Northwest Terascale Research Projects: Using jet substructure to find new physics at the LHC

Topics to be discussed

There has been substantial recent progress in recognizing signals for new physics in hadron collider events against the background of standard model events by looking at substructure of jets. One tries to find combinations of subjets in a jet that match to the decay of a sought heavy particle while at the same time discarding subjets that likely represent contamination from initial state radiation or low transverse momentum jets from the underlying event. These methods are most effective when a sought heavy particle is highly boosted in the sense of having a transverse momentum that is large compared to its mass. Among the methods that have proven successful are jet filtering, trimming, and pruning.

This workshop seeks to encourage further progress by concentrating on three related topics.

  1. Can the jet grooming methods be applied to signals that present busy final states, with multiple hard jets or subjets? For example, one may look for a Higgs boson recoiling against a t t-bar pair. On one side of the event is the Higgs boson, which decays to b b-bar. That presents a fairly simple structure. But on the other side is the t t-bar pair, decaying (say) into b + q + q-bar plus b + l + neutrino. Besides the lepton, there are four subjets that one wants to find, along with radiation from these subjets and initial state and underlying event radiation that make the analysis more difficult. Other examples involve the decay chains of SUSY particles. If we demand that the starting two SUSY particles are highly boosted, then there are two `fat' jets to analyze and each contains multiple subjets. What is the best way to perform such an analysis?
  2. Generically, if one makes a pair of heavy objects, the transverse momentum of each one is similar in magnitude to the heavy object mass. That is, the heavy objects are not highly boosted. One pays a price in number of available signal events by asking for a large boost factor. To what extent can one extend methods developed to find the decays of highly boosted heavy objects in order to find the decays heavy objects that are not so highly boosted? For example, one can look for the production of a squark and an antisquark that are very heavy but not highly boosted. Then their decay products can be high transverse momentum light parton jets together with highly boosted heavy particle jets. To reconstruct everything, one would still analyze a region of rapidity and azimuthal angle to find relevant jets inside the region, but the region would be too big to be itself reasonably called a jet. Then one might speak of event grooming instead of jet grooming, but one might make use of some of the same techniques. (If one previously analyzed a `fat jet', then here one would analyze something that looked like Jabba the Hut.)
  3. How well do generic methods like jet pruning and trimming work in a busy environment? If one has a specific signal in mind, such as a SUSY decay chain, can the ideas behind the generic methods be adapted to a specific search, overcoming the difficulties of working in a busy environment?

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Last updated 19 October 2010

Davison E. Soper
Institute of Theoretical Science
University of Oregon
Eugene OR 97403 USA
soper@uoregon.edu