Initially, atoms were thought to be the smallest units of matter. Later 3 sub-atomic particles, protons, neutron and electrons were discovered, and all atoms were made of this.

In the 1940s, hundreds more of these subatomic particles were found through experiments with the particle accelerator. This needed some organisation. These particles then began to be categorized into the following types

NOTE: Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay.

In the 1960s, physicists realized that protons and neutrons are further made of ‘quarks’ these quarks are of 6 kinds, up, down, bottom, top, strange and charm. Their properties include electric charge, mass colour and spin. They form composite particles called hadrons the most stable of which are protons and neutrons.

The quarks are: up, down, top, bottom, strange and charm.

Up, charm and top quarks have a charge of +2⁄3, while down, strange and bottom quarks have a charge of -1⁄3. Each quark has a matching antiquark. Antiquarks have a charge opposite to that of their quarks.

Up and down quarks are found inside protons and neutrons. Since they have a relatively lower mass, they are normally not subject to particle decay, and can be found easily. They are the most common quarks. They are held together by bosons called gluons. Two up quarks and one down make a proton (2⁄3 + 2⁄3 - 1⁄3 = +1 charge) while two down quarks and one up make a neutron (2⁄3 - 1⁄3 - 1⁄3 = 0 charge). The other four flavours are not seen naturally on Earth, but they can be made in particle accelerators. Some of them may also exist inside of stars.

Below: diagram of proton and a neutron

Top and bottom quarks are the heaviest and second heaviest elementary particles. The top quark interacts primarily by the strong interaction, but can only decay through the weak force. It decays to a W boson and either a bottom quark (most frequently), a strange quark or, on the rarest of occasions, a down quark. Although bottom quarks are not very common, they are found in particles such as B mesons, among others. All bottom quarks will decay into charm quarks or up quarks

Charm and strange quarks are the third and fourth heaviest quarks. They are found in hadrons such as J/ψ meson (J/ψ), D mesons (D), charmed Sigma baryons (Σc). Example of hadrons containing strange quarks include kaons (K), strange D mesons (Ds), Sigma baryons (Σ), and other strange particles.

There are 6 kinds of leptons. They may or may not have a charge. They consist of the electron, the muon, the tau (charged); the electron neutrino, the muon neutrino, and the tau neutrino (uncharged). They can be classified into electronic leptons, muonic and tauonic leptons. Since the electron is much lighter than the other charged particles, the muon and the tau rapidly decay into electrons.

Electrons are the most common and most stable charged leptons, and they occur in every atom. They fundamentally govern all chemical reactions, along with being responsible for the transfer of charges. They are negatively charged.

Muons are not stable, and 200 times heavier than an electron, and so is not known to exist in the free state. They rapidly decay into electrons. It was the second lepton to be discovered and are created in particle accelerators.

Taus have properties similar to those of electrons but are much heavier. They are the only leptons that have the sufficient mass to decay into hadrons.

Neutrinos are neutral leptons that are not affected by the electromagnetic force, that are only affected by the weak force. They are created by certain types of radioactive decay, when cosmic rays hit atoms, in nuclear reactions near the sun and many more. They are of 3 kinds, electron neutrinos, muon neutrinos and tau neutrinos.

Bosons are a class of subatomic particles. They are a category of subatomic particles divided on the basis of spin. Particles with half-integer spin are called fermions, and those with a full integer spin are called bosons. The name was coined by Paul Dirac, after the Physicists, Satyendra Nath Bose.

 Satyendra Nath Bose

The 5 main Bosons are W, Z, gluon, photon, Higgs and the proposed but not included graviton.

Bosons are classified into gauge and scalar. Gauge bosons are force governing bosons. Photons, Z, W and gluons are gauge bosons. There is only one scalar boson, the Higgs boson. See fundamental forces.

The gluon governs the strong force and keeps the up and down quarks attached in the neutron and proton, and the neutron and proton in the nucleus. Without this, the protons in the nucleus would repel each other, and the matter would not exist.

The photon governs the electromagnetic force.

The W and Z govern the weak force. Without them, particle decay would not occur.

The Higgs gives mass to all the other particles. As of march 28^th 2015 at 14:11 PM IST, the Higgs Boson is the latest boson to be discovered and is often called the single biggest discovery in modern science.

The graviton is a hypothetical boson that governs gravity. The standard model, as well as general relativity, allow a particle as this to exist, although its existence is not scientifically proven yet. Therefore, gravity is not a part of the standard model. It was a major part of the plot of the 2014 Hollywood feature film Christopher Nolan’s ‘Interstellar’ which uses gravitons to communicate across interstellar distances and through Einstein-Rosen bridges.

The standard model is a widely accepted model of the inner workings of the universe. Our understanding of the universe has evolved from the four elements of air, water, soil and fire to the modern model of Quantum mechanics, in just a few hundred years. These advancements are milestones of human achievement, and reminders that we are incredible creatures, who have never rested with even 99.99% of an answer, and are always, asking questions, and learning. Although these achievements and the kind of money spent on research may seem in vain, I believe that they are uniting factors as they define the humans as a race. I would like to conclude by quoting Erwin Schrodinger,

 “Quantum physics thus reveals a basic oneness of the universe.”

Fin.