Chapter VIII: Searches for supersymmetry at √
8.6 Interpretation
We present results of the search using method A as it provides slightly bet- ter sensitivity. The two-dimensional (MR,R2) distributions for the search regions in the Multijet, Electron Multijet, and Muon Multijet categories ob- served in data are shown in Figures 8.16-8.21, along with the background prediction from method A. We observe no statistically significant discrepan- cies and interpret the null search result using method A by determining the 95% confidence level (CL) upper limits on the production cross sections of the SUSY models presented in Sec.3.8using a global likelihood determined by combining the likelihoods of the different search boxes and sidebands.
Following the LHC CLsprocedure [184] precisely defined in Sec.7.5, we use the profile likelihood ratio test statistic and the asymptotic formula to eval- uate the 95% CL observed and expected limits on the SUSY cross section σNLO+NLL. Systematic uncertainties are taken into account by incorporat- ing nuisance parametersθ, representing different sources of systematic un- certainty, into the likelihood function L(data|µ,θ). For each signal model the simulated SUSY events are used to estimate the effect of possible sig- nal contamination in the analysis control regions, and the method A back- ground prediction is corrected accordingly. As before, the template prob- ability density for the signal, normalized to unit probability, is multiplied by µσNLO+NLLLeboxb-tag, where µ is the signal strength parameter, σNLO+NLL is the SUSY signal cross section, Lis the integrated luminosity correspond- ing to the size of the data set, andeboxb-tagis the signal selection efficiency for a given category and b-tagged jet multiplicity. To determine a confidence interval for µ, we construct the profile likelihood ratio test statistic ˜qµ of Eqn. 7.14 as a function of µ. Then for example, a 68% confidence interval forµcan be taken as the region for which the test statistic is less than 1. By allowing each nuisance parameter to vary, the test statistic curve is wider, reflecting the systematic uncertainty arising from each source, and resulting in a larger confidence interval forµ.
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Figure 8.16: The (MR,R2) distribution observed in data is shown along with the background prediction obtained from method A for the Multijet event category in the 0 b-tag (upper) and 1 b-tag (lower) bins [227]. The two- dimensional (MR,R2) distribution is shown in a one-dimensional represen- tation, with each MR bin marked by the dashed lines and labeled near the top, and each R2 bin labeled below. The ratio of data to the background prediction is shown on the bottom panels, with the statistical uncertainty expressed through the data point error bars and the systematic uncertainty of the background prediction represented by the shaded region.
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Figure 8.17: The (MR,R2) distribution observed in data is shown along with the background prediction obtained from method A for the Multijet event category in the 2 b-tag (upper) and≥3 b-tag (lower) bins [227]. A detailed explanation of the panels is given in the caption of Fig.8.16.
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Figure 8.18: The (MR,R2) distribution observed in data is shown along with the background prediction obtained from method A for the Muon Multi- jet event category in the 0 b-tag (upper) and 1 b-tag (lower) bins [227]. A detailed explanation of the panels is given in the caption of Fig.8.16.
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Figure 8.19: The (MR,R2) distribution observed in data is shown along with the background prediction obtained from method A for the Muon Multijet event category in the 2 b-tag (upper) and ≥ 3 b-tag (lower) bins [227]. A detailed explanation of the panels is given in the caption of Fig.8.16.
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Figure 8.20: The (MR,R2) distribution observed in data is shown along with the background prediction obtained from method A for the Electron Multi- jet event category in the 0 b-tag (upper) and 1 b-tag (lower) bins. A detailed explanation of the panels is given in the caption of Fig.8.16.
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Figure 8.21: The (MR,R2) distribution observed in data is shown along with the background prediction obtained from method A for the Electron Multi- jet event category in the 2 b-tag (upper) and≥3 b-tag (lower) bins [227]. A detailed explanation of the panels is given in the caption of Fig.8.16.
First, we consider the scenario of gluino pair production decaying to third- generation quarks. Gluino decays to the third-generation are enhanced if the masses of the third-generation squarks are significantly lighter than those of the first two generations, a scenario that is strongly motivated in natural SUSY models [74,119,229, 230] discussed in Sec.3.8. Prompted by this, we consider the three decay modes:
• eg→bbχe0;
• eg→ttχe0;
• eg→btχe+1 →btW∗+χe01or charge conjugate,
where W∗ denotes a virtual W boson. Due to a technical limitation in- herent in the event generator, we consider these three decay modes for
|meg −mχe01| ≥ 225 GeV. For |meg−mχe01| < 225 GeV, we only consider the eg→bbχe0decay mode.
We perform a scan over all possible branching fractions to these three decay modes and compute limits on the production cross section under each such scenario. The production cross section limits for a few characteristic branch- ing fraction scan points are shown on the left of Fig.8.22as a function of the gluino and neutralino masses. We find a range of excluded regions for dif- ferent branching fraction assumptions and generally observe the strongest limits for the eg → bbχe01 decay mode over the full two-dimensional mass plane and the weakest limits for the eg → ttχe01 decay mode. For scenarios that include the intermediate decayχe1± → W∗±χe01 and small values ofmχe0
the sensitivity is reduced because the LSP carries very little momentum in1
both the NLSP rest frame and the laboratory frame, resulting in small values ofEmissT and R2. By considering the most conservative limit obtained for all scanned branching fractions, we calculate an exclusion limit valid for any assumption on the branching fractions, presented on the right of Fig.8.22.
For an LSP with mass of a few hundred GeV, we exclude pair production of gluinos decaying to third-generation quarks for mass below about 1.6 TeV.
This result is a unique attempt at deriving a branching fraction independent limit on gluino pair production at the LHC for the scenario in which gluino decays are dominated by three-body decays to third-generation quarks and a neutralino LSP.
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Figure 8.22: (Left) the expected and observed 95% confidence level (CL) up- per limits on the production cross section for gluino pair production decay- ing to third-generation quarks under various assumptions of the branching fractions. The two gray dashed diagonal lines correspond to|meg−mχe01| = 25 GeV, which is where the scan ends for the eg → bbχe01 decay mode, and
|meg −mχe01| = 225 GeV, which is where the scan ends for the remaining modes due to a technical limitation inherent in the event generator. For
|meg−mχe01|<225 GeV, we only consider theeg→bbχe01decay mode. (Right) the analogous upper limits on the gluino pair production cross section valid for any values of the gluino decay branching fractions [227].
In Fig.8.23, we present additional interpretations for simplified model sce- narios of interest. On the left, we show the production cross section limits on gluino pair production where the gluino decays to two light-flavored quarks and the LSP, and on the right we show the production cross section limits on top squark pair production where the top squark decays to a top quark and the LSP. For a very light LSP, we exclude top squark production with mass below 750 GeV.