Neutrinos (Part 2): Overview of the standard model.

The standard model of elementary particles displays all the elementary particles that have been discovered till date, this post deals with how the particles are placed in the standard model and how these particles are related to each other.

What are elementary particles?

Elementary particles are the smallest physical entities of matter, we stumbled across atoms with our quest to find the elementary particles but a few experiments suggested that atoms are not the end, there were subatomic particles called electrons, neutrons and protons which make up the atoms. A question now arises, are these subatomic particles elementary?

Discovery of electrons

J J Thomson’s cathode ray experiment in 1897 showed that there were particles smaller than the atom namely, the electron. The experiment was quite simple, an empty glass tube with electrodes on its ends and a fluorescent background showed rays moving from the negative terminal (Cathode) to the positive terminal (Anode). These rays were later called as electrons as they possessed a negative electric charge.

Rather than looking into the specifics of this experiment let us look at it from another perspective, why was it possible to observe electrons? or to rephrase it, why did we detect electrons in the cathode ray experiment?

If we look at the setup of the experiment, electric terminals are placed at the ends of the glass tube and we supply energy across the terminals in the form of voltage, this voltage could strip electrons from the surface of the terminals and hence we could observe them. We could only observe these particles because we could supply enough energy to generate them.

Discovery of Quarks

Along with the electrons, protons and neutrons were also discovered to be subatomic particles, if these particles are to be elementary they should not have any smaller constituents. To test this particle accelerators were built. A particle accelerator takes a stream of protons to very high energies and directs them to collide with a target (such as a metal plate) or other particles that are accelerated in the opposite direction (usually another stream of protons). As these particles are moving at speeds close to that of light, tremendous amount of energy is released on collision but what is this energy? Well, they are smaller particles called quarks.

We will not get into the deeper aspects of quarks (as that requires a post of its own) but let’s look at how we discovered quarks. A particle accelerator gives tremendous amount of energy to protons to make them move at near light speeds, on collision the quarks are observed (actually particles called mesons were seen which are composed of a quark and an anti-quark, quarks cannot exist in isolation as they have a fractional charge). The up and down quarks make up protons and neutrons. Quarks exist in 6 different types, also known as flavors (yes, the vocabulary used in particle physics is quite strange).

Similar to how electrons were observed by applying voltage, quarks were observed by accelerating particles to high speeds, we can only observe particles if we can supply enough energy to break them. It was relatively easy for electrons as we need energy in the range of a few eV (electron Volts) but for quarks, the particle accelerators need a few GeV (Giga electron Volt) of energy to operate.


The electron is categorized into a family called leptons there are 2 other particles Muon (\mu) and Tau ( \tau) which are only different from the electrons by mass, a muon is around 200 times heavier than an electron and a tau is 17 times heavier than a muon. Other than the mass difference and the way the particles interact there is not much difference between these 3 particles hence they are grouped together as a family called leptons. Neutrinos too belong to the family of leptons but we will deal with them later. Like quarks there are 6 flavors of leptons.

If we look at the standard model, we can see the particles arranged 3 generations (indicated by the columns I,II and III)

These generations refer to the energy of the particles and also refers to the amount of energy required to observe the particle, this forms a hierarchical structure. The rows indicate the charge of the particles, you may observe that in both quarks and leptons the rows differ by a charge of 1 unit.

Forces of interaction

Nature has four fundamental forces:

  1. Strong Nuclear Force
  2. Weak Nuclear Force
  3. Electromagnetic Force
  4. Gravitational Force

These forces have a particular range within which they are predominant.

Quarks and Leptons interact due to these forces (gravitational force does not have a place in the standard model so we will leave that out for now), they interact by exchanging particles called bosons, and there are different kinds of boson for each force, accordingly these particles are described as force carriers or intermediate vector bosons.

Strong Interactions

The up and down quarks in neutrons and protons are very closely bounded, this indicates a very strong attractive force acting between quarks. The bosons which mediate strong forces are called gluons (g) as they “glue” quarks together. Quarks exchange streams of gluons all the time and a particle collision between protons breaks this gluon exchange and makes new exchange pairs which we observe as new particles. As only quarks can exchange gluons, they are grouped together as one family.

Electroweak Interactions

The electromagnetic force and weak nuclear force together take part in electroweak interactions. Electrons exchange bosons called virtual photons (\gamma ) which mediate the force between them. The rest of the leptons interact using the W^{\pm} or the Z bosons. The W^{\pm} are charged while Z are uncharged. Electroweak interactions, as the name suggests are weak forces between particles, they are called weak because these interactions are rare events such as radioactive decay or neutrino interactions.

Virtual photons are exchanged between electrons, electrons repel each other but the same virtual photons are involved in electron-positron attraction, but in a nutshell virtual photons are lepton-lepton interactions.

The W^{\pm} and Z bosons are slightly different, they are involved in interactions where quarks decay into leptons or the other way, that is they are quark-lepton interactions. Neutrino interactions come under the category of quark-lepton interactions.

The structure of the standard model is based on the kind of interactions between the particles, this forms the basis for the different families each particle is assigned to. As we now have a good idea about the structure of the standard model we can dive into the interactions between these particles and represent them using Feynman diagrams. This will be the motive for Part 3.

Guest post written by Rohan VJ
Rohan VJ is a student at St.Joseph’s College, Bengaluru. He is interested in theoretical physics, and also handles @our.inexplicable.universe on instagram.

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