Elementary Particles and Particle Physics Theory

Vishesh Ramesh on elementary particles and particle theory

Short Description of Topic

History

Particle physics, and subatomic physics, has had a long history with many different and diverse scientists playing a part. The idea that matter is composed of elementary particles can be traced as far back as the 6th century B.C.E, and to scientists and philosophers of ancient Greece, India, the Middle East, and Western Europe. These early hypotheses rose from philosophical speculation and reasoning as opposed to experimentation.

John Dalton in the 19th century provided his theory of the atom, which was then believed to be the utmost fundamental particle. By the end of the 19th century, however, it was discovered that atoms were composed of yet smaller particles, when the electron and its charge was discovered via experimentation. Ernest Rutherford’s gold scattering experiment confirmed the existence of the nucleus and the proton, and further experimentation in the early 20th century led to the confirmation of the existence of the neutron.

By the middle of the 20th century, even smaller particles composing atomic nuclei were hypothesized, as the nuclear strong force was also discovered and established as a fundamental force alongside gravity and electromagnetism. Around this time, the understanding of fundamental particles also deepened, particularly with Erwin Schrödinger’s work in quantum physics and the establishment of wave-particle duality of the photon and Einstein’s work on the photoelectric effect showing photons to carry electromagnetic force.

In the mid-20th century, Marcus Fierz and Wolfgang Pauli formulated and refined the spin-statistics theorem, establishing bosons and fermions. In the time period between 1935 and 2000, positrons, muons, mesons, leptons, baryons, bosons, and neutrinos were discovered at a breathtaking and chaotic pace. The Standard Model is a theory that has its origins in this time period, born of a desire to unite these new subatomic fundamental particles and the four fundamental forces, and in the process of creating it, several more particles were hypothesized and most later discovered. The current formulation of the Standard Model was finalized in the 1970’s.

Elementary Particles

Elementary particles can first be grouped into fermions, particles with mass composing matter and antimatter, and gauge bosons, force carrier particles with no mass. In addition to these two groups, there lies the Higgs Boson in a category of its own, classified as a as a scalar boson.

Fermions: Particles of Matter and Antimatter

Fermions are all fundamental particles that compose matter or antimatter, and therefore have mass. All matter is composed of quarks and leptons, which share a spin of +1/2 in common. Antimatter is composed purely of antiquarks and antileptons.

Matter

Quarks

There are six types of quarks: Up, Down, Charm, Strange, Top, Bottom. All six quarks have a spin of +1/2. Up, charm, and top quarks have a charge of +2/3, while down, strange, and bottom quarks have a charge of -1/3.

Leptons

Leptons are particles with spins of 1/2. Leptons can be broken down into two major classes: charged leptons and neutral leptons. The electron, muon, and tau are the charged electrons, all with a charge of -1. Each of these have a respective neutral lepton, which are termed neutrinos. The speed at which neutrinos travel is hotly contested because it's extremely important and extremely difficult to measure. Many scientists believe neutrinos travel at the speed of light, which would pose a challenge to the idea that only massless particles can travel at the maximum speed of the universe which is the speed of light.

Antimatter

As matter is composed of quarks and leptons, antimatter is composed of antiquarks and antileptons. The antiparticle counterparts of quarks and leptons are mostly identical in property magnitudes but opposite in sign.

Antiquarks

For the six flavors of quarks, there exist antiquarks. Antiquarks have the same magnitude in properties as their quark counterparts, but flipped signs. The charges of each antiquark is the negative of its counterpart's charge.

Antileptons

For every charged lepton flavor there is a corresponding antilepton flavor, so there exist the anti electron, commonly known as the positron, the antimuon, and the antitau. There is still uncertainty however as to whether or not neutrinos have antiparticles. The electron antineutrino, muon antineutrino, and tau antineutrino, are all theorized, but some particle physicists argue that neutrinos may be their own antiparticles. There is yet much research to be done on neutrinos to further understand them.

Gauge Bosons: Carriers of Fundamental Forces

There are four fundamental forces: electromagnetic force, gravity, strong force, and weak force. Each of these are hypothesized to have a carrier particle. These particles are classified together as Gauge Bosons.

Photon: Carrier of Electromagentic Force

The photon is the guage boson responsible for mediating the electromagentic force. Electromagnetism and the photon are the most well understood force and respective carrier. The photon's mediation of the electromagnetic force can best be explained by the photoelectric effect.

Gluons: Carrier of Strong Force

Gluons mediate the strong force, which is the force between quarks. The attraction between quarks, strong force, is what allows quarks to come together to form hadrons, which can be classified into baryons(combinations of three quarks) and mesons(combinations of a quark and an antiquark). The most well-known baryons are protons and neutrons which form the atomic nucleus. Gluons and the strong force are reponsible for both the attraction between neutrons and protons, and the attraction between quarks that allow neutrons and protons to form. There are altogether eight variations, or colors, of the gluon.

W and Z Bosons: Carrier of Weak Force

There exist three gauge bosons that mediate the weak force between W and Z Bosons: W⁺, W⁻, and Z⁰. The weak force, or weak nuclear force, is the interaction responsible for the radioactive decay of subatomic particles, which plays a crucial role in nuclear fission. These bosons are not massless, but have some mass. The two W bosons are repsonsible for the absorption and emission of neutrinos in nuclear decay, while the Z boson is responsible for the transfer of momentum, spin, and energy when the neutrinos scatter after decay.

Graviton: (Hypothesized) Carrier of Gravitational Force

Gravitons are the hypothesized particle to carry the fundamental force of gravity. Yet to be discovered, they are expected to have a spin of 2, no mass, have light-particle duality, and travel at the speed of light.

Scalar Bosons: The Higgs Boson

The Higgs Boson, first theorized in the 1960's, was confirmed recently to exist through research conducted at the Large Hadron Collider. Termed "the God particle", the higgs boson has extremely low mass, no spin, and is extremely short-lived before decaying. This particle still is under the spotlight in particle physics, as much is theorized about the higgs boson to complete the standard model and particle physics theory, but yet to be confirmed.

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