Theory

Theories of particle physics

What is an Elementary Particle?

It is the most fundamental constituent of matter and cannot be divided any
further. Let us give an example:

Different types of elementary particles differ from each other in their mass and interactions.

The Standard Model of Particle Physics

There are three fundamental types of interactions:

  • Electromagnetic  Interactions.  They  occur in electronic circuits but also in your eyes right now as you read this.
  • Weak Interactions.  These take place in radioactive decays in nuclear reactors.
  • Strong Interactions.  You do not experience them in everyday life but without them nucleons would fall apart and stars, planets and life wouldn’t exist.

And then there is gravity, but that’s an entirely different story…

All known elementary particles are organised within the Standard Model:

What are neutrinos?

A short history of neutrinos

Neutrinos were first proposed in 1930 by Wolfgang Pauli. In his words:

TODAY I DID SOMETHING TERRIBLE WHICH NO THEORETICAL PHYSICIST SHOULD EVER DO. I PROPOSED SOMETHING WHICH CAN NEVER BE VERIFIED EXPERIMENTALLY

And yet, the neutrino was detected 26 years later at the Hanford and Savannah River nuclear reactors…

Hanford experiment. Credit: http://lappweb.in2p3.fr/neutrinos/anhistory.html

To this date they remain a mystery to our knowledge.

Neutrinos are special

  • They are the most common type of matter in the universe.
  • They are the lightest elementary matter particles we know about. Actually, they are so light that we don’t yet know their exact mass.
Animal characters credit: Mozilla foundation

For comparison, if the electron had the mass of a cow, the neutrino would be as light as an ant.

  • They are the most  feebly-interacting  particles in the Standard Model and only experience the weak force.
  • Neutrinos come in three different types which physicists call flavours.

Neutrinos oscillations

We know that on Earth we receive 70 000 000 000 electron neutrinos in a square centimetre (the size of your pinky nail) every second, just coming from the core of the Sun!  However, in the mid-1960s, when scientists measured the neutrinos coming from the Sun with their detectors on Earth, they noticed that two thirds of them were missing!

What happened to these neutrinos?  We know that all the neutrinos should have reached us, as there is nothing to stop them on the way.

Did they disappear?

What became of them?

It took scientists 30 years to figure this out: as the neutrinos travel, their flavours can change!

By the time they reach Earth, our neutrinos have transformed into a mixture of electron, muon and tau flavoured neutrinos.  These last two types were invisible to these pioneering detectors. This behaviour was a huge shock! Imagine picking a basket of apples from a tree, but after a long journey home, you discover bananas and strawberries in there as well!

This is now known as neutrino oscillation and its discovery was rewarded with the Nobel prize in Physics in 2015.

Why study neutrinos?

While we have observed neutrino oscillations, many of the neutrino properties are still unknown to us, and many questions remain unresolved:

  • What are their masses ?
  • Could they be both matter and antimatter at the same time ?
  • Do additional neutrinos exist ?

A better understanding of neutrinos can help us gain a deeper insight of:

  • the nuclear reactions in the stars and how they age
  • the near complete absence of antimatter in the visible Universe
  • how particles become massive
  • the evolution of the Universe, from its first moments to the formation of galaxies
Credit:Andrew Pontzen/Fabio Governato

Neutrino interactions

But, how do neutrinos talk to other particles? In most cases, neutrinos will bounce off a particle in a similar way to a tennis ball bouncing off a wall: it is an elastic interaction.  This occurs when neutrinos hit a neutron, or an electron or a proton (diagram b).  However, if an electron neutrino* hits a proton, it can also disappear and become a positively charged electron, called a positron (diagram a).  Nevertheless, a low-energy electron neutrino will not disappear when hitting a proton if it changes its flavour!  As mentioned earlier, this can occur when the neutrino oscillates.

Physicists represents the particle interactions using the so-called Feynman Diagrams, introduced by Nobel Prize winner Richard Feynman in 1948.

In these diagrams, the straight lines represent the particles that interact while the squiggly one refers to the particle that is exchanged in the interaction. Since neutrinos only interact via the weak force, they exchange the corresponding W and Z bosons.

*To be more precise, this interaction involves an antineutrino which is the antiparticle associated with the neutrino.