1. What is Higgs theory anyway? The search for the Higgs boson was the top priority in Particle Physics for years – but why? The 'Standard Model' describes forces as being due to the exchange of particles. These are shown in this table: The Weak force is different from the others because its particles have mass. But the equations describing the forces want all the particles to be massless. The mass of the W boson can be calculated, once the strength of the force and mass of the other particles is known. We can check this! The theory predicts a new particle, the Higgs Boson. It is unlike any seen before: ● Interactions with other particles are proportional to their masses ● It is not spinning, unlike all other particles. Well, no, not really. The cartoons at the top show the Higgs field being formed. While the little ball stays balanced in the middle the field is massless. But when it inevitably rolls down the field becomes dense – really dense. The Higgs field filling all of space has a density far higher than that of lead; more like a neutron star. We know it is there because the strength of the Higgs decay H→ZZ shows 'weak charge' being absorbed by the field. But it should have a gravitational pull; the universe ought to be collapsed by the mass of this field, while instead it is expanding. We do not understand this. We are still looking for a complete theory of gravity and particle physics. But it is clear that the Higgs boson plays a big part in that story. The clinching evidence came on 4 th July 2012 with the announcement of a Higgs boson by ATLAS and CMS at the LHC. One plot with a 'bump' of events from the Higgs boson at 126 GeV is shown below. This particle is new and hard to study but already we see a pattern of interactions proportional to particle masses emerging. Decays to W,Z and γ all look as they should, and there is weaker evidence for decays to “matter” particles like the b-quark. By studying angular distributions we can ask if it is spinning: the answer seems to be no, as predicted. Is the W boson mass right? Predicted: 80.359 ±0.011 GeV/c 2 Measured: 80.385 ±0.015 GeV/c 2 The difference is three parts in ten thousand, and they agree within the errors quoted. The prediction is shown below, as a function of the top quark mass. Measuring these has been a huge effort but the result gives strong confidence in the Higgs theory. The Higgs (et al.) theory says the whole universe is filled with a sea of weak force – empty space is really full. We hardly notice because we are in it all the time. The W and Z boson are strongly affected and become heavy while the electron is just given a small mass. This sea, called the Higgs field, interacts with all the matter particles in it too, by different amounts. We see that as their mass. The problemThe solution? How to test it? Check the W mass So we know it all? Check the W mass Find the Higgs boson How does the Higgs field work? Imagine a room full of politicians, and the prime minister trying to walk across it. He is slowed down by all the people trying to talk to him. But he cannot stop either; the people with him are walking on. He has gained mass The same politicians will form a group to discuss a rumour. This is just like the Higgs field can produce a Higgs boson if given the energy. Higgs' prediction