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What is a Higgs boson and did CERN find one or not?

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For the past fifty years physicists have had an 'almost' complete theory that predicts how elementary particles interact that has become known as the 'standard model'. There was just one lousy little problem: Electrons have mass and the simple version of the standard model could not explain why this is so.

Over the years many people have proposed theories that attempted to explain how certain particles have mass. Most of which have fallen by the wayside as they have been disproved by experimental results. The Higgs boson is a particle whose existence is predicted by a particularly elegant theory which would explain most of the missing pieces in the standard model.

Contrary to the press version of the experiment, the Higgs boson does not give particles mass, nor does it explain why they have mass. Rather the existence of the Higgs boson is predicted by the Higgs mechanism which in turn explains why electrons and certain other particles can have mass.

Adding the Higgs mechanism to the 'standard model' of particle physics provides a complete theory of quantum mechanics that is consistent with (almost) all the experimental observations to date. That does not necessarily make the theory 'true' but it certainly makes it a useful theory.

So did CERN find one?

The short answer is that what CERN has observed is consistent with the existence of a Higgs boson but that the number of candidate 'Higgs events' falls short of the number required to announce the result with more than about 50% certainty.

A slightly longer answer is that experiments like ATLAS and CMS produce results that are ambiguous. The particle is highly unstable lasting only a fraction of a second before it decays. The experiments cannot see the particle directly, they can only see the debris left when the particle decays. A single event has multiple explanations. The decay of a Higgs particle might have given rise to a particular set of tracks but so might many other processes. The only way to be sure is to perform the experiment over and over again until there are enough events to distinguish statistically.

If you are interested in the real physics here, I found this pice by David Derbes to be a pretty useful primer on the subject [H/T Atrios.]

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