Abstract
The results of many LHC searches for supersymmetric particles are interpreted using simplified models, in which one fixes the masses and couplings of most sparticles then scans over a few remaining masses of interest. We present a new technique for combining multiple simplified models (that requires no additional simulation) thereby highlighting the utility and limitations of simplified models in general, and demonstrating a simple way of improving LHC search strategies. The technique is used to derive limits on the stop mass that are model independent, modulo some reasonably generic assumptions which are quantified precisely. We find that current ATLAS and CMS results exclude stop masses up to 340 GeV for neutralino masses up to 120 GeV, provided that the total branching ratio into channels other than top-neutralino and bottom-chargino is small, and that there is no mass difference smaller than 10 GeV in the mass spectrum. In deriving these limits we place upper bounds on the branching ratios for complete stop pair decay processes for many values of the stop, neutralino and chargino masses. These are available with this paper.
Highlights
Offer useful insights but, strictly speaking, only apply to a particular set of sample points in a large parameter space [18]
We find that current ATLAS and CMS results exclude stop masses up to 340 GeV for neutralino masses up to 120 GeV, provided that the total branching ratio into channels other than top-neutralino and bottom-chargino is small, and that there is no mass difference smaller than 10 GeV in the mass spectrum
When combined, simplified models provide a powerful tool for constructing limits that are robust, conservative and as model independent as possible
Summary
In the context of SUSY results at the LHC, the most common definition of the term “simplified model” is a subset of the phenomenological MSSM, in which the masses of all sparticles are fixed, except for a few of interest, and the decay rates of the sparticles are fixed to trivial values, usually a branching ratio of 100% into a single final state. In the case of analyses optimised for top-neutralino decays, the searches frequently make use of kinematic top reconstruction, reducing the acceptance for bottom-chargino decays This is only one example of how the approach used on one model will hurt sensitivity to the other. A phenomenologist should only use individual simplified models to constrain the overall cross-section for a given process, i.e. the product of the stop pair production cross-section times branching ratio for that particular decay mode. Binding limits on the stop mass itself can still be recovered when simplified model analyses exist for all feasible decay modes If this is true, one can take each point in the parameter space of interest (e.g. each value of stop and neutralino mass) and check whether there is a choice of branching ratios that satisfies all of the simplified model constraints simultaneously. If no solution can be found, the corresponding mass values can be excluded independently of the branching ratios
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