Fundamental Particles (The Standard Model) Nathan Brown June 2007

Part I Fundamental Particle – The Meaning

It is understood that the universe is composed of particles.

Democritus (born 450 BC, Greece) first introduced the particle nature of creation. The term he used to describe the particle translates in English as “atom.”

Since Democritus, much work was done that led to the understanding that the “atom” was actually composed of three smaller particles (electrons, protons, and neutrons). As late as the 1930s these subatomic particles were thought to be indivisible, elementary (“having no smaller constituent parts”) or structureless.

Since the 1930s, it was determined that the proton and neutron, along with 100+ other particles, are composed of yet smaller particles known as quarks/antiquarks. These particles composed of the new quarks/antiquarks are collectively called hadrons. Electrons, part of a larger group collectively known as leptons, continue to appear structureless/fundamental or elementary in nature.

Currently, the leptons and quarks appear to qualify as the true fundamental/elementary particles; meaning without structure.

Part II The organization of known fundamental particles – The Standard Model

“The standard model of particle physics is a theory which [currently] describes three of four known fundamental interactions between the elementary particles that make up all matter…

It is a quantum field theory developed between 1970 and 1973 which is consistent with both quantum mechanics and special relativity.”

“…the standard model falls short of being a complete theory of fundamental interaction, primarily because of it’s lack of inclusion of gravity, the fourth known fundamental interaction.”

The particle nature of the standard model consists of two groups: - Matter Particles - Force-Mediating Particles

Matter Particles

The matter component of the Standard Model is comprised of twelve particles.

These particles all have an intrinsic spin value of ½, making them conform to the Pauli Exclusion Principle.

All matter particles of the Standard Model also have corresponding antimatter particles. These particles breakdown into groups of quarks (up, down, strange, charm, top, and bottom) and leptons (electron, muon, tau, and corresponding neutrinos). Quarks and leptons are further grouped into sets known as generations.

Quarks carry color charges (red, blue, or green) so they participate in strong interactions.

The up, charm, and top quarks carry the electric charge (+2/3). The down, strange, and bottom quarks carry the electric charge (-1/3). This allows the quarks to participate in electromagnetic interaction.

Leptons are color neutral and do not participate in strong interaction.

The electron, muon, and tau particles carry the electric charge (-1) and participate electromagnetic interaction.

Neutrinos have no electric charge and do not participate in electromagnetic interactions.

Quarks and leptons carry flavor charges and participate in weak nuclear interactions.

Force-Mediating Particles

Force-mediating particles of the Standard Model group into three categories that correspond with three of the four fundamental interactions. All three mediating particles are bosons and have intrinsic spins of (1).

This allows these particles to not conform to the Pauli Exclusion Principle.

Photons are electromagnetic force mediators involving charge particles. Photons are considered massless particles.

W +, W -, and Z o gauge bosons are weak nuclear mediators involving particles of different flavors (quarks and leptons). W +, W -, and Z 0 bosons are massive particles.

Gluons are strong nuclear force- mediators involving color charged particles (quarks). Gluons are considered massless particles.

Part III The Higgs particle – The Challenge

The Standard Model predicts the existence of one more particle known as the “Higgs boson.” The Large Hadron Collider (LHC) at CERN is hoped to confirm the existence of the Higgs boson in the near future.

References Google images