Walking, talking, and quacking like a Higgs boson

The ATLAS Collaboration at Europe’s Particle Physics Lab CERN, have reported a study of the Higgs boson, the elementary particle discovered at CERN’s Large Hadron Collider (LHC) in 2012. The results were presented at the biennial International Conference of High Energy Physics (ICHEP), hosted this year by the University of Prague but held entirely online because of the Covid-19 crisis. They have found that the “strength” with which the Higgs interacts with other particles agrees extremely well with the predictions of our best theory, the so-called Standard Model of Particle Physics.

Figure 1:  A depiction of a proton-proton collision in the ATLAS Experiment at CERN’s Large Hadron Collider resulting in production of a Higgs particle and a Z boson.  The Higgs boson decays into two other Z particles; one of the Zs decays into a pair of muons indicated in red and one of the Z bosons into an electron-positron pair shown in green (figure from the ATLAS Collaboration).

In 2012 physicists at CERN discovered a new particle with properties consistent with those predicted for the Higgs boson. In particular one could measure how often the Higgs boson would disintegrate or “decay” into other types of particles. But these and other properties were measured with limited precision, and as the world of elementary particles is large and complex, one could still question whether the new particle was really the Higgs. Over the last 8 years, studies by the ATLAS Collaboration as well as the competitor experiment called CMS have continued to reduce any doubt about whether the new particle is in fact the Higgs boson.  

The latest results from ATLAS for the coupling strengths are shown in Fig. 2 below. In both the upper and lower plots, the data points show on the vertical axis a quantity related to the “coupling strength” of the Higgs to other known elementary particles, namely, the muon (μ), tau lepton (τ), b-quark, W/Z bosons and the top-quark (t), while the horizontal axis indicates the mass of those particles. The dashed line shows the relation between these quantities predicted by the Standard Model. The data points are seen to agree very well indeed, with remaining small discrepancies consistent with estimated measurement uncertainties as indicated by the vertical bars on the points.

Figure 2:  Measurements of the coupling strength of the Higgs to other particles versus the mass of the particle with which it interacts.  The dashed lines indicate theoretical predictions of the Standard Model (figure from the ATLAS Collaboration).

Is the level of agreement enough to prove that the particle we’ve found is the Higgs?  Technically no, there is always some small room for doubt. But one should keep in mind that if the particle were to have some other non-Higgs explanation, then there would be no reason to expect anything like the pattern found in the figure above. So if it walks, talks and quacks like a Higgs, then we can regard this crucial part of the Standard Model to be well confirmed.

To scrutinise subtle signs of deviations of data taken at the LHC with respect to the Standard Model predictions, the Higgs measurements reported in this study can be reconciled in the framework of an Effective Field Theory. The framework helps in understanding how the signatures of new phenomena manifest in our detector even when the new phenomena occurs at distances even smaller than those directly probed by the LHC. This is an active area of work ongoing in the ATLAS Collaboration, stay tuned for updates in the future.

The ATLAS Collaboration is an international consortium of 181 universities, including 5 ESRs from INSIGHTS. Rahul Balasubramanian and supervisor Wouter Verkerke contributed to this analysis. INSIGHTS Scientific Coordinator Glen Cowan (Royal Holloway, University of London), also played a role in the analysis through development of the statistical methods used as well as Chair of its Editorial Board.