Simplified Models for a First Characterization of New Physics at the LHC

TL;DR

The paper addresses the challenge of characterizing potential SUSY-like new physics at the LHC with limited early data. It introduces four simplified models with a small set of masses and branching ratios to provide a detector-independent description of jets, leptons, and missing energy, enabling rapid comparison across full theories such as the MSSM. Through two detailed examples, the authors show how simplified-model fits constrain production modes, mass splittings, and decay patterns, and how deviations from these fits can indicate necessary extensions or alternative mechanisms. The approach offers a practical, model-agnostic target for theorists and a robust, early-data framework for guiding subsequent experimental and phenomenological investigations.

Abstract

Low-energy SUSY and several other theories that address the hierarchy problem predict pair-production at the LHC of particles with Standard Model quantum numbers that decay to jets, missing energy, and possibly leptons. If an excess of such events is seen in LHC data, a theoretical framework in which to describe it will be essential to constraining the structure of the new physics. We propose a basis of four deliberately simplified models, each specified by only 2-3 masses and 4-5 branching ratios, for use in a first characterization of data. Fits of these simplified models to the data furnish a quantitative presentation of the jet structure, electroweak decays, and heavy-flavor content of the data, independent of detector effects. These fits, together with plots comparing their predictions to distributions in data, can be used as targets for describing the data within any full theoretical model.

Figures (24)

• Figure 1: Particle and parameter content of the simplified models. From top to bottom: The two leptonic decay models, originating from production of either a quark-partner or gluon-partner; the two b-tag models, originating from either a quark-partner or a gluon-partner. Please see text and Section \ref{['sec:fourModels']} for further discussion.
• Figure 2: Left: Production cross-sections for gluino pair production, gluino-squark associated production, and squark-pair production, for light and heavy flavors of squarks. The mass on the $x$-axis is the mass of the produced particles. We have assumed equal masses for squarks and gluinos unless otherwise indicated. Right: The corresponding cross sections for UED production, with all KK particle masses taken equal. The MSSM generations are made in Pythia 6.4 Sjostrand:2006za and the UED generations using MadGraph/MadEvent 4 Alwall:2007stUEDMadGraph.
• Figure 3: Topologies typical of electroweak decays.
• Figure 4: Comparisons of lepton count observables between "data" (error bars) and the simplified model Lep(G) with parameter set A ($B_W=0$, red solid line) or B ($B_{LSP}=0$, green dashed) from Table \ref{['tab:ex1_G-LCM_bestFitTable']}.
• Figure 5: Jet counts (in 0- and 2-lepton regions) between "data" (error bars) and the simplified model Lep(G) with parameter set A ($B_W=0$, red solid line) or B ($B_{LSP}=0$, green dashed) from Table \ref{['tab:ex1_G-LCM_bestFitTable']}.