Authors: Chengcheng Han, Mihoko M. Nojiri, Michihisa Takeuchi, and Tsutomu T. Yanagida
Reference: arXiv:1609.09303v1 [hep-ph]
If you’ve been following the ongoing SUSY searches, you know that much of the phase space accessible at colliders like the LHC has been ruled out. Nevertheless, many phenomenologists are working diligently to develop alternative frameworks that maintain the compatibility of the Minimal Supersymmetric Standard Model (MSSM, one of the most compelling SUSY models) with recent experimental results. If evidence of SUSY is found at the LHC, it could help us start answering questions about naturalness, dark matter, gauge coupling unification, and other BSM questions, so it’s no wonder so many researchers are invested in keeping the SUSY search alive.
This particular paper discusses recent 13 TeV ATLAS results in the l+jets+ETmiss channel where an excess of up to 3.3σ above Standard Model expectations were seen in the initial 13TeV Run 2 dataset. Although CMS hasn’t reported any significant excess in this channel, both experiments see a moderate excess in the 0 lepton channel, so there’s some strong motivation for phenomenologists to explain these preliminary results as the presence of a new particle, namely a SUSY particle.
This ATLAS search is aimed at stop (the SUSY partner of the top quark) production where the stop then decays into a top and neutralino (a mixed state of the higgsino and gaugino, ie the SUSY partners of the Higgs and the gauge boson), and the top then decays to leptons. The stop is a particularly interesting SUSY particle, because it plays a critical role in the naturalness of SUSY models; most natural SUSY models predict a light stop and higgsino. The analysis group defined 7 (non exclusive) potential signal regions for this search, and excesses above 2σ were seen in 3 of them: DM_low, bC2x_diag, and SR1. The selection cuts for these regions are shown in Table 1. This paper discussed models to explain the DM_low excess of 3.3σ, but similar models could be used to explain the other excesses as well. The authors sought to create models which are compatible both with these results and the previous stop parameter limits set by ATLAS and CMS.
They first explored the two simplest one-step stop decays. Depending on what the Lightest Supersymmetric Particle (LSP) is, these decays can have different constraints, so they conducted a scan over the entire parameter space. There are two cases for this type of decay: (1) the LSP is a bino (SUSY partner of weak hypercharge boson) and the Next Lightest Super Symmetric Particle (NLSP) is the stop,leading to the simple decay shown in Figure 1a, or (2) the LSP is a higgsino which can be charged or neutral, with each possibility having different masses, which leads to the split decay shown in Figure 1b.
We can see in Figure 2 that most (or all in the case of the higgsino) of the preferred 2σ phase-space for these models are ruled out by existing constraints on stop production, so unfortunately these models aren’t particularly promising. Consequently, the authors designed an additional model essentially combing these two processes, where the LSP is a bino and the NLSP is a higgsino. This allows for both one step and two step decays, as shown in Figure 3. The results for this model are much more exciting; almost all of the preferred 2σ phase space exists outside of the existing constraints, as shown in Figure 4!
2 additional references are included in the Figure 4 plot. 3 benchmark models for different combinations of higgsino and stop masses are indicated with red crosses, and all of them fall in this allowed phase space. Furthermore, direct dark matter detection limits from the LUX experiment are shown as a dashed black line. The left side of this line has been excluded by LUX, so one of the considered benchmark models is not allowed. The second benchmark model falls near the exclusion line, so upcoming dark matter results will play an important role in telling us if this SUSY model can actually explain the excess!
So, SUSY hasn’t been found at the LHC, but its not dead yet! There are promising excesses in the current ATLAS dataset which are consistent with benchmark MSSM models with expected LSP candidates. We look forward to new data from the LHC and other experiments to tell us more!
References and further reading:
- Stephen P. Martin, “Supersymmetry primer” (arXiv:hep-ph/9709356)
- Sven Krippendorf, Fernando Quevedo, Oliver Schlotterer, “Cambridge Lectures on Supersymmetry and Extra Dimensions” (arXiv:1011.1491)
- John Ellis, “Supersymmetry, Dark Matter, and the LHC” (slides)