N-Jettiness: An Inclusive Event Shape to Veto Jets

TL;DR

The paper introduces N-jettiness, a global event shape variable that effectively vetoes additional jets in collider events, enabling precise theoretical predictions for exclusive N-jet cross sections at the LHC and Tevatron.

Contribution

It proposes N-jettiness as a new inclusive event shape to improve jet vetoes and provides a factorization framework for better theoretical control.

Findings

N-jettiness effectively vetoes extra jets in collider events.

The method allows for well-controlled theoretical predictions.

Provides a factorization formula with inclusive jet and beam functions.

Abstract

Jet vetoes are essential in many Higgs and new-physics analyses at the LHC and Tevatron. The signals are typically characterized by a specific number of hard jets, leptons, or photons, while the backgrounds often have additional jets. In such cases vetoing undesired additional jets is an effective way to discriminate signals and background. Given an inclusive event sample with N or more jets, the veto to have only N energetic jets defines an "exclusive" N-jet cross section. This strongly restricts the phase space of the underlying inclusive N-jet cross section and causes large double logarithms in perturbation theory that must be summed to obtain theory predictions. Jet vetoes are typically implemented using jet algorithms. This yields complicated phase-space restrictions and one often relies on parton-shower Monte Carlos, which are limited to leading-logarithmic accuracy. We introduce a global event shape "N-jettiness", tau_N, which is defined for events with N signal jets and vanishes in the limit of exactly N infinitely narrow jets. Requiring tau_N << 1 constrains radiation between the N signal jets and vetoes additional undesired jets. This provides an inclusive method to veto jets and to define an exclusive N-jet cross section that can be well-controlled theoretically. N-jettiness yields a factorization formula with inclusive jet and beam functions.