Matt Strassler [January 17, 2013]
Might there be more than one type of Higgs particle? Has our recent discovery been just the first of several that are still to come?
In many alternatives to the Standard Model, the number of types of Higgs particles is bigger than one. Many theories with a composite Higgs have several; the simplest supersymmetry theories have five, and more complex ones can have seven or nine. The biggest argument against this possibility might seem to come from the fact that so far the new particle is behaving like a Standard Model Higgs — a Higgs in a theory with just one Higgs particle. So shouldn’t we already be pessimistic about the possibility that there is more than one Higgs particle?
The answer is no, and that’s because of a famous “decoupling” theorem, which shows that in a wide variety of theories with multiple types of Higgs particles, one type of Higgs particle will closely resemble a Standard Model-type Higgs. The other Higgs particles may either be rather heavy or interact very weakly with ordinary matter and with the Standard Model-like Higgs, making them harder to discover. At the Higgs Symposium, Howie Haber himself devoted a significant part of his talk to this important theorem, whose first version he derived with Yosi Nir in 1989. He reminded the audience that the only way to tell the difference between a theory like this and the Standard Model itself is to either
- measure the Standard Model-like Higgs particle very precisely to discover the small differences between that particle’s properties and those predicted by the Standard Model, or
- directly discover additional types of Higgs particles.
We’ve been doing a lot on (1) over the past few months, and will continue to do so for at least the rest of the decade. Meanwhile efforts to do (2) are also taking place, but they are really just getting started. As I emphasized in my own talk, there are many opportunities for discoveries of other Higgs particles during 2013 and 2014, as the 2011-2012 data is more thoroughly analyzed. At this stage, we have to consider the question of whether there are other types of Higgs particles completely open.