Top quarks are especially interesting at the LHC because they are the most massive fundamental particle known, suggesting an intimate association with electroweak symmetry breaking and possible new-physics scenarios.
The top quark decays via two channels: t → Wb → lνb or t → Wb → qqb. When a tt pair is created in an experiment with energy roughly equal to the quark–antiquark rest mass, the decay products appear well separated in the detector. With the higher energies at the LHC, however, particles are often given a “boost” in momentum when produced so the decay products of a tt pair have extra momentum along the directions of the top and antitop, and are found in opposite hemispheres of the detector.
While higher energies allow the experiments at the LHC to probe for new physics as never before, they also bring new challenges. For example, what if the top quark is so boosted that the three jets from the decay t → Wb → qqb merge to a point where they are indistinguishable from each other and appear as one large jet? With the high energy at the LHC, this boosted situation happens quite often and must be accounted for when reconstructing top-quark decays. Analyses involving top quarks or other “boosted objects” at the LHC, now include approaches that allow for these effects (CERN Courier September 2012 p35).
The special techniques for measuring boosted top quarks are particularly important when searching for new resonances, where a new heavy particle decaying primarily into tt pairs could be observed as a bump in the relevant invariant mass spectrum. The higher the mass of the new particle, the more likely it is that the top-quark decay products will merge in the detector.
ATLAS recently performed searches for tt resonances in final states with one or no leptons. In the former case, the lepton is allowed to be much closer to the b quark than in non-boosted analyses. In the other hemisphere of the detector, a wide massive jet with underlying structure is required. Using these boosted techniques, the sensitivity to a new heavy-gauge boson increased by nearly 700 GeV.
With the expected energy upgrade of the LHC the frequency of boosted final states will increase and even more sophisticated methods will be needed to search for physics beyond the Standard Model. The future, is certainly boosted.