The inside story of the potential Higgs signal

Offering simple reading material about experimental results in high energy physics, and commenting on their importance and implications, is a good idea and one of the motivations for running this blog, but simple explanations are not always enough to make a story entertaining. A story told from the eyes of the person in front of which events develop is always a tad more fascinating.  

Now, all that to say that the potential signal of a MSSM Higgs boson I discussed a week ago in this post  has been explained in detail at the cosmic variance blog  by one of the makers of the analysis.

John Conway (pictured on the left next to our detector), a colleague in CDF and a very smart experimental physicist who has been working on this very issue for twenty years now, tells the story from the inside. Quite a nice post, and highly advisable.

It remains to say that the Higgs boson signal CDF sees is most likely a fluctuation. Which is unfortunate for two reasons: one, because what could be a Nobel-prize discovery is not there. Two, because the fact that CDF has an excess of events at around 160 GeV of reconstructed Higgs mass means that the 95% C.L. limit the experiment is able to set on the production of that particle is much weaker than it could have been.

In these days -well, thirty years now- of failed searches for physics beyond the Standard Model, the minimal result one usually can still publish and be proud about is a so-called exclusion plot, where points of the parameter space for the studied new theory are excluded (at 95% confidence level, for instance) by the fact that the new particles predicted by the theory are NOT found.

Of course, as one reckons with the lack of a signal in the data and switches to setting a limit, one would then like to set the most stringent possible limit. When the data show no bump, the limit is better than when the data show upward fluctuations…

The plot on the left shows the plane of two parameters: the unknown value of the hypothesized MSSM Higgs boson mass, and the value of the parameter called “tangent beta”, that is the tangent of the ratio between the expectation values of the two higgs fields in the model. Huh, too technical ? Ok, think of tangent beta as a dial that defines how much the higgs bosons like to couple to (ok, say “attract”) particles of a certain kind, “down-type quarks and leptons”.

LEP 2 already excluded all the blue region of this plot. CDF has different characteristics, due to the different production mechanisms of the particle in proton-antiproton collisions (LEP collided electrons and positrons). The light purple region is the one CDF expected to exclude with the analyzed data. The dark purple region is instead what CDF can exclude: a much smaller region! So either the Higgs is there after all (at, say, tan(beta)=50 and Mh=160 GeV), or CDF has been very unlucky!