The Standard Model - An Introduction

This post is the first in a series of posts about the Standard Model of Particle Physics, so stay tuned 😉. And remember this is just an introduction.

“What is matter made of ?” was the question that was keeping our ancient Greek and Indian philosophers up at night, until the origin of atomic theory. This theory is said to have been introduced by philosophers Leucippus and Democritus in Greece (around 5th Century BC) and is also said to have origins in ancient India. Then with time both experimental and theoretical proof led to the speculation regarding the existence of atoms and molecules and in the late 19th Century atoms were finally defined.

Now came subatomic particles. Subatomic particles are even smaller particles that make up an atom. Initially 3 such particles were defined : Electron, Proton and Neutron. These were discovered through Cathode and Anode ray experiments. It was soon realised that protons and neutrons, unlike electrons were not fundamental particles. In fact they were made up of quarks which are considered to be one of the elementary particles. Until now a total of 6 flavors of quarks have been discovered. Flavor is a quantum number only for quarks and it is a law that flavor of a quark is always conserved in electro-weak interactions (i.e. electromagnetic and weak interactions). The 6 flavors are Up, Down, Charm, Strange, Top and Bottom. Quarks usually have a charge of +2/3 or -1/3. Up, Charm and Top Quarks have a charge of +2/3 and the other 3 quarks have a charge of -1/3. Up and Down quarks are the lightest quarks in terms of mass and the Top and Bottom quarks are the heaviest, whereas the Charm and Strange quarks are heavier than Up and Down but lighter than Top and Bottom. As we know that lighter the particle the more stable it is, so accordingly Up and Down quarks are the most abundant and naturally occurring in normal matter.

(Protons are made up of two Up and one Down quark and neutrons are made up of two Down and one Up quark.)

It was also found that just like the electron there were two more particles with very similar properties. The only different is that they were heavier than the electron. The heaviest one was named “Tau” and the other was named “Muon”. Analogous to these three particles three more particles were discovered and they were so similar to these that they were named after them, they are - Electron Neutrino, Muon Neutrino and Tau Neutrino (in increasing order of mass). These 6 particles were together grouped and called as Leptons. These were all grouped together because all these particles have half integer spin (+1/2 or -1/2) and also because they are very similar to each other in their respective properties. All the Leptons along with the Quarks are named as Fermions and these consist the of the Fundamental particles.

Below you can see a picture of the different particles, these are all the subatomic particles and just like the Periodic Table of Elements in chemistry these are known as the Standard Model of Elementary/Fundamental particles.

Now with developments in physics a new branch of physics - Particle Physics came into view. This branch deals with all the fundamental particles and their interactions. We know that fundamental particles need to interact with each other because otherwise this universe won’t exist, the protons would be ripped apart, there would be absolute nothingness. Now these interactions cannot take place on their own, there is a need for a “medium”, a “quanta of the force”, another “fundamental particle”. These particles were given the name Bosons. At present we have defined 4 fundamental forces out of which the force of gravity remains untouched (i.e. hasn’t been unified with the rest of the forces). The other 3 forces are Strong Nuclear Force, Weak Nuclear Force and Electromagnetic Force. These bosons or fundamental force particles are also know as the quantum of their respective forces. For the Electromagnetic force we have the Photon (γ), Gluon (g) for the Strong Nuclear Force and the W± and Z bosons for the Weak Nuclear Force. These bosons mediate all the interactions between fundamental particles. These group of bosons are also called Guage or Vector Bosons.

Then came the question of mass. It was well established that atoms get mass from the protons, neutrons and electrons; protons and neutrons get mass mostly from the binding energy of the quarks; but where do these quarks or for that matter any fundamental particles gets its mass from? Dr Peter Higgs theorised the existence of a scalar field originating due to a scalar boson. He said that all the particles interact with this field in different ways and thus get their mass. This field was given the name The Higgs Field and the particle was given the name The Higgs Boson sometimes also known as the God particle (I don’t like that name so let this be the last time this name is used in this article series).

Anitparticles were initially theorised by Paul Dirac these particles have the exact same properties as of a particular particle but with an opposite charge and spin. Paul Dirac first proposed the existence of of the antiparticles of electrons, which were called positrons. Soon after the prediction of these particles a physicist  Carl D. Anderson found that cosmic-ray collisions produced these particles in a cloud chamber and this led to the discovery of positrons. Subsequently it was theorised “Every particle has an anti-particle associated with it”. The quarks too have antiparticles associated with them, these are called antiquarks.

Apart from these 17 fundamental particles we have another group of particles called Hadrons. This group consists of all those particles that are held together by quarks by means of the Strong Nuclear Force. This group is further divided into 2 sub-groups - Baryons and Mesons. Baryons are the particles that consist of odd number of quarks. You may be surprised to know that protons and neutrons (which are made up of 3 quarks) are a part of this group; however these aren’t the only particles in this group. There are also particles that are made up of 5 quarks these are called pentaquarks. Now comes the Mesons’ group. This group consists of all particles made up of two quarks only, and that too one antiquark and one quark. There is a very important thing that one should know, a single quark can never exist on its own, it will only exist in bound state, this is because individual quarks are highly unstable. So whenever you have a single quark it will form a bound state with another antiquark and this new particle that has formed is called a meson. There are a variety of mesons present in our universe. Mesons are also constantly formed and absorbed inside the atom by the protons and neutrons. How exciting is that! This, my friends is the world of Particle Physics.

Now as I have said earlier this article is an extremely abridged version of an introduction to the Standard Model and the physics beyond, I will be individually taking up each topic in my subsequent articles.