So has this elusive particle been found? And why is finding it so important?
I asked Alan Barr of Oxford University's Department of Physics, UK physics coordinator for LHC's AATLAS experiment, what the world's highest energy particle accelerator saw, what it means for science, and what life might be like once physicists' Most Wanted has been safely consigned to the particle zoo…
OxSciBlog: Why is the search for the Higgs boson important?
Alan Barr: Over the last hundred years, physicists have studied the subatomic world in exquisite detail, and from their findings have constructed a very beautiful mathematical theory of nature.
This remarkable achievement can be likened to a machine in which each of the cogs is required to have its place to make the whole work. This Higgs particle is the "cog" responsible for mass – it is the physical manifestation of the field which is theorized to give weight to all of the other fundamental particles.
Without that field, the electrons and quarks would be massless, and would zip around at the speed of light.
OSB: What do the latest results tell us?
AB: The latest results show a significant excess of a particular type of event – collisions in which the detector has spotted two high-energy particles of light – photons – which have similar total energies.
That special energy is crucially important, because Einstein tells us that mass and energy are interchangeable. The total energy of the photons is equal to the mass of some new particle (times the speed of light squared).
The Higgs boson is expected to have a very brief lifetime before decaying into other particles, and in particular into a pair of photons. The total energy of the photons – about 126 billion electron volts – should correspond to the mass of the Higgs particle.
OSB: At what point can we say, definitively, if it exists or not?
AB: Physicists require high standards of proof, to ensure that their interesting results are not just lucky – or indeed unlucky – statistical flukes. The level of confidence required is a "five-sigma" excess, which means that there is a less than a one in a million probability that the signal could happen by chance. That's the level of significance that we require to claim discovery of a new particle.
OSB: What more work needs to be done?
AB: Whether or not this particle is the Higgs boson – or even a Higgs boson (many theories propose more than one) will take a little more time to work out. For the moment we have reached what seems to be the top-most summit by knowing that there is a new particle there.
We will next have to survey the ground and find out precisely what that something is. That means measuring its properties – like its spin, and how often it decays in different ways. Only by probing its interactions with the world around it will we know whether or not it is the Higgs particle that many were expecting.
OSB: How will finding/not finding the Higgs change physics?
AB: The full consequences will take years to work out in detail, but some things we can be sure of already. Firstly, the Standard Model has made a far-reaching prediction and has been very much up to the test. That speaks volumes for the power of elegant mathematical theories to describe the intimate details of the inner workings of the universe.
Secondly, it means that there will have to be a programme of detailed measurements of the properties of the particle, to be sure that they do not differ from those that have been predicted. Such subtle discrepancies are predicted in many theories as the hints of a new theory yet to be uncovered.
And finally, the existence of a particle like the Higgs boson asks deep questions about why the universe seems to be so exquisitely finely set-up for us to inhabit, almost as if it had us in mind from the beginning.
OSB: What about life after Higgs: what else could the LHC find out?
AB: If this particle really is the Higgs boson, then we will have precise knowledge about the particles and forces of the Standard Model. That really deserves something of a celebration. But we should be cautious. Remember that, if the astronomers are correct, Standard Model particles make up only about 5% of the contents of the universe.
The "Dark Matter" and "Dark Energy" which are believed to control the evolution of the universe as a whole have never yet been made in the lab. Understanding this other 95% of the universe is an exciting prospect, and is sure to exercise the combined intellects and technological prowess of new astronomers and particle physicists for many decades to come.
With luck, the first clues may come in the next few years, as we turn up the LHC to even higher energies and probe even further into new and undiscovered terrain.