How standard is the Higgs boson discovered in 2012?

Without a doubt, it is a Higgs boson, but is it the Higgs boson of the Standard Model? Run 2 of the LHC find out, says theorist John Ellis


The highlight of the first run of the Large Hadron Collider (LHC) was undoubtedly the discovery by the ATLAS and CMS Collaborations of a new elementary particle of a type never seen before. All the properties of this particle measured so far are consistent with those predicted for the Higgs boson of the Standard Model. It was predicted to have zero spin (angular momentum), and every alternative option tested has by now been ruled out with a high degree of confidence. It was predicted to couple with other particles proportionally to their masses, and this is strongly supported by the data. This is why the committee that awarded the 2013 Nobel Physics Prize to Francois Englert and Peter Higgs stated “Beyond any reasonable doubt, it is a Higgs boson.”

Physicists are now asking themselves follow-up questions. Is there any difference between its properties and those predicted in the Standard Model? Is it the only Higgs boson, or are there others? Many of its couplings to other particles have been measured, but what about its coupling to the heaviest known particle, the top quark? Or its couplings to lighter particles like the muon? What gives its mass to this Higgs boson? Is it truly an elementary particle, or is it made of some smaller constituents? Is it a portal to some new physics beyond the Standard Model, such as dark matter?

The next run of the LHC, starting in the spring of 2015, will set about answering some of these questions. For example, its higher energy will enable the LHC experiments to probe more deeply for deviations from the Standard Model predictions, and to search for heavier Higgs bosons. It will be possible to measure directly this Higgs boson's coupling to the top quark, and to box in its possible coupling to the muon. These measurements may reveal some substructure inside this Higgs boson, or provide some other evidence for physics beyond the Standard Model. Time will tell!