12.5 Fundamental Particles

12.5 Fundamental Particles

By the term fundamental particle, we mean a particle that has no internal structure, which means that it is indivisible. Presently, the fundamental constituents of matter are considered to be quarks (protons, neutrons and mesons are made up of quarks) and leptons (including electrons, positrons, and neutrinos). They are considered the basic building blocks of matter.

When a nucleus is smashed in an ultra high energy particle accelerator, or two high energy particles are collided, entirely new types of particles are created which apparently do not exist within the atoms of the ordinary matter. They are the outcome of the violent collisions needed to probe the basic structure of matter. More than a hundred new particles have been identified to be classified into families with similar properties. Many of these were accounted well with the scheme of theoretical physicist while the rest were named "strange particle". They are always created in pairs, e.g., when a pion ($\pi^{+}$) collides with a proton, two strange particles $K^{0}$ (Kaon), and $\Lambda^{0}$ (Lambda) are created. The nuclear reaction is:

All particles spin on their axes and the spin of charged particles makes them tiny magnets. The characteristic spin of electrons, protons, neutrons is $1/2$ and the spin of photon is $1$ and of pions is taken as zero. Half spin particles obey the Pauli's exclusion principle which says that only one particle of a kind occupies a given quantum state. These particles are called "fermions". The particles with zero or whole number spin do not obey this principle. They are called "bosons" as they obey Bose-Einstein statistics. Further major classifications are:

1. The nucleons and the heavier particles such as $\Lambda^{0}$ and $K^{0}$ which decay to nucleons are called "baryons" (heavy).

2. The particles that do not interact strongly with nucleons, and are called leptons (small) along with the electrons, tau and neutrino.

Both of the $\beta$-decay (beta-minus and beta-plus) processes provide evidence that the protons and neutrons are not the fundamental particles. By the 1860s many new types of particles similar to the neutron and proton were discovered, as well as many "missized" particles called mesons whose masses were mostly less than nucleon masses but more than the electron mass (other mesons, found later, have masses greater than nucleons). The strongly attractive particles are called $\pi$ meson or pions while the weakly interacting particles were named $\mu$ mesons or muons.

This discovery led to the conclusion that these particles could not be fundamental particles, and must be made up of even smaller constituents, which were given the name quarks.