Random Thoughts on the Universe

Friday, September 05, 2008

The Higgs Mechanism

With the LHC soon to come alive, and the very good possibility that it will discover the Higgs boson, we thought it would be a good opportunity to explain the Higgs mechanism...

In the 1960's particle physics had a problem with the mass of particles. Electromagnetic forces can be derived from requiring that the laws of physics satisfy certain symmetries (called gauge symmetry) and everything works out fine. When you add the weak nuclear forces - which are responsible for decays of nuclei among other things, you can still use gauge symmetries to derive the laws of physics.

Except for one problem - in order to satisfy the gauge symmetry the particles that are responsible for the force must be massless. And while the photon in electromagnetism is, the three particles that cause weak nuclear forces are extremely heavy! Something is very wrong in a theory which requires massless particles and is known to have three of the four heaviest known particles.

The solution is the Higgs mechanism (which was actually developed by two other people as well but Higgs got the credit and the fame). This model is very beautiful mathematically, but it is hard to translate into laymen terms.

When there are no Higgs particles around, all particles are massless and so gauge symmetry is preserved in the theory. But the Higgs is an odd creature, because it actually requires less energy to create one than to leave space empty. And so they start popping into existence until the entire Universe is filled with Higgs particles (but don't worry - when a certain density of Higgs are formed, they prevent further creations so we aren't overwhelmed by them).

In the Higgs model, particles that interact with the Higgs will have trouble moving through this 'Higgs soup' and so it requires more force to move them. This resistance to movement is what we observe as mass.

And so the end result is a theory has massless particles and satisfies gauge symmetry, and a real-world where there are so many Higgs particles colliding with matter that there appears to be mass. It really is quite a beautiful idea.

The only catch is that we have never seen a Higgs particle in the lab. Because the Universe is filled with them, we have nothing to compare with! The best hope right now is the Large Hadron Collider, which should have enough energy to either force the creation of a few extra Higgs particles so that we can study the increased density compared with the average density of Higgses, or to force the destruction of a few Higgs particles so that we can compare the lower density with the average density.

Either way it will require a huge amount of energy and very accurate detectors. But whoever discovers it first will forever change the Standard Model of particle physics...

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