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Tuesday 17 July 2012

Questions the Higgs Boson may or may not answer

In 1964 when Peter Higgs first proposed the mechanism that bears his name, I had just started secondary school. Little did I know that 48 years later I would be a member of the ATLAS experiment celebrating the discovery of a new particle that bears all the hallmarks of the particle predicted so long ago by Peter Higgs and others.

On 4 July 2012 the two experiments, ATLAS and CMS, at the CERN Large Hadron Collider announced that the search for this elusive particle is probably at an end. Over several decades, particle physicists have built up an understanding of what we believe are the fundamental constituents of matter, called quarks and leptons (such as the electron), and the forces that hold them together. These forces are characterised by the exchange of other particles such as the photon that is responsible for electric and magnetic forces. This understanding is encoded mathematically in what we call the Standard Model. The Standard Model makes precise predictions for the behaviour of the particles and has been tested to a very high accuracy.

ATLAS Experiment © 2008 CERN

However, the original Standard Model only makes sense mathematically if all the particles have zero rest masses. Were this true they would all travel at the speed of light and the formation of stars, galaxies and us would have been impossible. In fact we now know that all particles except the photon have rest masses, including neutrinos which were thought to be massless until recently. Peter Higgs and others got around this by proposing what is now called the Higgs Mechanism to allow the particles to have mass whilst still keeping the Standard Model mathematically self-consistent. The Higgs Mechanism suggests that the whole of space is permeated by a 'sea' of Higgs particles that stick to the other particles and slow them down, effectively giving them mass. Higgs particles stick to heavy particles like the top quark a lot, but not to the photon at all and so it remains massless. The theory predicted that with sufficient energy we should be able create these Higgs particles in an accelerator and watch them decay as we now appear to have done.

So that's it then?

Well no, of course not! First we have to establish that the new particle really is the Standard Model Higgs and not some exotic variation or something completely different. To do this we will have to measure very carefully how the particle decays and the number of different ways it decays. These are absolutely predicted by the model and so any variation will indicate that it is something else. However the first signs are that it is consistent with the Standard Model Higgs.

The CMS detector © CERN

Then we can say that the Standard Model is complete and that's it?

Well yes and no. The Standard Model as such will be complete but unfortunately the Standard Model doesn't answer all the questions or explain all of the universe! In fact, there is still a big problem with the Higgs itself. Because the Higgs 'sticks' to the other particles to give them mass, they should stick to each other and get heavier and heavier, far heavier than the new particle observed at CERN. But if they were that heavy the mechanism wouldn't work. In order to get out of this conundrum we need to go to theories 'Beyond the Standard Model'. One such model is called 'supersymmetry' which contains new particles that can cancel out the effect of the Higgs sticking together and maybe solve other problems such as dark matter. ATLAS and CMS have been already searching for evidence of supersymmetry but so far have not found any, but the hunt has really only just started.

Apart from that there are a host of other questions that the Standard Model doesn't address such as why are there 6 quarks and 6 leptons when we only seem to need 2 of each? How does the proton know it has to have exactly the same electric charge as the electron although it is made of different things? The Standard Model doesn't include gravity which is the one force most of us feel, especially in the morning! How can that be included? What are the so called dark matter and dark energy that seem to make up 96% of the universe?

Now that it looks like we have found the Higgs particle and we are clearly on the right track with the Standard Model we can start to try and answer some of these other questions. We hope that there is a rich landscape at the Large Hadron Collider that we have only just started to explore and that the recent announcements at CERN will be the first of many such events!

This guest blog was submitted by Prof. Steve Lloyd, Professor of Experimental Particle Physics at Queen Mary University of London.

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