What is the Higgs boson and why does it matter?

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As the world awaits news of the possible discovery of the Higgs boson, there remains a lot of confusion about what it is, why we have had to work hard to find it ? and why we should care. Here's why.

First, the short answer:??????????

If the Higgs is discovered, it will represent perhaps one of the greatest triumphs of the human intellect in recent memory, vindicating the construction of one of science's greatest theories and the most complicated machine ever built. That's the good news.

But if the Higgs is all that is found at the Large Hadron Collider (LHC), a huge amount will remain to be discovered. Crucial experimental guidance that physicists need to understand fundamental questions about our existence ? from whether all four forces in nature are unified in some grand theory to determining what may have caused the big bang ? will still be absent. Answering these questions may be beyond our technical and financial capabilities in this generation.

Now for the long answer:???????????

If our ideas about the Higgs boson turn out to be correct, then everything we see is a kind of window dressing based on an underlying fabric of reality in which we shouldn't exist. The particles that make us up ? which bind together to form protons, neutrons, nuclei and ultimately atoms ? have mass. Without the Higgs, these particles would be massless, like photons.

We all know from our own experience that how heavy something feels depends on where it is located. For example, objects that are heavy on land appear lighter in water. Similarly, if you try to push a spoon through treacle it appears heavier than if you push it through air.

The standard model of particle physics implies that there is a "Higgs field" that permeates all space. This field interacts with particles, and does so with varying strengths. Particles that interact more strongly experience more resistance to their motion and appear heavier. Some particles, such as photons, do not interact with the field at all and remain massless.

In this way, the mass of everything is determined by the existence of the field, and mass is an accident of our circumstances because we exist in a universe in which such a background field happens to have arisen.

Playing subatomic catch

But why a Higgs particle? Relativity tells us that no signal can travel faster than light. Incorporating this into quantum mechanics tells us that forces which we think of as being due to fields are actually transmitted between objects by the exchange of particles. The way particles transmit forces is a bit like a game of catch: if I throw a ball and you catch it, I will be pushed backwards by the act of throwing and you will be pushed backwards by the act of catching. Thus we act as if we repel each other.

So if there is a Higgs field, it turns out that there has to be a particle associated with this field, and this is the Higgs particle.

This seems a fanciful framework, rather like imagining angels on the head of a pin. What would drive scientists to imagine such a scenario? One of the greatest successes of the past 50 years was the unification of two of the forces of nature: electromagnetism and the weak interaction. In this "electroweak" theory, electromagnetic forces arise by the long-range exchange of massless photons, and the short-range weak force is due to the exchange of massive particles called W and Z particles, predicted in the 1960s and discovered in the 1980s at CERN, the European particle physics laboratory near Geneva, Switzerland, which is now the home of the LHC.

In order for this theoretical unification to make mathematical sense, all three particles have to be massless in the underlying theory, and therefore the forces they mediate would be almost identical. Only if the W and Z particles obtain a mass by interacting with a background field ? the Higgs field ? will the underlying unified theory explain why the two forces appear different at the scales we measure them today, while remaining mathematically consistent.

High mass

Theory suggests that the mass of a Higgs particle should be about 100 times the mass of the proton; however, the exact mass is not predicted.

For over 25 years since the discovery of the W and Z particles, experimental physicists have been trying to build particle accelerators with the energy necessary to produce a Higgs particle, if it exists. The Tevatron accelerator at Fermilab in Batavia, Illinois, was able to reach up to about 120 times the mass of the proton (about 120 gigaelectronvolts) but did not find the Higgs.

The LHC was designed to probe for Higgs masses heavier than this. If the Higgs particle is announced with a mass of 125?GeV, as the rumours suggest, it will be the crown jewel of our theoretical understanding of the electroweak unified theory, our own origins and the origin of almost all mass we measure in the universe.

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