Discovering the Higgs boson was not the end of the story. It was rather the beginning of a new chapter. A bit like discovering a new land in a sea expedition means a whole new terrain to explore.
Is it really the Higgs boson?
The first step of that exploration was to check whether the newly discovered particle was indeed the elusive Higgs boson, or something entirely different. But how to establish the identity of a particle?
Every type of particle is characterized by a set of properties: mass, electrical charge, lifetime etc. For the Higgs boson, mass was the only unknown. For a known mass, all the other properties can be calculated from theory. Measuring them experimentally and comparing them with the result of these calculations allows scientists to verify that they have really found the Higgs boson.
One of the first things to check was “spin”. Spin is a quantum-mechanical property of particles, a form of intrinsic angular momentum. All particles that make up matter – quarks and leptons – have a spin of 1/2, and all the force-carrying bosons have a spin of 1. The Higgs boson is unique in that it has a spin of 0, making it the only known elementary particle with no spin.
The spin of a particle can be established by looking at its decays. The new boson was discovered by observing decays into photons and Z bosons, which already provides a strong constraint: only a particle with a spin of zero or two can decay into both photons and Z bosons. Through careful analysis of angular correlations – patterns in the directions in which the decay products fly off – physicists were able to disprove the spin-2 hypothesis with high confidence, confirming the spin-zero nature of the new boson.
So despite the discovery announcement on 4 July 2012, it took until March 2013 – and two and a half times more data – for physicists to be sure that some kind of Higgs boson had been discovered.
The bulk of the studies performed over the past 10 years have been measurements of Higgs boson interaction strengths with other particles – called “couplings” by physicists – to see whether they match those predicted by theory.
Couplings to different particles correspond to the masses of these particles, since these masses result from interactions with the Higgs field. These couplings are seen experimentally by looking at Higgs boson production and decay: the heavier a particle – and the stronger the coupling – the more likely the Higgs boson is to decay into or be produced from it.
Strengths of the Higgs interaction with other bosons were determined at the moment of discovery by observing its production from gluons and its decay into photons and W and Z bosons.
The next step was to measure the strengths of interaction with matter particles.
The decay of the Higgs boson to pairs of tau leptons was discovered in 2016 through a combined ATLAS/CMS analysis. The coupling to the tau was measured and was found to be compatible with expectations. The next milestone was the measurement of the top quark coupling.
For top quarks, the only way to probe the coupling was through the production process, since the top quark is heavier than the Higgs boson, and, as a result, the Higgs boson cannot decay into a pair of top quarks. Higgs boson production from a fusion of two top quarks was measured in 2018 by observing the ttH process, a Higgs boson being produced together with two top quarks.
The Higgs boson decay into b quarks was also observed in 2018, six years after the discovery announcement, despite being the most likely: 58% of Higgs bosons decay to b quarks. But b quarks are produced in the LHC collisions in such numbers, that a search for a Higgs boson decay faced an overwhelming background, making the measurement extremely challenging.
With the above interaction confirmed, scientists now know that the Higgs field does indeed give mass to the tau lepton, and top and bottom quarks, particles from the heaviest of the three known families, or generations, of matter particles.
The third run of the LHC is going to provide physicists with a wealth of data to deepen the study of this mysterious particle.
Probing the Higgs boson with increasing precision may lead to new paths of research, including new physics. Observing its decay into pairs of second-generation matter particles, such as charm quarks, will help understand more about the Brout-Englert-Higgs mechanism and its mass-giving properties. And time will tell whether the Higgs boson can reveal answers about the evolution of the early universe, and even dark matter.
Read more in “What's next for Higgs boson research?”