The Large Hadron Collider has been in the news for years now in its search for (amongst other things) the Higgs Boson. News articles tend to refer to this as the “missing piece” of the prevailing model of particle physics (the Standard Model). They sometimes even refer to it as the “God particle”, but this name is somewhat whimsical – I haven’t heard any actual physicists call it that in my entire life. But they never explain what the Higgs Boson is, (or rather, might be, if it exists at all). That’s what I’m doing to attempt to do here.

Bosons and Fermions

Fundamental particles all fall into two categories, bosons and fermions. Groups of bosons obey what is called Bose-Einstein statistics, while groups of fermions obey Fermi-Dirac statistics. The difference between the two is that bosons like to clump together, and fermions like to stay apart. The broad framework that particle physicists use to describe the universe is one of particles emitting and absorbing other particles – similar to a group of people standing around throwing tennis balls to each other. In this picture, the particles representing the ‘people’ tend to be fermions, and the tennis balls tend to be bosons.

Now, you know that when you catch a ball you feel pushed back a little. Another way of saying this is that the ball has transferred some force to you. For this reason, bosons are often referred to as force carriers. The best-known example of a boson is the photon. This is the particle that carries the electromagnetic force.

The Lagrangian

In physics, we have an extremely useful way of writing down a mathematical object which neatly summarises all the properties of a physical system. This is called the Lagrangian. For example, you could write down the Lagrangian for the system of a pendulum. This simple expression would tell you everything you need to know about the pendulum – where it’s going to be at a particular time, how much energy it has got etc.

We can actually write down Lagrangians for entire fundamental forces of physics. For example, we can write down the Lagrangian for electromagnetism. This then tells us everything we need to know about electromagnetism. One thing the Lagrangian tells us is which types of particles interact with each other, and how strongly. If, when we write down the Lagrangian for a particular system of two types of particles A and B, we find that it contains something like kAB (k multiplied by A multiplied by B), this tells us that A and B interact with each other, with ‘strength’ k. This k is known as the coupling constant. Often, the coupling constant is actually the mass of some particular particle.

Spontaneous Symmetry Breaking

The mechanism which necessitates the prediction of the existence of the Higgs boson is called spontaneous symmetry breaking. This is a mechanism by which an object in a mathematically ‘symmetrical’ state can end up in a state which does not so obviously have this symmetry.

The easiest and most common example is that of a ball sitting on top of a hill. The ball can roll down the hill in any direction with equal probability – the system is symmetric in this way. Now, if the ball is nudged even a tiny bit, it will roll down the hill in some direction. The system is now no longer symmetric since there is a preferred direction (the direction the ball is rolling). The only residual clue that there was, at some point in the past, some symmetry, is that any other direction of rolling would look the same from the point of view of the ball.

This process (an abstract version of it) is of great importance in physics. Without this process, a couple of particles (the W and Z bosons) which we have observed to have mass are predicted to be massless. This would appear to mean that our theories are wrong. But, if we include the presence of some form of ‘hill’ in our Lagrangian and look at how symmetry might be broken, we can actually solve some of these problems.

The Higgs Boson

The simplest way to include spontaneous symmetry breaking in our theories in such a way that the W and Z bosons are predicted to have mass is via the Higgs mechanism, which predicts the existence of a new particle – the Higgs boson. This gives us objects in our Lagrangians that look like hAB, where h represents the Higgs boson. This can be very loosely thought of as a replacement for the mass m in the mAB type terms above. This tells us that particles A and B interact with h in such a way that h somehow represents the existence of mass. This is what people mean when they say ‘the Higgs boson gives all other particles mass’. The Higgs boson itself also must have mass – so it interacts with itself.

The standard model does not predict the mass of the Higgs boson. The search is on right now to find it, and depending on what the mass turns out to be, we can confirm or exclude a wide variety of physical theories. We might not even find it at all – that’s the most exciting prospect. We’d have to come up with something completely new!