As soon as the discovery of that famous new particle was announced at the Large Hadron Collider [LHC] last year, there were already very good reasons to think it was a Higgs particle of some type. I described them to you back then, as part of my “Higgs Discovery” series. But, as I cautioned, those arguments relied partly on data and partly on theoretical reasoning.
Over the past nine months, with additional data collected through December and analyzed through the present day, it has become more and more convincing that this particle behaves very much like a Higgs particle, along the lines I described following the Edinburgh conference back in January. One by one the doubters have been giving up, and few remain. This is a Higgs particle. That’s my point of view (see last week’s post — you heard it here first), the point of view of most experts I talk to [in a conference I'm currently attending, not one person out of about forty theorists and experimenters has dissented], and now the official point of view of the CERN laboratory which hosts the LHC.
Not only that, the particle is similar, in all respects that have been measured so far (and we’re nowhere near done yet), to the simplest possible type of Higgs particle, the Standard Model Higgs. It is therefore natural to call this a Standard Model-like Higgs particle, shifting the “-like” over a step. That wording emphasizes that although confidence is very high that this is a Higgs particle, we do not have confidence that it is a Standard Model Higgs, even though it resembles one. This is for two reasons.
First, with the data currently available, the measurements are not precise enough to rule out deviations from a Standard Model Higgs as large as 10%, 20%, or even as much as 50% in certain of the particle’s properties.
Second, many interesting speculative theories, despite being dramatically different from the Standard Model, nevertheless predict nature will exhibit a Standard Model-like Higgs particle — one that may be distinguishable from a true Standard Model Higgs only after the LHC has gathered much more data.
The reason this happens in so many theories is due to something called the decoupling theorem, which I mentioned in this article and this article; it demonstrates that even when the Standard Model is not the complete theory of physics at the LHC, Standard Model-like Higgs particles can arise in many different ways. And thus the fact that this Higgs particle is Standard Model-like, though obviously strong evidence against many theories that would have predicted something else, is not compelling evidence in favor of the Standard Model.
So when particle theorists like me wonder, “Does the Standard Model describe all phenomena at the LHC?”, we know the answer will not come easily.