The Higgs boson particle seems to have been discovered! But what is the Higgs and why was it predicted?
For every field, a boson
The current explanation of matter and space, the Standard Model, developed in the 1960s, is one of the most successful theories of all time (and space!).
It explains all the approximately 200 elementary particles that exist and how they interact with each other. Some of these particles, the Z and W vector bosons, were predicted by the model and subsequently produced in CERN in the 1980s. Their masses were predicted with 99.99% accuracy using the modification developed by Peter Higgs and others in 1964.
A major failing of the early Standard Model was that it could not predict the observed masses of these particles. Indeed, it seemed to predict that they would be massless, as if you and I were as solid as we in fact are but as light as ghosts. It also can’t explain dark energy and dark matter, which seem to make up about 90% of the universe. Their gravitational effect is evident but they can’t be observed.
The Higgs mechanism explained the masses of particles. In the 1960s, Higgs and others proposed that space is permeated by a field, the Higgs field, that clings to particles, giving them the property that we call mass.
Now, it is a truism in physics that for every field there is a particle called a boson, so it was predicted that there would be a Higgs boson. Bosons transmit the field, carrying some of its energy from one place to another. The prime example of this is the photon, and the discovery of its nature shows why physicists are so keen to discover other field bosons.
In the mid 19th century, building on the discoveries of Michael Faraday, James Clerk Maxwell showed that a changing magnetic field could induce an electric field and vice versa. And, crazily, when he multiplied the two constants associated with the electric and magnetic fields*, he got the speed of light!
This means that the electric and magnetic forces are not different but are aspects of each other. Changes in each are mediated by photons of light or, conversely, light is produced by changes in electromagnetic fields. This led to the discovery of many invisible forms of “light”, such as radio waves, microwaves, and X-rays, with an enormous influence on our lives.
This is an example of the unexpected consequences of much scientific research. Physicists are trying to repeat the triumph of Maxwell in uniting two forces and discovering the particles associated with the force field. This has already been done for the electromagnetic and weak forces, the particles transmitting the electroweak force being the W and Z vector bosons. So far, there has not been any influence on our lives from this unification, and there may not be, but we cannot know where a discovery may take us.
Why use a collider to hunt for the Higgs?
The electromagnetic (EM) force is very strong and infinite in extent: its associated particles, photons, are massless.
They are quite easy to produce and are therefore all around us. The weak force, though responsible for a type of radioactivity, is … weak! It’s about 7,000 times weaker than the EM force and only operates over a very short range — less than the diameter of a nucleus. Its bosons, W and Z, have a lot of mass, about 100 times a hydrogen atom, and they are very rare and short-lived. They can only be produced where there is a lot of energy, such as in a particle accelerator.
They were predicted back in 1968 and produced at CERN in 1983 in the Super Proton Synchrotron. Like the Large Hadron Collider, this smashed protons together at high speeds, converting them into pure energy, which then in a few cases “condensed” into W and Z bosons. These decayed into more stable particles in a characteristic way, enabling scientists to deduce their existence.
It wasn’t just luck that W and Z were discovered at CERN. The Higgs mechanism predicted particular masses for W and Z and it was only with the SPS that sufficient energies would be available to produce particles with these masses. CERN’s 1983 experiments were therefore a test for the Standard Model, which it passed.
The Higgs theory predicted a field and a particle, the Higgs boson. Predicting the mass of the Higgs was not straightforward but eventually most estimates settled on a value about 50% higher than the W and Z masses. Sufficient energy was not available from the SPS or from the Large Electron-Positron collider (LEP) that followed it. This had to wait for the construction of the Large Hadron Collider.
If a Higgs is produced, it is predicted to decay immediately into two Z particles, which will then decay into two muons each. These are easily detected because they behave like electrons, but 200 times heavier.
Both of the Higgs boson-hunting experiments at the LHC see a level of certainty in their data that is apparently worth calling a “discovery”. But they are not yet absolutely certain that what they have seen is a Higgs…
* then took the square root and divided the answer into 1. For those who want to try it themselves, c = 1/(?0?0), where ?0 = 4Ι x 10-7 and ?0 = 8.85 x 10-12. You should get c = 3 x 108 m/s approximately.
*** More: here.
The CERN rap! Explains the LHC in verse.
Thatcher and the Higgs boson
Back in 1993, the Conservative Science Minister, William Waldegrave, challenged physicists to come up with an analogy for the Higgs mechanism.
Professor David Miller of UCL produced the following:
“Imagine a room full of Tory party workers. Mrs Thatcher walks in and the workers near her are attracted and cluster round her, giving her a greater ‘mass’ and making it more difficult to get her moving. The party workers are like the Higgs field.
“Now imagine a rumour passing through the room. The party workers cluster round the source and as the rumour passes the cluster also moves.
“Since the clustering gave Thatcher her ‘mass’, the clusters also have mass: they represent the Higgs boson.”