Model-Independent Jets plus Missing Energy Searches
9/18/2008
253 citations (239 excluding self-citations). One of the early papers that laid the groundwork for the Simplified Models program by demonstrating that LHC searches could be designed for reinterpretability rather than tied to specific theoretical benchmarks.
The Problem
Jets plus missing transverse energy is the most generic signature of new physics at a hadron collider. Any model with a stable, invisible particle produced in association with colored particles produces this final state: supersymmetry, universal extra dimensions, little Higgs models with T-parity, and many others. By 2008, the Tevatron was running and the LHC was about to start, but searches for jets + MET were designed around specific SUSY benchmarks (typically mSUGRA/CMSSM grid points). A limit set in terms of mSUGRA parameters was nearly impossible to reinterpret for a different model.
The Key Idea
Instead of optimizing for a specific model’s spectrum, define bins in visible energy (HT) and missing energy (MET) and publish the cross-section limit in each bin. Any theorist working with any model that produces jets + MET can compute their signal’s distribution across the same bins and read off whether the model is excluded. The paper used gluino cascade decays as a case study but the binning is model-independent. The method shifts the burden of model-dependence from the experimentalist (who sets the limit) to the theorist (who computes the signal), where it belongs.
Impact
The approach influenced early LHC analyses at both ATLAS and CMS. CMS’s “Interpretation of Searches for Supersymmetry with Simplified Models” (261 citations) codified the reinterpretable-limits philosophy as official methodology. The idea of publishing limits in a format that enables broad reinterpretation became a guiding principle for the Simplified Models framework developed two years later.
The paper also enabled an unexpected application: dark matter searches at colliders. Goodman et al.’s “Constraints on Dark Matter from Colliders” (806 citations) applied the model-independent jets+MET approach to constrain dark matter models using effective field theory, showing that collider limits on invisible particles are competitive with direct detection for light WIMPs. This dark matter application of the jets+MET methodology became one of the most active areas of LHC phenomenology.
The follow-up paper, “Where the Sidewalk Ends” (99 citations), extended the strategy to the 7 TeV LHC with optimized kinematic variables and explicit recommendations for cut choices adopted by the experimental collaborations.
Recollections
Johan Alwall was a postdoctoral visitor at SLAC who had developed new techniques for Monte Carlo simulations to more accurately predict high-energy radiation off of events, radiation that traditional parton shower approaches had historically underestimated. Mariangela Lisanti was my first graduate student. My-Phuong Le was a graduate student of Joanne Hewett who joined the project.
The motivation came from a question that had been nagging me: if naturalness is a principle, where is the new physics? This was the LEP Paradox of Barbieri and Strumia — new particles should already have appeared in existing data if they were at the natural scale. One answer was that we were looking under a lamppost. The existing searches assumed top-down motivations for new particle spectra, optimizing for widely separated mass hierarchies predicted by specific theories like the CMSSM. But what about compressed spectra, where the new particles are close in mass and their decay products carry little energy? Compressed spectra are much harder to see because the visible energy released in each decay is small.
I realized that initial state radiation could make compressed-spectrum events visible. A gluino pair produced alongside a hard ISR jet would show up as a monojet-like signature with missing energy, even if the gluino’s own decay products were too soft to detect. This meant existing data might already contain new physics that compressed-spectrum searches could find. The paper was about building the framework: define bins in visible and missing energy, publish limits in those bins, and let theorists map any model onto the same grid. The follow-up, “Where the Sidewalk Ends,” was about figuring out how to do better — optimizing the search strategy for the 7 TeV LHC rather than just reinterpreting what existed.