What does the standard model of particle physics actually mean?

 We all know that matter is composed of molecules which can be further divided into atoms. Again, we can further divide atoms into neutrons, protons and electrons. But are they elementary particles? No, they are not. Electron is an elementary particle, but proton and neutron are not. They can be divided into much smaller particles called quarks..

So, now the question might arise that are electrons and quarks the only elementary particles present in the universe? Again the answer is no. That is where the standard model comes into action. Now the question might arise: What is this standard model thing? Well, standard model is a well established theory of particle physics which describes three of the four known fundamental forces of nature, namely: the electro-magnetic force, the strong nuclear force and the weak nuclear force. It unfortunately does not explain the gravitational force. The standard model is basically an arrangement of all the known elementary particles and mediator particles present in the universe. To make it clear let's see what the standard model looks like.

The Standard Model

Here you can see there are so many particles. So, let's classify them. There are basically two types of particles: The fermions and the bosons. Fermions are particles with half integral spin and make up matter. For instance, electron is a fermion and you can see in this image that it has a spin of half.

Bosons are basically particles with integral spin and mediate scalar or vector fields. For example, the photon is a boson which is a spin-1 particle.

Now, let's classify fermions. Fermions can be classified into quarks and leptons. The basic difference between leptons and quarks is that quarks interact through all the four fundamental forces but the leptons do not interact through the strong nuclear force.

Quarks: There are three generations of quarks as you can see in this image and a total of six quarks are present, namely; up, down, charm, strange, top and bottom. As it turns out only up and down quarks are stable and form daily matter. The other four quarks are very unstable and decay down to up and down quarks. (Note: quarks do not exist on their own in normal conditions, they are always found in bound form such as in protons and neutrons which are baryons). The quarks in the upper row have a charge of positive 2/3 and the ones in the lower row have a charge of negative 1/3. A proton is basically composed of two up quarks and one down quark, giving it a total charge of positive 1. And the neutron is basically composed of two down quarks and an up quark, giving it a net charge of 0 .

Leptons: Leptons can be classified into two types. The charged ones and the uncharged ones which are also called neutrinos. The charged ones include the electron, the muon and the tau particle and there is a neutrino associated with each one of them. Again, there are three generations of leptons as you can see in the image. Of the three charged quarks only electron is stable and so the muon and the tau decay down to electron. And of the uncharged ones only the electron neutrino is stable.

We can see that only the first generation of fermions are stable. Well, why is that? Scientists are still trying to find an answer to this question. Again there also comes the question: Why are there only three generations of fermions at all? Scientists are trying to answer that question also.

Another thing which is quite interesting is that each and every fermion has its own anti-particle which is the same particle with the same mass but oppositely charged. For example, the anti-particle counterpart of electron is called positron.

Now let's classify bosons. Bosons are of two types: The gauge bosons or the vector bosons and the scalar bosons.

Vector bosons: They are also called force carriers as they mediate force fields which are vector fields.

For example, the gluon particle mediates the strong nuclear force field, the photon mediates the electro-magnetic force field and the W and Z bosons mediate the weak nuclear force field.

Scalar bosons: They mediate scalar fields. It includes the famous Higgs boson which gives mass to every particle.

After all of that, mysteries still remain regarding the standard model.