Where does the Higgs boson come from?

When the Chelyabinsk meteor – dubbed Chebarkul – entered Earth’s atmosphere at around 17 km/s, it started to heat up due to friction. After a point, cracks already present on the chunk of rock weighing 9,000-tonnes became licensed to widen and eventually split off Chebarkul into smaller parts.

While the internal structure of Chebarkul was responsible for where the cracks widened and at what temperature and other conditions, the rock’s heating was the tipping point. Once it got hot enough, its crystalline structure began to disintegrate in some parts.

Spontaneous symmetry-breaking

About 13.75 billion years ago, this is what happened to the universe. At first, there was a sea of energy, a symmetrically uniform block. Suddenly, this block was rapidly exposed to extreme heat. Once it hit about 10¹⁵ kelvin – 173 billion times hotter than our Sun’s surface – the block disintegrated into smaller packets called particles. Its symmetry was broken. The Big Bang had happened.

The Big Bang splashed a copious amount of energy across the universe, whose residue is perceivable as the CMBR.

Quickly, the high temperature fell off, but the particles couldn’t return to their original state of perfect togetherness. The block was broken forever, and the particles now had to fend for themselves. There was a disturbance, or perturbations, in the system, and the forces started to act. Physicists today call this the Nambu-Goldstone (NG) mode, named for Jeffrey Goldstone and Yoichiro Nambu.

In the tradition of particle physics treating with everything in terms of particles, the forces in the NG mode were characterised in terms of NG bosons. The exchange of these bosons between two particles meant they were exchanging forces. Since each boson is also a particle, a force can be thought of as the exchange of energy between two particles or bodies.

This is just like the concept of phonons in condensed matter physics: when atoms part of a perfectly arranged array vibrate, physicists know they contain some extra energy that makes them restless. They isolate this surplus in the form of a particle called a phonon, and address the entire array’s surplus in terms of multiple phonons. So, as a series of restlessness moves through the solid, it’ll be like a sound wave moving through it. Simplifies the math.

Anyway, the symmetry-breaking also gave rise to some fundamental forces. They’re called ‘fundamental’ because of their primacy, and because they’re still around. They were born because the disturbances in the energy block, encapsulated as the NG bosons, were interacting with an all-pervading background field called the Higgs field.

The Higgs field has four components, two charged and two uncharged. Another, more common, example of a field is the electric field, which has two components: some strength at a point (charged) and the direction of the strength at that point (neutral). Components of the Higgs field perturbed the NG bosons in a particular way to give rise to four fundamental forces, one for each component.

So, just like in Chebarkul’s case, where its internal structure dictated where the first cracks would appear, in the block’s case, the heating had disturbed the energy block to awaken different “cracks” at different points.

The Call of Cthulhu

The first such “crack” to be born was the electroweak force. As the surroundings of these particles continued to cool, the electroweak force split into two: electromagnetic (eM) and weak forces.

The force-carrier for the eM force is called a photon. Photons can exist at different energies, and at each energy-level, they have a corresponding frequency. If a photon happens to be in the “visible range” of energy-levels, then each frequency shows itself as a colour. And so on…

The force-carriers of the weak forces are the W+, W-, and Z bosons. At the time the first W/Z bosons came to life, they were massless. We know now because of Einstein’s mass-energy equivalence that this means the bosons had no energy. How were they particulate, then?

Imagine an auditorium where an important lecture’s about to be given. You get there early, your friend is late, and you decide to reserve a seat for her. Then, your friend finally arrives 10 minutes after the lecture’s started and takes her seat. In this scenario, after your arrival, the seat was there all along as ‘friend’s seat’, even though your friend took her time to get there.

Similarly, the W/Z bosons, which became quite massive later on, were initially massless. They had to have existed when the weak force came to life, if only to account for a new force that had been born. The debut of massiveness happened when they “ate” the NG bosons – the disturbed block’s surplus energy – and became very heavy.

Unfortunately for them, their snacking was irreversible. The W/Z bosons couldn’t regurgitate the NG bosons, so they were doomed to be forever heavy and, consequently, short-ranged. That’s why the force that they mediate is called the weak force: because it acts over very small distances.

You’ll notice that the W+, W-, and Z bosons make up for only three components of the Higgs field. What about the fourth component?

Enter: Higgs boson

That’s the Higgs boson. And now, getting closer to pinning down the Higgs boson means we’re also getting closer to pinning down the Higgs mechanism as valid, a quantum mechanical formulation within which we understand the behaviours of these particles and forces. This formulation is called the Standard Model.

(This blog post first appeared at The Copernican on March 8, 2013.)