Where the Sidewalk Ends: Jets and Missing Energy Search Strategies for the 7 TeV LHC
2/25/2011
99 citations (89 excluding self-citations). The direct sequel to the Jets + MET paper, optimized for the LHC’s first real data.
The Problem
By early 2011, the LHC was delivering 7 TeV collisions and the first inverse femtobarn of data was within reach. The question was no longer how to design reinterpretable searches in principle but how to optimize them in practice: which kinematic variables give the best signal-to-background separation across the widest range of new-physics spectra, where should the cuts be placed, and how far into mass parameter space can the early data reach?
The Approach
The paper maps sensitivity across simplified model mass planes for gluino and squark topologies, testing different kinematic variables (HT, MET, Meff, razor variables) and cut strategies. Rather than optimizing for a single benchmark point, the analysis identifies cut choices that maintain good acceptance across a broad range of mass splittings and decay topologies. The title refers to the boundary in mass parameter space where a given search strategy loses sensitivity: knowing where the sidewalk ends tells you where a different strategy needs to take over.
Impact
The recommendations were adopted by ATLAS and CMS in their early jets + MET analyses. The paper is part of a connected program: the Jets + MET paper (253 citations) established the model-independent philosophy, this paper operationalized it for 7 TeV, and the Simplified Models paper (946 citations) systematized the framework across all search channels. Together they represent a progression from concept to practice to standard.
Recollections
Daniele Alves and Eder Izaguirre were my graduate students at Stanford. We had realized that the reanalyses of Tevatron data we’d hoped for from the Jets+MET paper were not going to be forthcoming, so we shifted focus to the LHC, which was about to deliver its first collisions. The core idea was that no single search strategy covers the full kinematic range of gluino cascade decays. By combining multiple signal regions, each optimized for a different part of the mass plane, we could cover the full topology space and identify where each strategy loses sensitivity — where the sidewalk ends.
The Tevatron analysis had revealed something startling: there were potentially huge gaps in the existing limits that allowed gluinos to be as light as 120 GeV. I joked that the LHC could erupt in a “super-fire” — a fire from supersymmetric particles being produced at enormous rates — if this was actually the case. The possibility that new physics was hiding in plain sight, missed because searches were optimized for the wrong spectra, gave the work real urgency.
That urgency paid off directly. The gaps we identified and the search strategies we developed let us produce some of the first new-physics interpretations of LHC data in the paper “It’s On: Early Interpretations of ATLAS Results in Jets and Missing Energy Searches” with Eder and Mariangela Lisanti, using nanobarns of data from the very first jet energy calibration runs. The title was not subtle about our excitement.