Physicists have always tried to see what the world is made of. Rutherford use alpha radiation to discover the nucleus. Today we have the atomic model that has protons and neutrons bound in the center and the tiny electrons orbiting around it.
That is not the end. Rutherford probe was low energy. It was good enough to probe atoms. Today, high-power accelerators can explore much finer structure of stuff.
Protons and neutrons that make up the atomic nuclei consist of quarks. We infer from experimental results of electrons scattering off protons that there are three quarks in protons and neutrons. The quarks turn out to be just two flavors: up and down. Thus we can say that all matter is composed of three types of stuff: electrons, up quarks, and down quarks.
There are many other particles found in detectors of particle accelerators around the world. How exactly do we "see" these tiny particles? Invention of particle detectors, starting from the bubble chamber (Nobel Prize for Donald Glaser), has allowed us to see the trails of the charged particles. Think of a jet plane's condensation trail. The plane may be far away so it's too tiny to observe on the ground, but we can easily see the trail of vapor. (See P Watkins' Seeing Particles)
When chemists discovered many elements, they construct the periodic
table of elements which arrange these elements nicely into columns and
rows. For particles, Murray Gell-Mann used Group Theory to categorize
them into patterns. Some of these have three quarks, just like nucleons.
The three-quark bags are grouped together and are called
Quarks are confined inside these 3q and q-qbar bags. They are never observed individually yet.
How do things interact? We can picture the interactions as the exchanges of "force" particles. There are four fundamental forces that can be used to described all the known interactions in the world. These are
Strong nuclear force is exchange of gluons. This type of interaction happens only among quarks, who have color quantum numbers (r, g, b). A gluon emission changes the quark's color. Gluons do not exist as free particles. The nuclear strong force keeps quarks together to form protons and neutrons. The strong force also keep the nucleons together to form nuclei.
In high energy scattering, gluons play important roles. Experiments have shown that gluons are abundant. The probability of gluon emission increases rapidly with energy.
Photons are exchanged between particles with electrical charges. Photons are massless and chargeless. Nuclei and electrons combine via this electromagnetic force. The EM force is also responsible for atoms forming molecules.
Weak nuclear force is when a boson of type W+, W-, or Z is emitted from a particle. These are all very heavy. Z (91.2 GeV) is slightly more massive than the W's (80.4 GeV). This weak nuclear force is what governs the beta decay of radionuclei. A quark in a proton (neutron) emits a W+ (W-) boson and is converted into a neutron (proton). The W boson quickly decays into a two leptons (usually electron and electron neutrino).
Finally, gravity help atoms and molecules form planets and stars. Gravity, however, is not included in the Standard Model. For that we have to come up with something else.
This is all we see. This is matter. There is a lot more we do not see: electromagnetic radiation beyond the visible region. Small stuff like quarks have never been detected.
Quarks, leptons, and force mediators altogether form the Standard Model (SM). SM, constructed in 1970-1973, consists of the electroweak model of the electromagnetic+weak forces and quantum chromodynamics (QCD) of the strong force. The SM is highly successful in describing experimental results (e.g., it predicted W and Z bosons, charm and top quarks). However, it does not explain why there are three generations of quarks and leptons, and how all these particles have their masses.
A big quest in particle physics is therefore the attempt to understand how these particles come to be. That is, what is the mechanism behind particles acquiring mass? Beyond this, there is much more that we do not have explanations for:
The Standard Model proposes the Higgs as the particle responsible for mass generation of the elementary particles. The Higgs was first theorized by Peter Higgs, Francois Enclert, and Robert Brout in 1964. The Higgs would have mass of over 100 GeV and carries zero charge and zero spin (so it is called a boson).
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© roppon picha @ ucd npg