Q&A: The Higgs boson

Scientists at the Large Hadron Collider (LHC) have discovered a new sub-atomic particle consistent with the long-sought Higgs boson.

The particle's confirmation would stand out as one of the great scientific achievements of the 21st Century so far.
But what exactly is the Higgs boson, and why have particle physicists spent more than 40 years searching for it?

What is the Higgs boson?

The Higgs so far definitively exists only in the minds of theoretical physicists. There is a sturdy theory for how much of the Universe works - all of the particles that make up atoms and molecules and all the matter we see, most of the forces that direct them, and a small zoo of more exotic particles. This is called the Standard Model. However, there is a glaring hole in the theory: it does not explain how it is that some of those particles gain their mass. The Higgs mechanism was proposed in 1964 by six physicists, including the Edinburgh-based theoretician Peter Higgs, as an explanation to fill this hole.

What is so important about mass?

Professor Jim Al-Khalili explains what the Higgs boson is and why its discovery is so important

Mass is, quite simply, a measure of how much stuff an object - a particle, a molecule, or a Yorkshire terrier - contains. If not for mass, all of the fundamental particles that make up atoms and terriers would whiz around at light speed, and the Universe as we know it could not have clumped up into matter. The Higgs mechanism proposes that there is a field permeating the Universe - the Higgs field - that allows particles to obtain their mass. Interactions with the field - with the Higgs bosons that come from it - are purported to give particles mass. This is not unlike a field of snow, in which trudging through impedes progress; your shoes interacting with snow particles slows you down.

How do scientists search for the Higgs boson?

Ironically, the Standard Model does not predict an exact mass for the Higgs itself. Particle accelerators such as the LHC are used to systematically search for the particle over a range of masses where it could plausibly be. The LHC works by smashing together two beams of the sub-atomic particles called protons at close to light-speed. This generates a vast shower of particles that are only created at high energies. The Higgs will probably never be observed directly, but scientists at the LHC have been looking for a Higgs that fleetingly exists in this soup of particles. If it behaves as researchers think it will, it should decay further into yet more particles, leaving a trail that proves its existence.

It is not the first machine to hunt for the particle. The LEP machine, which ran at Cern from 1989-2000, ruled out the Higgs up to a certain mass, and until it was switched off in 2011, the US Tevatron accelerator searched for the particle above this range. On Monday, the Tevatron team released their final analysis, which tantalisingly points to a particle much like the one that the LHC data suggests.

Experiments at Cern - the Large Hadron Collider at work

LHCb ATLAS ALICE CMS

When will we know if we have found it?

Particle physicists are a notoriously conservative bunch when it comes to saying they have found something. If you flip a coin 10 times and get eight heads, you might think the coin is somehow "loaded". But only after hundreds of flips can you say so with the kind of certainty that physics requires for a formal "discovery".
The first hurdle is to definitively nail down the particle's mass - showing up as a kind of "bump" in the data - and that part looks to be just around the corner. What is next is to make sure that it behaves as the theory predicts it should - how it interacts with other particles and in turn decays in to yet more particles. This is very much the frontier of high-energy physics and a complete and certain entry into the Standard Model is probably a long way off yet.

And what then?

Most professional physicists would say that finding the Higgs in precisely the form that theory predicts would actually be a disappointment. Large-scale projects such as the LHC are built with the aim of expanding knowledge, and confirming the existence of the Higgs right where we expect it, while it would be a triumph for our understanding of physics, would be far less exciting than not finding it. It is those kinds of surprises that have led to great revolutions in science.
Rest easy, though - if the trend continues and this simplest version of the Higgs takes pride of place in the Standard Model, many big questions remain. After all, the Standard Model explains matter as we know it, but there is much reason to believe that matter only makes up 4% of the observable Universe. The rest - dark matter and dark energy - may prove even harder to pin down. It is as if we are near to completing one side of a Rubik's cube and being reminded the other five are all a jumble.

The Standard Model and the Higgs boson

• The Standard Model is the simplest set of ingredients - elementary particles - needed to make up the world we see in the heavens and in the laboratory
• Quarks combine together to make, for example, the proton and neutron - which make up the nuclei of atoms today - though more exotic combinations were around in the Universe's early days
• Leptons come in charged and uncharged versions; electrons - the most familiar charged lepton - together with quarks make up all the matter we can see; the uncharged leptons are neutrinos, which rarely interact with matter
• The "force carriers" are particles whose movements are observed as familiar forces such as those behind electricity and light (electromagnetism) and radioactive decay (the weak nuclear force)
• The Higgs boson came about because although the Standard Model holds together neatly, nothing requires the particles to have mass; for a fuller theory, the Higgs - or something else - must fill in that gap