What is the Standard Model in Particle Physics?

In this article, we are going to learn the basics of Standard Model of Particle Physics. However, before we do so, we need to clear the context.

In Physics we try to answer the very basic questions of our existence, e.g. what our universe is made of, what are the fundamental forces and laws governing our world, etc.

To do so, two groups of physicists work together:

Theoretical Physicists – Physicists who give ideas, conduct thought experiments, work out the maths, and even predict things that have yet to be detected. Einstein and Stephen Hawking have been one of the most famous theoretical physicists till date. Einstein’s thought experiments are now almost folklore amongst scientists.
Experimental Physicists – Physicists who work in labs and try to verify the theories and hypothesis made by Theoretical Physicists. And sometimes even find new unknown phenomenon.

For more than last 100 years physicists have been trying to find out how the universe works and what it is made of, by breaking stuff and then analysing it.

For thousands of years people used to think that atom is the smallest unit of physical existence. But then we came to know that atom itself is made up of nucleus and electrons, and that nucleus is in turn made up of particles called protons and neutrons.

However, that was not the end of story. Rather we just opened up a pandora’s box. The deeper we looked, the smaller particles we witnessed.

In fact, so much has been our obsession with breaking things with higher and higher energy, that we have build particle accelerators of humongous sizes – the largest one being the Large Hadron Collider (LHC) in Geneva, which is almost the size of a city.

In it we pace up protons to speeds almost equal to the speed of light and then smash them together head-on. Physicists then study the aftermath of the collisions and try to find new particles. The latest discovery being the Higgs Boson (popularly known as the God Particle).

However, this discovery was expected. In fact, experimental physicists were expecting to find Higgs Boson. That was because the Standard Model of Particle Physics predicted that such a particle must exist. This model even provided the experimental physicists with the information regarding the expected properties of such a particle.

So, Standard Model of Particle Physics is a model of the expected fundamental particles in our universe. It has all the fundamental particles we already know, and some that we expect to find. So, it’s a work in progress.

It is one of the prime candidates of the Theory of Everything, a theory that can explain everything in this universe. Experimental Physicists the world over are trying to test, and thus verify (or reject) this model.

Note

There are many candidates for the coveted Theory of Everything, e.g. String Theory, Standard Model, etc.

Moreover, there are various versions of these theories and models too, e.g. there are various versions of String Theory, some predicting 10 dimensions, and some 11.

There’s a model of Super-symmetry that builds on the Standard Model and doubles up the number of fundamental particles that our universe should have.

Now, we have some basic understanding of what we are talking about here. So, now we can dive deeper into the nitty-gritties of the Standard Model.

Table of Contents

Details of the Standard Model of Fundamental Particles
Various elementary particles of Standard Model

Details of the Standard Model of Fundamental Particles

Standard model is a set of very small, and (yet) indivisible elementary particles, which form our universe. It also explains how these particles interact with each other to give rise to fundamental forces of nature (weak and strong nuclear force, electromagnetic force, gravity), mass, etc.

There are basically two types of fundamental particles in Standard Model:

Fermions – Further subdivided into Quarks and Leptons. They make up the matter around us, so are also called as Matter Particles.
Bosons – Further subdivided into Vector Bosons, Scalar Bosons, and Tensor Bosons. (Tensor Basons have yet not been found experimentally). Vector Bosons are responsible for the four fundamental forces, and hence are called Force Carriers. Scalar Bosons, i.e. Higgs Boson(s) is/are responsible for imparting mass to some of the other fundamental particles.

As already mentioned, there are four fundamental forces of nature as per Standard Model:

Weak nuclear force
Strong nuclear force
Electromagnetic force
Gravity

Here’s a diagram representing all those particles that constitute the Standard Model:

As already told, it’s a work in progress and this model predicts some particles that have yet to be found. The most recent addition to this model has been the Higgs Boson (God particle).

While Graviton is a particle that will explain the fundamental force of Gravity. It is predicted by Standard Model. But it still has not been found in experiments.

Anti-Matter

All the particles in the Standard Model also have an anti-matter version – fundamental particles that have opposite charge but are the same otherwise.

How do we know it?

Because anti-matter particles have been produced and observed in particle colliders. However, they combine quickly with the matter particles and annihilate each other, producing pure energy.

Now let’s study about these elementary particles in more detail.

Various elementary particles of Standard Model

Electron (e)

Molecule is the smallest unit of any chemical compound, e.g. H₂O (Water molecule), O₂ (Oxygen molecule), etc. These molecules are in turn made up of elements that can be found on the periodic table.

Atom is the smallest unit of any element on a periodic table, e.g. Hydrogen atom (H), Oxygen atom (O), etc. However, an atom is not the smallest unit of matter (though scientists used to think so for many centuries).

Every atom has a nucleus (having protons and neutrons), and electrons revolving around it.

Now, these electrons are part of the Standard Model, because these are indivisible fundamental particles (as of now). In fact, electrons were the first discovered fundamental particle of the Standard Model.

Muon (μ) and Tau (𝜏)

Muon (μ) and Tau (𝜏) are heavier siblings of electron. We only see them in particle colliders, when we smash protons and atoms together. That’s because these are relatively heavier, and decay into lighter particles pretty quickly.

Photons (γ)

The electrons are bound to the nucleus of an atom by electromagnetic force. Photons are the particles that are associated with electromagnetic force. They are denoted by the Greek symbol gamma, γ.

So, electrons and the nucleus of an atom exchange photons and thus are attracted towards each other. This is what gives rise to electromagnetic force.

Light

The light we see is also made up of photons. So, light is basically an electromagnetic wave.

Quarks and Gluons

We already know that nucleus of an atom is made up of neutrons and protons. But even these particles are not fundamental particles, as they can further be subdivided.

Protons and Neutrons are made up of Quarks, which are fundamental particles of nature.

Proton is made up of 2 Up Quarks and 1 Down Quark.
Neutron is made up of 1 Up Quark and 2 Down Quarks.

Nucleus is held together by another fundamental force of the Standard Model, called the Strong Nuclear force. Fundamental particles called Gluons (g) carry this Strong Nuclear force.

Six types of quarks

There are basically 6 types of quarks that you will find in the Standard Model.

Up quark (u)
Down quark (d)
Strange quark (s)
Charm quark (c)
Bottom quark (b)
Top quark (t)

We only see Strange, Charm, Bottom, and Top quarks in particle colliders, when we smash protons together at high speeds. That’s because these are relatively heavier, and decay into lighter particles pretty quickly.

w and z fundamental particles

We know that the heavier versions of electrons and quarks decay rapidly. This happens via an exchange of w and z fundamental particles. These particles are responsible for carrying the Weak Nuclear force.

This force is also responsible for neutrons and protons to transform into one another, and thus is responsible for the process of nuclear fusion (a process that fuels every star in our galaxy).

We can observe these w and z fundamental particles only in high-energy particle colliders.

Note

Unlike photons and gluons, w and z particles have mass.

Neutrinos (ν)

These fundamental particles only interact with other fundamental particles via the weak nuclear force. They are represented by the Greek symbol nu (ν).

Billions of neutrinos produced by the sun, pass through our body every day without interacting with the atoms of our body.

To detect these neutrons large tanks of water and other solutions were kept deep inside mountains. Even then scientists only detect only a handful of neutrinos interactions. They are pretty rare.

Neutrinos are of three types, each associated with a certain version of electron:

Electron Neutrino (ν_e)
Muon Neutrino (ν_μ)
Tau Neutrino (ν_𝜏)

Higgs Boson (H)

Higgs Boson is a particle in the Standard Model that is related with the Higgs field. Particles that interact with this field have a mass – the greater the interaction, the more the mass. However, some particles do not interact with it at all and hence are massless, e.g. photons.

Various kinds of Higgs Boson

As per some theoretical physicists, instead of only one Higgs Boson, there maybe more of them. One scientist has come up with a hypothesis that includes 5 different Higgs Bosons, that may explain the phenomenon of dark matter. It’s still being tested in Large Hadron Collider (LHC).

Higgs Boson is only produced 4-5 times in a billion collisions of protons in Large Hadron Collider. That’s because protons must strike in a very particular manner for them to produce a Higgs Boson.

Moreover, even when it is produced, its life is very small. So, it’s hard to notice, and finding various kinds of Higgs Bosons may take some time (even if we assume that they exist).

Winding Up

Standard Model is still a long way from becoming the Theory of Everything, that can explain everything in the world around us. Scientists are still testing it in the particle accelerators around the world.

We still have many questions that need an explanation, for example:

How to integrate Standard Model with the fundamental force of Gravity?
How matter particles interact with force carriers?
What is dark matter, which is much more common in our universe than normal matter?

But the future looks promising. And then it’s not as if we are totally reliant on this model to explain the realities of our universe. There are many other rival candidates, such as String Theory, M Theory, etc. And also, many versions of the Standard Model, e.g. Supersymmetry.