The Higgs boson
A landmark discovery
What is the Higgs boson and why does it matter?
Physicists describe particle interactions using the mathematics of field theory, in which forces are carried by intermediate particles called bosons. Photons, for example, are bosons carrying the electromagnetic force. In 1964, the only mathematically consistent theory required bosons to be massless. Yet, experiment showed that the carriers of the weak nuclear interaction – the W and Z bosons – had large masses. To solve this problem, three teams of theorists: Robert Brout and François Englert; Peter Higgs; Gerald Guralnik, Carl Hagen, and Tom Kibble independently proposed a solution now referred to as the Brout-Englert-Higgs (BEH) mechanism.
The BEH mechanism required the presence of a new field throughout the universe which gave mass to some of the bosons. Existence of this field could be verified by discovery of its associated particle – the Higgs boson. On 4 July 2012, the ATLAS and CMS experiments at CERN announced that they had independently observed a new particle in the mass region of around 125 GeV: a boson consistent with the Higgs boson. On 8 October 2013, the Nobel Prize in Physics was awarded jointly to theorists François Englert and Peter Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider".
The discovery of the Higgs boson opened up whole new windows in the search for new physics, since its properties are predicted to be different in different theoretical models. Supersymmetry, for example, predicts the existence of at least five different types of Higgs bosons. Will the Standard Model continue to survive the precision measurements of the LHC or will an improved model appear? Only through continued study will physicists be able to answer this question.
What have we learned since discovery?
After discovery, physicists began to study the properties of the newly-discovered particle to understand if it was the Standard Model Higgs boson or something else. First came the confirmation of the mass of the Higgs boson: the final unknown parameter in the Standard Model. This was one of the first parameters measured and found to be approximately 125 GeV (roughly 130 times larger than the mass of the proton). With this mass, the Higgs boson could decay several different ways.
In the Standard Model, the Higgs boson is unique: it has zero spin, no electric charge and no strong force interaction. The spin and parity were measured through angular correlations between the particles it decayed to. Sure enough, these properties were found to be as predicted. At this point, physicists began to call it “the Higgs boson.” Of course, it still remains to be seen if it is the only Higgs boson or one of many, such as those predicted by supersymmetry.
Subsequent studies by ATLAS and CMS have found that the Higgs boson interacts with both bosons and fermions (particles that make up matter), confirming the prediction by the Standard Model that all elementary particles acquire mass via the all-pervasive Higgs field. The stronger a particle interacts with the Higgs field, the heavier is. Physicists have also been studying the strength of these interactions, which could be impacted by new physics beyond the Standard Model.
The first direct probe of fermionic interactions was to tau particles, which was observed in the combination of ATLAS and CMS results performed at the end of Run 1. During Run 2, the increase in the centre-of-mass energy to 13 TeV and the larger dataset allowed further channels to be probed. Further, the Higgs boson has been observed decaying to bottom quarks and being produced together with top quarks. This means that the interaction of the Higgs boson to fermions has been clearly established.
One of the neatest ways to summarise the current understanding of the Higgs boson is by comparing its interaction strength to the mass of Standard Model particles (see figure on left). This clearly shows that the interaction strength depends on the particle mass: the heavier the particle, the stronger its interaction with the Higgs field. This is one of the main predictions of the BEH mechanism in the Standard Model.
Physicists are not only trying to verify that the properties of the Higgs boson agree with those predicted by the Standard Model – they are specifically looking at what properties would provide evidence for new physics! For example, constraining the rate that the Higgs boson decays to invisible or unobserved particles provides stringent limits on the existence of new particles with masses below that of the Higgs boson. They are also looking for Higgs boson decays to combinations of particles forbidden in the Standard Model. So far, none of these searches have found anything unexpected, but the search is still on!