  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.
