Jet substructure has emerged as a critical tool for LHC searches, but studies so far have relied heavily on shower Monte Carlo simulations, which formally approximate QCD at the leading-log level. We demonstrate that systematic higher-order QCD computations of jet substructure can be carried out by boosting global event shapes by a large momentum $Q$ and accounting for effects due to finite jet size, initial-state radiation (ISR), and the underlying event (UE) as $1/Q$ corrections. In particular, we compute the 2-subjettiness substructure distribution for boosted $Z\to q\overline{q}$ events at the LHC at next-to-next-to-next-to-leading-log order. The calculation is greatly simplified by recycling known results for the thrust distribution in $e^{+}{e}^{-}$ collisions. The 2-subjettiness distribution quickly saturates, becoming $Q$ independent for $Q\gtrsim 400\mathrm{GeV}$. Crucially, the effects of jet contamination from ISR/UE can be subtracted out analytically at large $Q$ without knowing their detailed form. Amusingly, the $Q=\infty$ and $Q=0$ distributions are related by a scaling by $e$ up to next-to-leading-log order.

• Received 25 April 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.092001