Sunday, 30 August 2015

Weekend plot: SUSY limits rehashed

Lake Tahoe is famous for preserving dead bodies in good condition over many years,  therefore it is a natural place to organize the SUSY conference. As a tribute to this event, here is a plot from a recent ATLAS meta-analysis:
It shows the constraints on the gluino and the lightest neutralino masses in the pMSSM. Usually, the most transparent way to present experimental limits on supersymmetry is by using simplified models. This consists in picking two or more particles out of the MSSM zoo, and assuming that they are the only ones playing role in the analyzed process. For example, a popular simplified model has a gluino and a stable neutralino interacting via an effective quark-gluino-antiquark-neutralino coupling. In this model, gluino pairs are produced at the LHC through their couplings to ordinary gluons, and then each promptly decays to 2 quarks and  a neutralino via the effective couplings. This shows up in a detector as 4 or more jets and the missing energy carried off by the neutralinos. Within this simplified model, one can thus interpret the LHC multi-jets + missing energy data as constraints on 2 parameters: the gluino mass and  the lightest neutralino mass. One result of this analysis is that, for a massless neutralino, the gluino mass is constrained to be bigger than about 1.4 TeV, see the white line in the plot.

A non-trivial question is what happens to these limits if one starts to fiddle with the remaining one hundred parameters of the MSSM.  ATLAS tackles this question in the framework of the pMSSM,  which is a version of the  MSSM where all flavor and CP violating parameters are set to zero. In the resulting 19-dimensional parameter space,  ATLAS picks a large number of points that reproduce the correct Higgs mass and are consistent with various precision measurements. Then they check what fraction of the points with a given m_gluino and m_neutralino survives the constraints from all ATLAS supersymmetry searches so far. Of course, the results will depend on how the parameter space is sampled, but nevertheless  we can get a feeling of how robust are the limits obtained in simplified models. It is interesting that the gluino mass limits turn out to be quite robust. From the plot one  can see that, for a light neutralino, it is difficult to live with m_gluino < 1.4 TeV, and that there's no surviving points with  m_gluino < 1.1 TeV. Similar conclusion are  not true for all simplified models, e.g.,  the limits on squark masses in simplified models can be very much  relaxed by going to the larger parameter space of the pMSSM. Another thing worth noticing is that the blind spot near the m_gluino=m_neutralino diagonal is not really there: it is covered by ATLAS monojet searches.  

The LHC run-2 is going slow, so we still have some time to play with  the run-1 data. See the ATLAS paper for many more plots. New stronger limits on supersymmetry are not expected before next summer.

Saturday, 15 August 2015

Weekend plot: ATLAS weighs in on Higgs to Tau Mu

After a long summer hiatus, here is a simple warm-up plot:

It displays the results of ATLAS and CMS searches for h→τμ decays, together with their naive combination. The LHC collaborations have already observed Higgs boson decays into two 2 τ leptons, and should be able to pinpoint h→μμ in Run-2. However,  h→τμ decays (and lepton flavor violation in general) are forbidden in  the Standard Model, therefore a detection would be an evidence for exciting new physics around the corner.  Last summer, CMS came up with their 8 TeV result showing a 2.4 sigma hint of the signal. Most likely, this is just another entry in the long list of statistical fluctuations in the LHC run-1 data. Nevertheless, the CMS result is quite intriguing, especially in connection with the LHCb hints of lepton flavor violation in B-meson decays.  Therefore, we have been waiting impatiently for a word from ATLAS. ATLAS is taking his time, but finally they published the first chunk of the result based on hadronic tau decays. Unfortunately, it is very inconclusive.  It shows a small 1 sigma upward fluctuation, hence it does not kill the CMS hint.  At the same time, the combined significance of the h→τμ signal increases only marginally, up to 2.6 sigma.

So, we are still in a limbo.  In the near future, ATLAS should reveal the 8 TeV h→τμ measurement with leptonic tau decays. This may clarify the situation, as the fully leptonic channel is more sensitive (at least, this is the case in the CMS analysis).  But it is possible that for the final clarification we'll have to wait 2 more years, once enough 13 TeV data is analyzed.