The Higgs boson

After physicists at CERN reveal “tantalising hints” of the Higgs boson, we take a look at the particle that's expected to be finally found in 2012.

Topic of the moment: the Higgs boson

What is it?
Put simply, the Higgs boson is thought to be the elementary particle responsible for the existence of mass.

The concept was introduced into particle physics in the 1960s as a means of solving the problem of why some force-carrying particles have mass but others don’t.

In the “Standard Model” of particle physics, the electromagnetic force is carried by photons, which are familiar to us as particles of light, and the weak nuclear force is carried by particles called W+, W- and Z bosons.

Abdus Salam, Sheldon Glashow and Steven Weinberg found that the electromagnetic and weak forces are different manifestations of a single phenomenon – at an energy of around 100 GeV, they unify into what is known as the electroweak force. However it was not understood why the photon is massless and the W and Z particles are massive.

What is now known as the Higgs mechanism was proposed by Peter Higgs and others as a way of explaining why this should be the case.

In the Standard Model, quantum numbers such as electric charge are dependent on 'coupling' to the appropriate field – in this case the electromagnetic field. The Higgs mechanism introduces another – the Higgs field – into the theory. Particles that couple to this field gain a mass, while those that don’t couple to it will remain massless.

The Higgs particle is a quantum of the Higgs field in a similar sense to a photon of light being a quantum of an electromagnetic field.

There are also some versions of the Higgs mechanism in which there is a field but no particle.

Why look for it?
Although incomplete – for example it excludes gravity and does not allow for the possibility of neutrinos having mass, for which there is some experimental evidence – the Standard Model has made a range of successful predictions.

It predicted the existence of several other particles such as gluons, the W and Z bosons, and certain types of quark before they were experimentally observed.

The Higgs is the last of the particles whose existence is predicted by the Standard Model that has not yet been observed.

Stephen Hawking has previously suggested that it will be a more interesting result if the Higgs is not found, as physicists will need to start again from scratch in developing a correct model of fundamental particles.

But if it is found it will serve as further evidence backing up the Standard Model.

How does the search for it work?
It's thought that the mass of the Higgs – which isn't predicted by theory and has to be determined by experiment – is very high, so the particle would take a huge amount of energy to produce. This is just one of the main aims of the Large Hadron Collider at CERN, the most powerful particle accelerator yet built.

The LHC collides extremely high-energy beams of (mainly) protons and antiprotons. When particles make contact with their antiparticles, they annihilate each other and leave only energy. This then produces a shower of particles, and the Higgs is expected to be among these – if the energy is high enough.

The particle detectors in the LHC are set up to look for the signature of the Higgs emerging from this shower.

Earlier experiments at CERN and at the Tevatron in the US have excluded a mass above 285 GeV  and in the range 149–206 GeV.

CERN's announcement of December 2011 hints of the Higgs at around 124–126 GeV. However the signal at this mass level was at best 2.6 standard deviations above the mean of the background fluctuations, whereas a discovery is only considered to have been made when it reaches five standard deviations – the much-hyped “five sigma” level.

So when will we find it?
CERN’s director-general Rolf-Dieter Heuer has said that we should know by the end of 2012 whether the Higgs exists or not. It promises to be an interesting year – one way or another.

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