So, how do I feel about the Higgs discovery? Am I excited, indifferent or even just a little disappointed? Before CERN’s announcement on the 4 July 2012, I had asked myself on many occasions whether I hoped the Higgs would be discovered or not. After all, if there were no such thing as the Higgs field, or Higgs mechanisms that supposedly gave particles their mass, and hence no Higgs Boson (the particle that is no more than a brief condensation of Higgs field energy) then we would need to revise our theories of the subatomic world… and that would be pretty exciting. Well, it seems like that won’t be necessary (for now) because experiments have confirmed what theory predicted all along.
The seminar room at CERN listening to the announcement of the Higgs discovery on 4 July 2012. Image courtesy of CERN
So how important was the discovery of the Higgs? Like all other physicists, I am slowly getting used to the idea that pervading the whole of space there really is an invisible and, until 4 July 2012, undetected mysterious presence known as the Higgs field. It affects the Universe, as far as we know, only at the tiniest scales through the way it allows subatomic particles to move through it. Some, like photons (the particles of light), pass through as though it wasn’t there at all and some, like neutrinos, only feel it very weakly, while others move through it as though it were treacle and so are unable to travel very fast. The way we see this is through these particles having different masses. Thus the heaviest quarks, the Top and Bottom, have their relatively large masses because they interact strongly with the Higgs field, whereas the lightest quarks, the Up and Down (that make up all the stuff we see in the Universe) couple relatively weakly with the field.
The data from CMS, one of the two experiments at the LHC. It shows the famous 'Higgs' bump at 125 GeV. Image courtesy of CERN
So, it is really the Higgs field and the way it interacts with matter that is important, not the Higgs boson. That is simply a very short-lived lump of energy that is the particle manifestation of the field – it doesn’t have a size or a shape. When CERN announced they had discovered the Higgs boson they did not mean they had ‘seen’ one. What they meant was something rather more obscure. Because this particle only lives for a tiny fraction of a second before it decays into something else, it is these products that experimenters were looking for; and billions of these decay products are created in the high-energy collisions of the two proton beams in the LHC. So, how do they know which ones might have been produced by a Higgs particle. Well, what they do is record all the candidate particles that may have been produced by a Higgs boson created in the mêlée and plot the number of such events against their energy. This plot is called a cross section and shows whether, at certain energies, there is a sharp rise in the number of events. This means that there is a certain narrow energy range (corresponding to presence of a Higgs boson) when these decay products are produced in more abundance that at other energies. This manifests itself as a bump in the cross section. The sharper and more defined the bump, the more likely it is that that energy is special. So when physicists talk about confidence levels and sigma values they are referring to the sharpness and clarity of this bump in the cross section. And they are very very confident.
One final point is that the particle physicists at CERN were very careful in their statement to say that although the evidence was overwhelming of the discovery of a new particle, they could not yet be sure that it was the Higgs boson. But I reckon they are just being very cautious for now, and rightly so. But in my view, it if looks like the Higgs, smells like the Higgs and is exactly where they expected to find the Higgs, then it is the Higgs.
This visualization shows the separation between dark matter (blue) and ordinary matter (red) in a collision of two galaxies in the Bullet Cluster. Credit: NASA/CXC/CfA/STScI
So what next? Are we going to have to wait another twenty or so years before the next big discovery – before the next vital missing piece in the jigsaw is found? I hope not and I doubt it very much. The past few decades have been about designing, building and testing the Large Hadron Collider and its giant detectors that are used to capture and record the many billions of subatomic collisions produced inside them. In reality, it has only taken a couple of years to find the Higgs boson since the experiment began in earnest. With the Higgs confirmed, this also gives us confidence and strong hints as to where to look to answer the next big question. And for many, that is to find the particles that are believed to make up dark matter.
We have known for decades that galaxies would simply not hold together were it just for the gravitational pull of all the matter we see in them: the stars, planets, dust and gas. There had to be some other invisible form of matter that contributed the rest of the gravitational glue that stopped the stars from floating away from each other. It is thought that there is much more of this dark matter than all the matter we can see. One strong possibility is that dark matter is made up of a type of particle that has so far eluded detection. And the LHC is perfectly placed to find this particle if it exists. The discovery of the Higgs gives up confidence and points us in the right direction to look for this particle.
I just hope we are surprised. It’s all very well finding the Higgs and confirming that our current theories are on the right tracks, but it would be so much more exciting were we to find something completely unexpected.
Let’s wait and see.
Jon Butterworth on Why Does the Higgs Decay? [Guardian Article]
What would happen if you put your hand in the LHC? [60 Symbols Video]
The ATLAS Boogie (How we really find the Higgs!) [YouTube Video]