Higgs bosons, global fits, MSSM, and dark matter May 31, 2007Posted by dorigo in astronomy, news, physics, science.
Today’s Higgs boson session of the CDF week started with a very nice talk by Sven Heinemeyer on Higgs physics, which after a brief overview of the Standard Model Higgs, focused mostly on its cousins in the framework of the Minimal SUperSYmmetric models.
I must warn readers that this post cannot be at the level I usually keep these discussions, because of the complexity of some of the issues and the impending talks I am trying to follow with a small part of my brain today. Maybe later I will summarize it here!
Anyway, after a brief but comprehensive introduction to the theoretical framework of electroweak symmetry breaking, Sven showed some details of the way the indirect limits on the Higgs mass have been cooked up by global fits to electroweak observables. This is relevant for the famous “blue band plot” (check the latest version in the LEP electroweak working group pages), which shows what values of the Higgs mass are consistent with the measurement of electroweak processes that are influenced by it. It turns out that there are a few results pulling the most favored Higgs mass in the fit down, and only one pulling it up strongly enough. The point was made that if the forward-backward asymmetry measurement made by LEP with b-quarks from Z decay was taken out of the global fit, the Higgs mass would become really incompatible with the direct lower limit set by LEP II.
You can see it in the plot above (not exactly the one showed in Sven’s talk, but one containing similar information), where each individual SM observable provides an error bar showing which Higgs mass it favors, given the correctness of the standard model. The grey vertical band marks the preferred region, and each observable provides an independent (how much so I am not totally sure) range for M(h). I think Sven’s point is valid, but one could well single out other results (such as the one for the left-right asymmetry, A_LR) which pull down strongly, and the favored M(h) would move up quite a bit – the plot has a logarithmic x axis, and so points on the left appear to have large error bars while they are in fact more precise!
Then the famous Higgs reach plot (see below) was mentioned, to drive home the point that we “promised” exclusion limits tighter than LEP II by 2008 and some sensitivity up to 135 GeV with the full Run II statistics. \begin(rant) … I was surprised and somewhat saddened to see that plot being made fun of, and not by the speaker! It looked like the audience did not give a lot of credibility to those predictions any more, despite many of them being the very authors of the underlying studies. Well, I still do.
[The plot above has been discussed in length elsewhere on this site. In short, it shows the integrated luminosity per each of the two Tevatron experiments (y axis) which is needed to exclude or discover the Higgs boson as a function of the Higgs mass.]
In my view, people tend to forget that CDF and D0 have consistently shown in the past that results reach and exceed expectations if we give them time. Nowadays, the Tevatron experiments are doing worse than what the plot implies for low values of the Higgs mass, but many of the improvements on the relative analyses (those looking for a light Higgs boson decaying to b-quark pairs) have not been implemented yet! I think the pessimism that seemed to hide behind the slight amusement of the audience fights openly with the fact that our collaboration is strongly pursuing the Higgs boson in all the available channels, and is investing 70% of its resources into those analyses. \end(rant).
After a rant, let me come back to Heinemayer’s talk. He asked himself: what is the preferred Higgs mass in the MSSM when electroweak measurements are all taken into account ? This is a nice exercise which for some reason I have not seen done before. Sven showed the result of these computations. It looks like the best fits all point to a preferred mass of about 110 GeV, for several choices of the MSSM parameters, and tan(beta) values of 10 or 50. So, if one were to compare this result with the preferred Higgs boson mass value in the Standard Model, which is now at 76 GeV, one would conclude that the MSSM seems favored with respect to the SM… But this means missing a crucial point! In fact, the price one pays in delta chi^2 is very high in the MSSM fits, as shown by the steepness of the family of curves in the graphs he showed. In contrast, despite the fact that the preferred mass in the SM is lower, it is still giving a good fit with masses up to 140 GeV or so. That is to say, it is not so important what is the preferred value of your fits, as much as how wide is the region which still has a reasonable probability.
Now, to get to another important point made in the talk: Sven explained that the so-called “benchmark scenarios” for MSSM models (discussed, for instance, in http://xxx.lanl.gov/abs/hep-ph/0202167), which pick some reference values for many of the model parameters and only allow a variation of M(A) and tan(beta), are all ignoring the “bounds” from cold dark matter models. Now what this all means is the following. Standard cosmology, which has shown several successes in the recent past and which “fits” a very precise amount of dark matter in the matter-energy mix, is compatible with the presence in the universe of supersymmetric particles, which would provide the missing mass while not producing unwanted effects that would have been detected by past or curent experiments. The amount of dark matter is actually convertible -if one buys the extrapolation- in a mass and a typical cross section for these candidate superparticles. The point is then that the “benchmark scenarios” have MSSM parameters which do not agree well with the hints from the amount of cold dark matter cosmologists favor.
To mend that inconsistency, Sven discussed a scenario called NUHM – for “Non-unified Higgs model”, which implies non-unified scalar fermion and scalar Higgs parameters. I am not in the position of discussing the details of this scenario, because it is based on work that Sven has not published yet. Anyway, the choice of parameters of NUHM provides a better match to the amount of cold dark matter, while still being in fair agreement with electroweak and B physics measurements performed by particle accelerators.
Below I would have like to show the plots contained in Sven’s talk, which have a color code showing which regions of M(A) and tan(beta) in the NUHM benchmark scenario are fitting better with all data. In the plots, one notices that the best fits are in a region which will be hard to reach by the Tevatron experiments in their direct Higgs searches. I could not attach the plots to this post, because – as I discovered by talking to Sven during the coffee break – also theorists have restricted information at times. He has not published this work yet, and quite naturally I have to wait until he does to adobe this site with them…
Finally, Sven mentioned that the NUHM scenario is compatible with M(A)=160 GeV and tan(beta)>45 -the values that the CDF H->tau tau tentative signal (which, remember, is most likely only a 2-sigma fluctuation) points to. This also has some important consequences. If the A particles have a mass of 160 GeV, then the lightest neutral higgs boson, h, should be just around the corner for direct searches at low mass: the favored value for its mass would then be about 115 GeV, just above the LEP II limit, and at reach of CDF and D0. Also, another observable which is influenced by SUSY effects, the branching fraction of Bs mesons to muon pairs, would be larger than 2×10^-8, and so possibly at the reach of the Tevatron experiments with an extended running of the machine.
If you read the post down to this last line, you are probably a physicist, and you could be interested in exploring the information on the Higgs boson which has been made available on the FeynHiggs web page: http://www.feynhiggs.de .