Periodic Reporting for period 1 - JANUS (Opening new frontiers in multi-scale evolution of collider events: a dual pathway to precision)
Reporting period: 2022-11-01 to 2025-04-30
The Large Hadron Collider (LHC), a 27-kilometre ring of superconducting magnets, is the world's largest and most powerful particle accelerator. Inside the LHC, two high-energy particle beams travelling in opposite directions collide at speeds close to the speed of light. Fundamental research about critical observables in the fields of Higgs and jet physics depend on theoretical calculations describing the evolution of the system. The ERC-funded JANUS project will enhance the accuracy of these calculations by establishing connections between two important fields, resummation techniques and parton shower Monte Carlo event generators, computer programs that simulate the final states of high-energy collisions down to the level of individual stable particles. JANUS conducts theoretical and algorithmic research aimed at deepening our understanding of the complex structure of strong interactions and at developing novel computational methods for the accurate prediction of scattering events at the LHC and future particle colliders.
The JANUS project lies at the intersection of QCD factorisation/resummation and Monte Carlo event generators. Its primary goals are to deepen our understanding of the all-order structure of scattering observables in perturbative QCD and to bridge QCD resummation with Monte Carlo parton showers, aiming to develop event generators with higher perturbative accuracy. The JANUS team, in collaboration with an international network of researchers, has been advancing this effort on two fronts.
The first front focuses on developing advanced field-theoretic and computational methods to achieve high-precision predictions for collider observables in infrared regimes. This includes NNLL and N3LL resummation of jet observables in multi-leg scattering processes and reactions with heavy quarks, as well as exploring new theoretical techniques to describe the infrared behavior of energy and charge correlators. Insights from these studies are also applied to construct slicing methods for fixed-order perturbative calculations in these scattering processes. The resulting methodology will enable state-of-the-art theoretical predictions for key observables relevant to collider phenomenology at the LHC and future particle colliders.
The second research front focuses on developing Monte Carlo generators with improved perturbative accuracy. A key aspect is the parton shower stage, where energetic particles produced in the high-energy collision undergo copious QCD radiation. The PI and collaborators have laid the theoretical groundwork for NNLL-accurate parton shower algorithms, leading to a novel NNLL algorithm for the description of collinear fragmentation and to the first NNLL parton shower for QCD final states in lepton-lepton collisions.
Another critical component of Monte Carlo generators is the computation of higher-order QCD corrections to the hard scattering process, that is essential for precision phenomenology at the LHC. In this area, the JANUS team and collaborators are working on the formulation of a new class of collider observables to resolve the properties of multi-jet final states at lepton and hadron colliders. The infrared limit of these observables constitutes a key element in the computation of NNLO QCD corrections to processes with hadronic jets at the LHC within Monte Carlo event generators.
The first front focuses on developing advanced field-theoretic and computational methods to achieve high-precision predictions for collider observables in infrared regimes. This includes NNLL and N3LL resummation of jet observables in multi-leg scattering processes and reactions with heavy quarks, as well as exploring new theoretical techniques to describe the infrared behavior of energy and charge correlators. Insights from these studies are also applied to construct slicing methods for fixed-order perturbative calculations in these scattering processes. The resulting methodology will enable state-of-the-art theoretical predictions for key observables relevant to collider phenomenology at the LHC and future particle colliders.
The second research front focuses on developing Monte Carlo generators with improved perturbative accuracy. A key aspect is the parton shower stage, where energetic particles produced in the high-energy collision undergo copious QCD radiation. The PI and collaborators have laid the theoretical groundwork for NNLL-accurate parton shower algorithms, leading to a novel NNLL algorithm for the description of collinear fragmentation and to the first NNLL parton shower for QCD final states in lepton-lepton collisions.
Another critical component of Monte Carlo generators is the computation of higher-order QCD corrections to the hard scattering process, that is essential for precision phenomenology at the LHC. In this area, the JANUS team and collaborators are working on the formulation of a new class of collider observables to resolve the properties of multi-jet final states at lepton and hadron colliders. The infrared limit of these observables constitutes a key element in the computation of NNLO QCD corrections to processes with hadronic jets at the LHC within Monte Carlo event generators.
The research conducted by the JANUS team and their collaborators has already achieved significant advancements beyond the state of the art of the field, as outlined in the section "Work performed and main achievements". Ongoing work is expected to yield further key developments, with a detailed account to follow in due course.