Improved Treatment of Hard Radiation in New Physics Processes at the LHC

Abstract

Despite the continued success of the Large Hadron Collider, no clear evidence for the existence of new BSM particles has been identified to date, pushing the bounds on their masses to ever higher values. As such, increasing efforts have been made to constrain all remaining regions of parameter space where light new particles could still exist. To do so reliably requires accurate Monte Carlo simulations of signal events, often in the case that hard radiation is produced together with the new particles. In this thesis, we focus on using matrix-element corrections based on the Powheg formalism to improve the simulation of hard radiation produced in new physics events. The corrections have been implemented within the Herwig++ Monte Carlo event generator, both for squark-antisquark production at the LHC and a wide range of decay modes that occur in beyond the Standard Model physics scenarios. Taking supersymmetry as a test case, we find that corrections applied to radiation generated during either the production or decays of new particles each impact on the reach of analysis strategies sensitive to high transverse momentum jets, with the most important effect occurring when the former correction is applied in scenarios featuring a compressed new particle mass spectrum. Finally, we investigate the sensitivity of the LHC to supersymmetric scenarios using monotop signatures of a single top quark produced together with missing transverse energy. We present analysis strategies sensitive to compressed regions of parameter space, and compare their expected reach at the next run of the LHC to those of more traditional search
strategies.