Opening new frontiers in multi-scale evolution of collider events: a dual pathway to precision

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For the decades to come, the pioneering investigation of the fundamental laws of nature at the Large Hadron Collider (LHC) and at future collider experiments will rely on the interpretation of complex scattering events by means of accurate theoretical calculations, which are instrumental to find small signals of elusive new phenomena. These calculations must describe the evolution of the system from the few particles produced in the high-energy scattering to the tens or hundreds of low-energy particles observed in the detectors. This proposal aims at transforming the core of our theoretical understanding of such a multi-scale evolution, which nowadays is often too inaccurate to cope with the striking experimental precision. The goal of JANUS is to develop innovative ideas and theoretical methods to advance significantly both approaches to the problem: Resummations and Parton-Shower-Monte-Carlo (PSMC) generators. JANUS will establish a deep connection between the two fields, resulting in novel technology for the accurate modelling of multi-scale effects. Its main objectives are: - To formulate new theoretical methods to tackle the resummation of complex collider observables in reactions with more than two emitters and scales with state-of-the-art accuracy. - To lay the theoretical foundations for a new generation of PSMC algorithms, capable of describing accurately the hard scattering and subsequent parton-shower evolution for reactions with different jet multiplicities with a new standard of precision. This technology will be applied to obtain state-of-the-art theoretical predictions for a broad range of critical observables in the important fields of Higgs and jet physics, enabling their optimal exploration at the LHC and future collider experiments.