Elementary Particles

Subatomic particles that are thought to be indivisible are referred to as ‘Elementary Particles’. Atoms that comprise ordinary matter are not the smallest indivisible units of mass. Each atom is composed of discrete parts called electrons, protons and neutrons. These smaller, further elementary particles can be arranged to explain all known elements. It was eventually discovered that protons and neutrons were not truly elementary either, but were made up of still smaller particles called quarks. We now know that there are plenty of elementary particles, so many that particle physicists call it the ‘particle zoo’. The study of elementary particle physics endeavors to classify and understand all of the known particles, including massless particles like photons. There must be underlying theory must be able to explain the existence and interactions of all of the known elementary particles. As of now, Physicists are working on various theories such as the String Theory and the M-Theory to eventually come up with the ultimate theory to explain all matter.

The atomic nucleus model of Ernest Rutherford (1871–1937) and James Chadwick’s (1891–1974) discovery of the neutron suggested a universe consisting of three elementary particles: the proton, the neutron, and the electron. That simple model is still very useful in describing nuclear phenomena, because many of the other elementary particles are very short-lived and appear only briefly during nuclear reactions. To explain the strong forces that exist inside the nucleus between the Nucleons physicists developed Quantum Chromodynamics and introduced the Quark as the basic building block of Hadrons, the class of heavy subatomic particles (including neutrons and protons) that experience this strong interaction or short-range nuclear force. Physicists call the other contemporary family of elementary particles with finite masses Leptons, light particles (including electrons) that participate in electromagnetic and weak interactions but not the strong nuclear force.

The Quantum Theory was formulated to try to explain all of the known elementary particles and their interactions and fields. As work on the quantum theory developed, more detailed treatments of the various particles and the discovery of new particles necessitated a revision of the standard quantum interpretation. The standard model connected the electromagnetic force with the strong force and explained many of the particles and their various interactions. The Standard Model is the reigning model of Elementary Particle physics; it successfully combines the weak and strong forces into a consistent quantized theory. The standard model utilizes quarks to build protons, neutrons and other subatomic particles. Later theories like super-symmetry attempted to combine the standard model with the weak force to form a Grand Unified Theory (GUT). A more sophisticated theory that would link the ultimate force, gravity, to the three GUT forces is sometimes called the Theory of Everything (TOE).

Questions to Ponder

• How do Physicists distinguish elementary particles if most of them have no charge?
• Do elementary particles have a structure? So, what are they composed of?