining the outcomes of particle interactions and reactions in particle physics. These laws, such as the conservation of charge, baryon number, lepton number, and strangeness, provide constraints on how particles can interact and decay. For example, the conservation of charge dictates that the total electric charge before and after a reaction must be the same. Similarly, the conservation of baryon number means that the sum of baryon numbers must remain constant in a reaction. This law is crucial in understanding why protons, with a baryon number of +1, are stable and do not decay into other particles with zero baryon number. The conservation of strangeness in strong interactions (but not in weak interactions) explains why particles containing strange quarks have longer lifetimes than expected. These conservation laws are fundamental in predicting possible particle interactions and decays, and they are essential tools for physicists in exploring and understanding the subatomic world. The adherence to these laws in particle interactions reflects the underlying symmetries and principles governing the physical universe.