Unitarity bounds on effective field theories at the LHC

Abstract

Abstract Effective Field Theory (EFT) extensions of the Standard Model are tools to compute observables (e.g. cross sections with partonic center-of-mass energy $$ \sqrt{\hat{s}} $$ s ̂ ) as a systematically improvable expansion suppressed by a new physics scale M. If one is interested in EFT predictions in the parameter space where M <$$ \sqrt{\hat{s}} $$ s ̂ , concerns of self-consistency emerge, which can manifest as a violation of perturbative partial-wave unitarity. However, when we search for the effects of an EFT at a hadron collider with center-of-mass energy $$ \sqrt{s} $$ s using an inclusive strategy, we typically do not have access to the event-by-event value of $$ \sqrt{\hat{s}} $$ s ̂ . This motivates the need for a formalism that incorporates parton distribution functions into the perturbative partial-wave unitarity analysis. Developing such a framework and initiating an exploration of its implications is the goal of this work. Our approach opens up a potentially valid region of the EFT parameter space where M ≪ $$ \sqrt{s} $$ s . We provide evidence that there exist valid EFTs in this parameter space. The perturbative unitarity bounds are sensitive to the details of a given search, an effect we investigate by varying kinematic cuts.