The history of physics has been closely linked to light. From the first astronomers to Newton, Galileo, Einstein … most of the physics discoveries, experiments and developments have included light in one way or another. And the discovery of the Higgs boson, the biggest achievement on the field of physics over the 21st century so far, is not an exemption. But how can be the Higgs boson and light related? It is a subtle but crucial link.
The Higgs boson particle was predicted by the late sixties and it was supposed to be the last piece, let me call it the cornerstone, of the so-called Standard Model. This successful model reconciles both relativity and quantum physics to explain most of the physical phenomena in one simple framework. The Standard Model included not only the efforts of the most remarkable physicists of the past century such as Bohr, Schrodinger, Heisenberg, Pauli, Fermi, Dirac, Feynman but also from Einstein, Maxwell, Faraday, Newton and many others accounting for all the achievements in the history of physics related to electricity, magnetism and the nuclear forces. The model worked perfectly but it lacked of one piece: the Higgs boson.
The Higgs boson is difficult to detect: it is invisible, tiny, quite heavy (for being a particle), rare to produce and … extremely unstable. These characteristics made scientists to chase after it for more than 40 years without success. I think you can excuse them: since it is quite heavy you need to concentrate a huge amount of energy (and that means a huge amount of money as well) to find a single particle; since it is unstable you need to develop concise and advanced searching techniques; since it is rare you need a lot of patience. It seems like a rather impossible task, isn’t it? But wait, there are more than bad news; at least we know how to produce it, how it behaves and where to look for it.
The Standard Model predicts with extremely detail how this particle behaves. Specially, since it is an unstable particle, how it decays into other particles. And this gives us an important hint, a precious lead to hunt down our particle. Let me explain it with detail. The Higgs boson decays in a very short time, around 10-25 seconds. After this time, in average, it disappears giving rise to new particles as a product of its decay. The final state, the result of its decay, is unknown since quantum physics, a statistical theory, plays a relevant role on its fate. But the theory gives us all the different ways it can decay, i.e. the different set of particles that can appear after its decay, and its relative probability. Each of these final states is called a channel. The theory describes that one possibility is that the Higgs boson decays into four electrons, which is called the 4 electron channel. Another possibility is that it decays into two electrons and two neutrinos, or four muons, and so on… Since the probability of each channel is not the same, the Branching Ratio (see Fig. below) gives us the probability of a final state once a Higgs boson is produced. It tells us where to look for it.
For a 125 GeV Higgs boson (that’s the mass of this particle) the final products (channels) are two bottom quarks, two Z bosons, two W bosons, 2 tau leptons and two photons. Actually the latter is one of the so-called golden channels for the Higgs boson. Yes, photons, the carriers of light. And scientists call it golden because it is especially worthy.
It is an important channel since photons, unlike the particles in the other listed channels, are stable particles and the signal they leave on the detector is relatively clean. This make them easy to detect and measure, making it possible to compute their “invariant mass”, a magnitude related with the total mass of the system that is a powerful tool to discriminate from the background signal, i.e. the production of photons from other processes. This is something impossible when neutrinos or quarks are produced in the decay.
These photons are detected by the electromagnetic calorimeter, a device that captures photons and transform their energy into an electric pulse. With this operation we can compute the energy of the incoming photon and the invariant mass. That’s why electromagnetic calorimeters are a key part of any modern particle detector.
On 4 July 2012 the scientists at CERN presented the results of three years of search of the Higgs Boson (see Fig. below). Both ATLAS and CMS experiments showed scientific evidence of the existence of such a particle. As expected, the gamma (photon) channel had an enormous impact with a bump around 125 GeV consistent with the other channels and Standard Model predictions.
Photons, once again in the history of science, are playing a capital role in the discovery and confirmation of a fundamental physics theory.
Javier Santaolalla is a Physicist and Telecommunications engineer. He holds a PhD in particle physics from the Universidad Complutense de Madrid, Spain, with a research project of four years at CERN. He was part of the CMS collaboration where he contributed to the calibration of the muon detector and the study of electroweak processes. Nowadays, he works in outreach and science communication with the group Big Van. He presents the physics YouTube channel Date Un Voltio.