Standard Model of Particle Physics

We all know that every matter in this universe is made of electron, proton and neutron. These are the basic and fundamental particles of physics. Protons and neutrons are also made of quarks. Now, what is quark? Quark is the smallest particle which carries the characteristics of the matter (excludes electron). Till yet practically, it is to be considered that quark is the smallest particle.

Standard Model of Elementary Particles:

Standard Model of particle physics explains about various properties of particles and subatomic particles. But, here is a question,”Why do we need this model?”

We need this model, because with the help of this model we can easily explain about different particles. Some particles are responsible for the building blocks of the universe and some others are responsible for different force carriers. Earlier, scientists did not need this model, because at that time there were not lots of particles to be discovered. But, after revolution of quantum mechanics, field theory and particle accelerator (Large Hadrons Collider) collision, scientists got some new particles and the list of the particles were increased. That is why, now we need a standard model of the particles through which we can explain about various properties of the particles.

Now, what is a particle? Simply, particle is that smaller dot, which is responsible for the building block and different forces of the universe. Particles are divided into two categories –

  1. Fermions and
  2. Bosons


Fermions are those particles, which are responsible for creating various matters like you, me, planets, stars etc. Simply, Fermions create matters. They may be electron, proton or neutron. The name “Fermion” came from famous scientist Enrico Fermi which was given by Paul Dirac. Fermions can also be divided into two categories –

  1. Quarks and
  2. Leptons

1. Quarks:

Most of us know about quarks. These are those particles which are brain of protons and neutrons. Quarks are divided into six categories – Up, Down, Charm, Strange, Top and Bottom. The most important thing of quarks is that these are not stable, that is why they interact among them ( Up, Down, Strange, Top and Bottom) and form new particles. For example, one proton is made up of two Up quarks and one Down quark, neutron is made up of two Down quarks and one Up quark. The newly created particles from various quarks are called Hadrons. Hadrons are also classified into two types – Baryons and Mesons. Baryons are those which are created by combination of three quarks. Protons and neutrons are baryons, because they are created by combining Up and Down quarks. Mesons are those which are created by one quark and an antiquark. Pion, kaon etc are some of the Meson particles. The rest quarks Charm, Strange, Top and Bottom combine with each other to form various particles. Except proton, the hadrons family is unstable including neutron. That is why, it stabilize itself by combining with protons in the nucleus.

2. Leptons:

Leptons are that particles which are not formed by any quarks. They are very different. Leptons are also classified into two categories – Charged leptons and neutral leptons. Charged leptons are those which have a specific charge, like electron. Neutral leptons are those which do not have any electrical charge or neutral, like neutrino. Charged leptons interact with another particles to form different particles, but neutral leptons does not interact or rarely interact with any one. Some examples of leptons are – electron, tau and muon. Actually these are classified with three generations. First generation is electronic leptons (electron neutrino), second one is muonic leptons (muon neutrino) and third one is tauonic leptons (tau neutrino). These leptons are quite stable, that is why most of them can interact with different particles. Here is also an interesting thing that, when leptons and antileptons are collided with each other, then they destroy each other and  we get sufficient (100%) pure energy. As for example, electron has an anti electron particle which we call Positron. Their mass is same but charges are opposite to each other. When they are collided with a high-speed then we get pure energy. The collision between matter and antimatter is called Annihilation.

These are the classifications of Fermions. From the above explanation, we can understand that Fermions are responsible for creating the matters. But they do not cooperate with the different forces which are carried by Bosons.


These are those particles which are responsible for different forces of nature. Simply, all characteristics of different forces are carried by the Bosons. The name “Boson” came from an Indian scientist Satyendra Nath Bose. Actually, Paul Dirac proposed that name, because it follows the Bose-Einstein statistics which was originally proposed by Satyendra Nath Bose. The Bosons are classified into following categories –

  1. Photons
  2. W/Z Bosons
  3. Gluons and
  4. Gravitons

1. Photons:

Earlier, electric force and magnetic force were considered as different forces. But, James Clerk Maxwell derived some equations and showed some relationship between the two forces which were later called Maxwell’s Laws. From then, the two forces were combined to form a single force which was named as Electromagnetic force. Till now, we consider the two forces (electric and magnetic) as a single force. This force is carried by photons. That means, photons are responsible for this force. Without photons, there will be no electric force or magnetic force.

2. W/Z Bosons:

This is the combination of W bosons and Z bosons. Here, “W” means weak. W bosons are positive and negative which are actually antiparticles to each other. But Z bosons are neutral. The Three particles (W positive, W negative and Z) combine together and forms W/Z bosons. The W/Z boson is responsible for weak nuclear force. The range of weak nuclear force is almost 10-18 meters or 0.1% of diameter of a proton. Due to weak nuclear force, beta decay occurs. Because, when two protons collide with each other, then two particles are emitted. Due to the weak nuclear force one particle becomes positively charged positron and other becomes neutrally charged neutron. As in heavy atoms, the combination of protons and neutrons are not in balance, that is why weak nuclear force converts protons to neutrons and emit some radiation which is called Beta Decay.

3. Gluons:

The Gluons are those elementary particles which act like a “Glue” in the formation of proton and neutron. The term “Gluon” came from “Glue”, because it binds two positively charged particles together like glues. It is also called as Gauge boson. Gluons are responsible for the strong nuclear force. Strong nuclear force is the strongest force among all the four fundamental forces of nature. Because, it binds two positively charged particle together by exchanging meson particles, but we know that two same charged particle repel each other. Then how can they stay together? The protons feel a repulsive force from the neighbouring protons. But if the protons approach to a range of equal to or less than 10−15 meter or 1 femtometer to each other, then exchange of meson particles occur (like in table tennis we throw the ball to our competitor and he throws that back to us) and strong nuclear force is created among them which binds the protons and neutrons by overcoming the repulsive force. There is an interesting fact that, when the strong nuclear force is broken, then there produce enough gamma rays and neutrinos. That means, we can get sufficient gamma rays and neutrinos by breaking the strong nuclear force.

4. Gravitons:

For now, this is just a hypothetical particle. Mathematically, graviton carries the characteristics of gravitation. But till yet, practically we have not got any particles which carries the characteristics of gravitation. (more will be discussed in the next article “Theory Of Everything”).

These are various properties of the elementary particles which can only be easily understood by the Standard Model Of Particle Physics. Simply, Fermions are responsible for the building block of our universe and Bosons are responsible for the four fundamental forces of nature. The properties of graviton particle will be discussed in the next article.

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