Simplified Models for LHC New Physics Searches
5/13/2011
946 citations (666 excluding self-citations) as of early 2026. The most-cited paper I have been involved with.
The Problem
By 2010, LHC searches for new physics were being designed, performed, and presented in terms of hypothetical models’ fundamental parameters, often evaluated at the GUT scale. The dominant approach used ad hoc ansatze like the CMSSM/mSUGRA assumptions to reduce the MSSM’s 100+ free parameters to 4-5. These constrained scenarios had immense institutional support: analyses were optimized, benchmark points were agreed upon, and the collaborations had years of experience with them.
The danger was that this approach obscured the physical parameters actually being searched for. Analyses designed to identify complex particle spectra predicted by the CMSSM or similar theoretically motivated scenarios might not be sufficiently inclusive. If nature produced new physics with a spectrum that didn’t match the benchmarks, searches optimized for those benchmarks could miss it. The experiments risked looking under the lamppost rather than figuring out how to separate signal from background for broad classes of new particles.
Simplified models reframed the question. Instead of starting from a UV-complete theory and working down, they started from the collider signature and worked up: effective Lagrangians with just a few new particles, where the only free parameters are masses and production cross-sections that map directly to measurable quantities. The shift was from “what does this theory predict?” to “what can this search see?”
The Approach
The paper catalogs simplified models organized by experimental signature rather than by theoretical origin. The major categories are jets plus missing energy, heavy flavor (tops, bottoms, taus), leptons, photons, and exotic signatures like displaced vertices and high multiplicity final states. Each model specifies a minimal particle content, a production mechanism, and a decay topology. A gluino decaying through an off-shell squark to jets and a neutralino, for instance, is fully characterized by the gluino mass, the neutralino mass, and the production cross-section. An experimentalist can set limits on this two-dimensional mass plane, and a theorist working with any UV completion that produces the same topology can read off constraints directly.
The key innovation was organizational, not computational: mapping the space of possible signatures systematically and defining a common language for presenting results.
Impact
The simplified model framework became the default language for LHC new-physics searches. Of the paper’s 946 citations, roughly half are ATLAS and CMS experimental analyses that present their results on simplified model mass planes. When CMS published its comprehensive “Interpretation of Searches for Supersymmetry with Simplified Models” (261 citations), it codified the approach as official CMS methodology. ATLAS’s summary papers on squark and gluino searches adopted the same framework.
The paper also spawned an ecosystem of reinterpretation tools. CheckMATE (363 citations), SModelS (228 citations), and MadAnalysis 5’s public analysis database (217 citations) were all built to work with simplified model limits, allowing theorists to test new models against existing experimental results without re-running detector simulations. These tools would not exist without a standardized format for presenting limits.
The approach extended beyond supersymmetry. The dark matter community adopted simplified models as their primary framework for LHC searches, producing the “Dark Matter Benchmark Models for Early LHC Run-2 Searches” (776 citations) and “Simplified Models for Dark Matter Searches at the LHC” (503 citations). Both papers cite this work as foundational and extend the simplified model methodology to dark matter mediators, effective field theory comparisons, and mono-X signatures. The Particle Data Group’s Review of Particle Physics cites the paper in its discussion of BSM search methodology.
The paper’s influence is less a citation count than a standard adopted by an entire field. Fifteen years later, every ATLAS and CMS search for new physics publishes limits on simplified model mass planes. The framework outlasted the specific models it was designed to test.
Recollections
The workshop grew out of a joint ATLAS/CMS/Theory meeting at CERN in June 2010, where the experiments made a direct request: they needed representative topologies from the theory community to ensure their searches covered the relevant phase space. It had been nearly a decade since theorists had systematically proposed signatures to cover a broad range of new physics, and the LHC was about to deliver real data.
I organized the “Topologies for Early LHC Searches” workshop at SLAC, September 22-25, 2010, together with Mariangela Lisanti, Rouven Essig, Tim Tait, Natalia Toro, and Philip Schuster. About 100 theorists and experimentalists attended. We split into working groups by signature type: I led Jets, Mariangela took Leptons, Philip handled Photons, Natalia covered Heavy Flavor, Tim ran Exotica, and Rouven managed Taus. Each group cataloged its simplified models with codes (J.005 for a jet topology, E.002 for an exotic signature) on lhcnewphysics.org, which served as the living document before the paper was finalized.
We presented a draft at a follow-up CERN meeting in November 2010 and posted the final paper to arXiv in May 2011. Coordinating 95 authors and producing a coherent document from a workshop’s worth of parallel sessions was its own challenge. Getting the notation and conventions consistent across all the topologies required more editorial work than any physics calculation.
What I’m most proud of is not the paper itself but the translation layer it created between theory and experiment. That was the point, and it worked.