Top quark jets at the LHC

Abstract

We investigate the reconstruction of high $p_T$ hadronically decaying top quarks at the Large Hadron Collider. One of the main challenges in identifying energetic top quarks is that the decay products become increasingly collimated. This reduces the efficacy of conventional reconstruction methods that exploit the topology of the top quark decay chain. We focus on the cases where the decay products of the top quark are reconstructed as a single jet, a “top jet.” The most basic “top-tagging” method based on jet mass measurement is considered in detail. To analyze the feasibility of the top-tagging method, both theoretical and experimental aspects of the large QCD jet background contribution are examined. Based on a factorization approach, we derive a simple analytic approximation for the shape of the QCD jet mass spectrum. We observe very good agreement with the Monte Carlo simulation. We consider high-$p_T$ $t\overline{t}$ production in the standard model as an example, and show that our theoretical QCD jet mass distributions can efficiently characterize the background via sideband analyses. We show that with $25{\mathrm{fb}}^{-1}$ of data, our approach allows us to resolve top jets with $p_T\ge 1\mathrm{TeV}$, from the QCD background, and about 1.5 TeV top jets with $100{\mathrm{fb}}^{-1}$, without relying on $b$-tagging. To further improve the significance we consider jet shapes (recently analyzed in [10]), which resolve the substructure of energy flow inside cone jets. A method of measuring the top quark polarization by using the transverse momentum of the bottom quark is also presented. The main advantages of our approach are (i) the mass distributions are driven by first principle calculations, instead of relying solely on Monte Carlo simulation; (ii) for high $p_T$ jets ($p_T\ge 1\mathrm{TeV}$), IR-safe jet shape variables are robust against detector resolution effects. Our analysis can be applied to other boosted massive particles such as the electroweak gauge bosons and the Higgs.