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So, is Higgs finally here? *July 4, 2012*

*Posted by apetrov in Uncategorized.*

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Today is a big day at CERN. There are two collaborations that presented their latest results on the search of the Higgs boson. Did they finally discover the Higgs boson?

Let’s first figure out what it all means? What two collaborations? What is Higgs boson? And, most importantly, *what do we mean by “discovered”*?

First things first. The two collaborations that I’m talking about are CMS and ATLAS, two huge detectors and hundreds of professors, postdocs and graduate students working to get and work the data that come out of it. The collaborations looked at almost three years (although 2010 does not really count and 2012 is still going — but with fantastic pace) and found signals of the Higgs boson, a particle that was predicted to be there in the minimal Standard Model.

Why is that we need Higgs boson and why is it there? The Standard Model of particle physics is described by its symmetries — or the symmetry group (SU(2)xU(1)) under which matter contents transform. This symmetry tells us *how* particles interact — and in fact, that makes Standard Model quite a constrained system. So the introduction of this symmetry is very important. However, this symmetry also tells us that all particles that are described there should be massless! What should one do? The idea is to break that symmetry, of course. The problem is *how* to break that symmetry. One cannot simply add symmetry-breaking terms (that would wreck the whole original setup), one has to do it indirectly. So the idea was proposed to introduce a field that interacts with all fields that are present in the Standard Model. That field also interacts with itself and forms a condensate (i.e. provides non-zero value for energy density of the vacuum) once, roughly speaking, the temperature of the Universe after the Big Bang drops below certain value. This mechanism gives mass to both electroweak gauge bosons (particles that represent weak force) and quark and leptons. The mechanism itself was first proposed in 1962 by Philip Warren Anderson. The model of spontaneous symmetry breaking was independently developed in 1964 by three groups, Robert Brout and Francois Englert; Peter Higgs; and Gerald Guralnik, C. R. Hagen, and Tom Kibble. The particle that manifests this effect is the famous Higgs boson. Read more about it here. I’ll talk more about it in my later posts.

CMS went first (which is a bit unusual, as Atlas, for some alphabetical reason, would always be first to deliver their talk). The talk was delivered by Joe Incandela, a CMS spokesperson. A gist of their talk is that they looked at several possible decay channels of the Higgs boson. First, Higgs can decay to two photons (H → γγ). They see a significant bump at mH = 125 GeV, but only in the combination if different reconstruction techniques. The overall significance is over 4 sigma. Next, they talked about H → ZZ channel. This channel is tougher, as they need to reconstruct Z’s that decay in different decay channels. Now, if they combine their data in H → γγ and in H → ZZ they find that statistical significance for signal that the Higgs is there at 5 sigma. However, once they combine all data, especially the H → ττ channel, their combined statistical significance goes slightly down to 4.9 sigma. This is just below discovery by the standards of Physical Review Letters, a very influential physics journal. But this is in very significant.

The next talk was by ATLAS. Fabiolla Gianotti, ATLAS spokesperson, gave that talk. They also see excess in H → γγ channel, but their statistical significance in that channel is lower, 4.5 sigma. Also, they include the so-called look-elsewhere effect — and then their statistical significance goes down to 3.5 sigma. Then, she discussed the H → ZZ channel. They see the excess with 3.4 sigma significance at mH about 125 GeV. Now, the combined results have excess at mH = 126.5 GeV with (local) statistical significance of 5.0 sigma. This is a discovery.

In passing, ATLAS also see a bump in their 4-lepton channel at approximately 90 GeV. Still not clear what it is….

This discovery is very significant. It tells us that our ideas on how electroweak symmetry is broken are at least partially correct. This is also the first truly elementary particle discovered since the Z-boson. There are still many questions, both experimental and theoretical, about the analyses presented today at CERN. What is going on with the H → ττ channel? Is it really a Standard Model Higgs boson? Or some other scalar particle. We’ll sure to study those things indeed.

P.S. The theorists who described the effects were there and not only were acknowledged by the experimental speakers, but also got to say a couple of words at the end.

The 90 GeV bump is Z -> 4 leptons via internal conversion.

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