July 4th 2013 was the first anniversary of the announcement by the ATLAS and CMS collaborations of the discovery of a Higgs-like particle. Now is a good point to reflect on what we now know about the Higgs, what we have yet to find, and what else our experiments are working on.
To mark the anniversary, UK physicists were manning a stand at the Royal Society Summer Exhibition. The Exhibition is rather like a University open day but in the grand setting of the Royal Society headquarters in London, with about 30 select research teams presenting current work in science and technology. I recommend it highly – especially if you are a scientifically curious teenager. Our mission was to try to put over the ideas that lead to the existence of the Higgs boson being proposed, and to explain why it took over 40 years from the Higgs particle proposal to its discovery. If you want to know more about it, look at our booklet, which also introduces a few of the very many people who worked on the discovery.
The Higgs mechanism was proposed to fix a problem in the hugely successful ‘Standard Model’ of particle physics. While it accounts for the fundamental particles that build up our world, and their interactions, it rather embarrassingly only works if all the particles are massless. Peter Higgs and others proposed a way of solving that problem. They suggested the universe is filled with a field we call the Higgs field, and that some particles interact with it to generate their masses. Those particles will only accelerate a little when a force is applied, almost as if the field is ‘sticky’, and so appear massive.
The proposal was elegant – but science requires evidence. Higgs pointed out that if the field exists then there should be a new particle associated with it. The particle would have a distinguishing property; while all the other fundamental particles we know act like a quantum mechanical version of a spinning top, and so have a preferred direction in space, the Higgs would not, as this direction would mean that the masses of particles would depend on their direction of motion. The Higgs would be the first fundamental spin-less particle. This gave a clear behaviour you can test for.
Our next question was the mass of the particle, telling us how powerful our accelerators need to be. Sadly, the theory could not predict that. This was one reason why the quest for the Higgs took over 40 years; the earliest searches were at accelerators that lacked sufficient energy. From the 1990s, an accelerator near Chicago did have enough energy to make a Higgs – but at such a low rate they would have had to take data for many years before they had enough evidence for it.
Large Hadron Collider(LHC) has both sufficient energy and rate of collisions. In a little over two years, ATLAS and CMS had enough Higgs’s to be able to say they saw evidence for Higgs-like decays. These sat above similar background decays from other processes, but with high enough statistics to rule out a cruel random fluctuation; and what is more, the excess was seen in several very different decays, consistent with a particle of the same mass in each case, and in both experiments.
We continued to take data until Christmas 2012. For some channels, in the absence of a Higgs, the observed excess would only occur randomly in approximately one in a billion experiments. We have clear evidence in decays to so-called “bosons”, which carry forces; we are now getting to the point where we expect to see evidence for the decays to “fermions”, which are the particles that make up matter. The Lancaster group is looking for the decays of the particle into “tau-leptons” for example, and also the way it interacts with charm quarks.
Importantly, the decays very strongly suggest the particle has no spin. For this reason, we now claim we have seen “a” Higgs; but questions still remain. Many theories suggest there could be a whole family of these particles, the others being harder to find because they have higher masses. While the rate at which the different decays occur is roughly consistent with the expectations, a lot more data is needed. Most importantly, the Higgs should also interact with itself. To confirm if that is the case will require upgrades to the Large Hadron Collider, and possibly a new collider smashing electrons and anti-electrons together, as has been proposed in Japan.
Other questions remain. Lancaster physicists are addressing how the universe today is mainly matter, with antimatter thankfully rare. In the early universe, matter and antimatter were produced in almost equal quantities, and the physics we know should have left far less matter in the universe than we see. We are trawling the ATLAS data to see evidence for processes that favour matter over antimatter, exhibiting so-called CP violation. These studies can give hints of particles too massive to be made directly in our collisions. We are also involved in searches for particles that may help explain the mysterious “dark matter” in astronomy that makes up the majority of the matter in the Universe.
One year on, the Higgs has become part of our everyday work. It still poses challenges and interesting questions, but we hope it is just a step on the way to even more interesting discoveries and understanding when the LHC returns with twice the energy in 2015.
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