Billions of Euros have been ploughed into the construction of the largest experiment known in the history of mankind. The Large Hadron Collider (officially due to be “switched on” September 10th 2008) will eventually create proton-proton collision energies near the 14 TeV mark by the end of this decade. This is all highly impressive; already the applications of the LHC appear to be endless, probing smaller and smaller scales with bigger and bigger energies. But how did the LHC secure all that funding? After all, the most expensive piece of lab equipment must be built with a purpose? Although the aims are varied and far-reaching, the LHC has one key task to achieve: Discover the Higgs Boson, the world’s most sought-after particle. If discovered, key theories in particle physics and quantum dynamics will be proven. If it isn’t found by the LHC, perhaps our theories are wrong, and our view of the Universe needs to be revolutionized… or the LHC needs to be more powerful.

Either way, the LHC will revolutionize all facets of physics. But what is the Higgs boson? And why in the hell is it so important?

I’ve read many very interesting articles about the Higgs boson and what its discovery will do for mankind. However, many of these texts are very hard to understand by non-specialists, particularly by the guys-at-the-top (i.e. the politicians who approve vast amounts of funding for physics experiments). The LHC physicists obviously did a very good job on Europe’s leaders so this gargantuan particle accelerator could secure billions of euros/dollars/pounds to be built.

There is a classic physics-politics outreach example that has become synonymous with LHC funding. On trying to acquire UK funding for the LHC project in 1993, physicists had to derive a way of explaining what the Higgs boson was to the UK Science Minister, William Waldegrave. This quasi-political example is wonderfully described by David J. Miller; Bryan Cox also discusses the same occasion in this outstanding TED lecture.

What is the Higgs boson? The Short Answer

Predicted by the Standard Model of particle physics, the Higgs boson is a particle that carries the Higgs field. The Higgs field is theorized to permeate through the entire Universe. As a massless particle passes through the Higgs field, it accumulates it, and the particle gains mass. Therefore, should the Higgs boson be discovered, we’ll know why matter has mass.

What is the Higgs boson? The Long Answer

Firstly we must know what the “Standard Model” is. In quantum physics, there are basically six types of quarks, six types of leptons (all 12 are collectively known as “fermions”) and four bosons. Quarks are the building blocks of all hadrons in the Universe (they are contained inside common hadrons like protons and neutrons) and they can never exist as a single entity in nature. The “glue” that holds hadrons together (thus bonding quarks together) is governed by the “strong force,” a powerful force which acts over very small distances (nucleon-scales). The strong force is delivered by one of the four bosons called the “gluon.” When two quarks combine to form a hadron, the resulting particle is called a “meson“; when three combine, the resulting particle is called a “baryon.”

In addition to six quarks in the Standard Model, we have six leptons. The electron, muon and tau particles plus their neutrinos; the electron neutrino, muon neutrino and tau neutrino. Add to this the four bosons: photon (electromagnetic force), W and Z bosons (weak force) and gluons (strong force), we have all the components of the Standard Model.

However, there’s something missing. What about gravity? Although very weak on quantum scales, this fundamental force cannot be explained by the Standard Model. The gravitational force is mediated by the hypothetical particle, the graviton.

The Higgs Field

The Standard Model has its shortcomings (such as the non-inclusion of the graviton) but ultimately it has elegantly described many fundamental properties of the quantum and cosmological universe. However, we need to find a way of describing how these Standard Model particles have (and indeed, have no) mass.

Permeating through all the theoretical calculations of the Standard Model is the “Higgs field.” It is predicted to exist, giving quarks and gluons their large masses; but also giving photons and neutrinos little or no mass. The Higgs field forms the basic underlying structure of the Universe; it has to, otherwise “mass” would not exist (if the Universe is indeed governed by the Standard Model).

As a particle travels through the Higgs field (which can be thought of as a 3D lattice filling the Universe, from the vacuum of space to the centre of stars), it causes a distortion in the field. As it moves, the particle will cause the Higgs field to cluster around the particle. The more clustering there is, the more mass the particle will accumulate. Going back to David J. Miller’s 1993 quasi-political description of the Higgs field, his analogy of the number of people attracted to a powerful politician rings very similar to what actually happens in the Higgs field as a particle passes through it (see the cartoon left and below).

Using the cartoon of Margaret Thatcher, ex-UK Prime Minister, entering a crowded room, suddenly makes sense. As Thatcher enters the room, although the people are evenly distributed across the floor, Thatcher will soon start accumulating delegates wanting to talk to her as she tries to walk. This effect is seen all the time when paparazzi accumulate around a celebrity here in Los Angeles; the longer the celeb walks within the “paparazzi field,” more photographers and reporters accumulate.

Pretty obvious so far. The Thatcher analogy worked really well in 1993 and the paparazzi analogy works well today. But, critically, what happens when the individual accumulates all these people (i.e. increase mass)? If they are able to travel at the same speed across the room, the whole ensemble will have greater momentum, thus will be harder to slow down.

The Higgs Boson

So going back to our otherwise massless particle travelling through the Higgs field, as it does so, it distorts the surrounding field, causing it to bunch up around the particle, thus giving it mass and therefore momentum. Observations of the weak force (exchanged by the W and Z particles) cannot be explained without the inclusion of the Higgs field.

OK, so we have a “Higgs field,” where does the “Higgs boson” come into it? The Higgs particle is simply the boson that carries the Higgs field. So if we were to dissect a particle (like colliding it inside a particle accelerator), we’d see a Higgs boson carrying the Higgs field. This boson can be called a Higgs particle. If the Higgs particle is just an enhancement in the Higgs field, there could be many different “types” of Higgs particles, of varying energies.

This is where the LHC comes in. We know that the Higgs boson governs the amount of mass a particle can have. It is therefore by definition “massive.” The more massive a particle, the more energy it has (i.e. E = mc2), so in an effort to isolate the Higgs particle, we need a highly energetic collision. Previous particle accelerator experiments have not turned up evidence for the Higgs boson, but this null result sets a lower limit on the mass of the Higgs boson. This currently holds at a rest-mass energy of 114 GeV (meaning the lower limit for the Higgs boson will be greater than 114 GeV). It is hoped that the high energy collisions possible by the LHC will confirm that the Higgs exists at higher masses (predicted in the mass range of 0.1-1 TeV).

So why is the Higgs boson important?

The Higgs boson is the last remaining particle of the Standard Model that has not been observed; all the other fermions and bosons have been proven to exist through experiment. If the LHC does focus enough energy to generate an observable Higgs boson with a mass over 114 GeV, the Standard Model will be complete and we’ll know why matter has mass. Then we will be working on validating the possibility of supersymmetry and string theory… but we’ll leave that for another day…

But does the Higgs boson give hadrons the ability to feel pain? I doubt it…

Special thanks to regular Astroengine reader Hannah from São Paulo, Brazil for suggesting this article, I hope it went to some way of explaining the general nature of the Higgs boson…