This Tuesday is an important day at Cern, the European Organisation for Nuclear Research. The scientists at the Large Hadron Collider (LHC) will present the latest results on the search for the Higgs boson, the fabled particle with the big job of explaining how nature's elementary particles acquire mass. The collider has been built to teach us all about how the tiny particles that make up everything in the universe behave. At 27km in circumference it represents the biggest, most powerful, microscope in the world – zooming in to reveal the goings-on at distances tiny compared even to the size of a single proton. At these femtoscopically small distances, we have very good reason to expect great things: either we will see a Higgs particle or we will see something else. Seeing nothing new is simply not an option.
So what makes us so sure the LHC is in a win-win situation? The "standard model" theory of particle physics is as dazzlingly ambitious as its name is bland – its remit is to explain how every single particle in the universe interacts with every other (with the sad exception of interactions due to gravity) and it does so using some simple rules that can be sketched on the back of an envelope. The standard model reveals that apparently different phenomena are really different aspects of the same thing (radioactive decay and electricity are linked, for example). Thus it is that the world we see, with all its diversity and complexity, is made out of just a handful of elementary particles hopping around according to some simple rules.
Underpinning all of this is the notion of "symmetry". It is symmetry that allows us to specify the rules that constrain the behaviour of the particles and it is the presence of symmetry that often moves physicists to speak of beauty. Symmetry implies that there are underlying patterns in nature that constrain the way particles behave. Knowing the symmetries in nature is key to explaining how the particles move around.
How does this all relate to the Higgs particle? Well, the symmetries of the standard model have been tested to Nobel prize-winning accuracy over the past four decades. But there is a huge fly in the ointment because the simplest realisation of the symmetries does not allow particles to have any mass. The Higgs particle was introduced to solve just this problem – and it works by cramming empty space full of Higgs particles. As particles move through apparently empty space they bounce around off the Higgs particles, zig-zagging their way along – the more they zig-zag, the more mass they have.
So, the Higgs particle saves the day and allows us to understand why the universe is both beautifully symmetric and made of massive particles. It is wise to play the role of the cynic and we might take the view that mass "just is" and that all those wonderful discoveries built on symmetry were just good luck. In other words, we could ask what happens if we take the standard model and reject everything that relates to the Higgs particle. Crucially, doing that does not work – we would be left with a car-crash of a model whose predictions are gibberish at the sub-femtometre scales probed by the LHC. This means that whatever happens we are going to need to dream up something new about the world.
That is what it means to say that the LHC is in a win-win situation: experiments in science rarely get such comfort. This something new could be the Higgs particle. But really, that is only a guess as to how things might work out. It has the virtue of being economical (we get to explain the origin of mass using only one new particle) but economy isn't much of a virtue and nature could be different.
The LHC experimenters are closing in on the standard Higgs particle. We already know enough to say that the results on Tuesday will either reveal its existence or almost exclude it. "Almost" because there will probably not be sufficient data to rule out a Higgs particle with a mass not much larger than 120 times the proton mass. But even that hiding place will be eliminated in 2012 and, by the end of next year, we should have either discovered the standard Higgs particle or decisively excluded it.
I have been waiting more than 20 years for this. Personally, I am most excited by the possibility that there is no Higgs particle and that nature has chosen a different path. If that is the case, then we are going to have to be patient for a little longer. It will be worth the wait.
Jeff Forshaw is a professor of theoretical physics, University of Manchester, and co-author with Brian Cox of The Quantum Universe: Everything That Can Happen Does Happen (Allen Lane)