Periodic Reporting for period 4 - hipQCD (Highest Precision QCD predictions for a new era in Higgs boson phenomenology)
Periodo di rendicontazione: 2023-12-01 al 2025-02-28
High-energy colliders allow us to study the fundamental particles and interactions that govern our universe. Extracting reliable information from these experiments requires highly sophisticated theoretical calculations capable of modelling the complex quantum environment of high-energy collisions.
Existing methods often lack the precision needed to fully exploit the LHC’s capabilities, limiting our ability to determine key Standard Model parameters ad features, and to identify potential signs of new physics.
Understanding the laws of nature is a central scientific endeavour but also a driver of technological innovation. Improving theoretical tools ensures that society’s investment in large-scale experiments such as the LHC
yields maximal scientific output, while the methods developed often benefit wider fields, from advanced computing to data analysis.
The hipQCD project addresses these challenges by developing innovative techniques for high-precision collider predictions and applying them to detailed phenomenological studies. Its main objectives include:
- devising new "subtraction frameworks" for higher-order Standard Model calculations;
- computing complex scattering amplitudes and studying their mathematical structure;
- performing precision analyses of Higgs and electroweak processes;
- testing the internal consistency of the Standard Model and searching for deviations that may indicate new physics.
Ultimately, the techniques and results developed in hipQCD aim to answer fundamental questions such as: What is the precise structure of Higgs interactions? and Do the particles and forces of nature behave exactly as predicted by the Standard Model, or do subtle discrepancies point toward new phenomena?
Existing methods often lack the precision needed to fully exploit the LHC’s capabilities, limiting our ability to determine key Standard Model parameters ad features, and to identify potential signs of new physics.
Understanding the laws of nature is a central scientific endeavour but also a driver of technological innovation. Improving theoretical tools ensures that society’s investment in large-scale experiments such as the LHC
yields maximal scientific output, while the methods developed often benefit wider fields, from advanced computing to data analysis.
The hipQCD project addresses these challenges by developing innovative techniques for high-precision collider predictions and applying them to detailed phenomenological studies. Its main objectives include:
- devising new "subtraction frameworks" for higher-order Standard Model calculations;
- computing complex scattering amplitudes and studying their mathematical structure;
- performing precision analyses of Higgs and electroweak processes;
- testing the internal consistency of the Standard Model and searching for deviations that may indicate new physics.
Ultimately, the techniques and results developed in hipQCD aim to answer fundamental questions such as: What is the precise structure of Higgs interactions? and Do the particles and forces of nature behave exactly as predicted by the Standard Model, or do subtle discrepancies point toward new phenomena?
The project has delivered substantial advances in both theoretical methodology and phenomenological applications relevant to precision studies at the Large Hadron Collider (LHC).
A major achievement is the completion of the formulation by group members and international collaborators of the nested soft-collinear (NSC) subtraction scheme, which now provides a robust, efficient, and general framework for computing next-to-next-to-leading order (NNLO) corrections in perturbative QCD across a broad class of collider processes.
This methodological breakthrough represents the foundation for many of the project’s subsequent results.
An important and unexpected development is the discovery that the NSC framework is sufficiently flexible to accommodate mixed QCD–Electroweak corrections. Building on this insight, we performed the first computation of mixed QCD–Electroweak corrections to
gauge-boson production at the LHC under realistic experimental conditions. We explored their implications for the extraction of Standard Model parameters, demonstrating that these effects are essential for achieving the target precision of current and future measurements.
We have also applied the NSC scheme to precision Higgs studies in key production channels, including associated Higgs production (VH) and vector boson fusion (VBF), with fully modelled Higgs decays. For VH production, we identified significant shortcomings in the standard
theoretical treatment of final states involving b-quarks and developed an improved calculation that systematically addresses part of these issues. Furthermore, we investigated the interplay between higher-order corrections and possible new-physics contributions,
ensuring that precision Standard Model effects are not misinterpreted as Beyond-the-Standard-Model (BSM) signals.
On the theoretical side, the group has obtained landmark results for multi-loop scattering amplitudes, which are central ingredients of precision collider predictions. Highlights include:
- the computation of two-loop amplitudes for di-photon production accompanied by an additional photon or a QCD parton — the latter representing the first complete two-loop calculation for a 2→3 process with the full colour structure of QCD;
- new progress toward even higher-order predictions, culminating in the first-ever results for three-loop 2 → 2 scattering amplitudes in QCD;
- the application of advanced numerical methods to compute intricate scattering amplitudes, leading, for example, to a complex two-loop electroweak amplitude relevant for dark-matter studies at the LHC;
- investigations into the structure of high-energy scattering, shedding light on the underlying dynamics that govern fundamental interactions in the high-energy limit.
In parallel, together with international collaborators we have performed dedicated studies of non-perturbative corrections, quantifying their impact in regimes where perturbation theory alone is insufficient.
This has allowed us to assess the limits of purely perturbative approaches and to understand how non-perturbative physics might affect the precision goals of current and future LHC analyses.
Complementing these theoretical advances, we have performed phenomenological studies addressing important open issues in collider physics. These include:
- the development of a theoretically well-defined and experimentally practical definition of a “quark jet,” a long-standing problem in precision jet physics;
- applications of our results to deepen our understanding of the Higgs boson, both within the Standard Model — for example through improved strategies to constrain its total width — and beyond it, by analysing the interplay of Standard Model and BSM effects in processes such as VBF.
Group members have also contributed important tools and resources to the wider community. Several of our results have already been used by other research groups for
precision phenomenology, demonstrating the impact and broad relevance of the work carried out within the project.
A major achievement is the completion of the formulation by group members and international collaborators of the nested soft-collinear (NSC) subtraction scheme, which now provides a robust, efficient, and general framework for computing next-to-next-to-leading order (NNLO) corrections in perturbative QCD across a broad class of collider processes.
This methodological breakthrough represents the foundation for many of the project’s subsequent results.
An important and unexpected development is the discovery that the NSC framework is sufficiently flexible to accommodate mixed QCD–Electroweak corrections. Building on this insight, we performed the first computation of mixed QCD–Electroweak corrections to
gauge-boson production at the LHC under realistic experimental conditions. We explored their implications for the extraction of Standard Model parameters, demonstrating that these effects are essential for achieving the target precision of current and future measurements.
We have also applied the NSC scheme to precision Higgs studies in key production channels, including associated Higgs production (VH) and vector boson fusion (VBF), with fully modelled Higgs decays. For VH production, we identified significant shortcomings in the standard
theoretical treatment of final states involving b-quarks and developed an improved calculation that systematically addresses part of these issues. Furthermore, we investigated the interplay between higher-order corrections and possible new-physics contributions,
ensuring that precision Standard Model effects are not misinterpreted as Beyond-the-Standard-Model (BSM) signals.
On the theoretical side, the group has obtained landmark results for multi-loop scattering amplitudes, which are central ingredients of precision collider predictions. Highlights include:
- the computation of two-loop amplitudes for di-photon production accompanied by an additional photon or a QCD parton — the latter representing the first complete two-loop calculation for a 2→3 process with the full colour structure of QCD;
- new progress toward even higher-order predictions, culminating in the first-ever results for three-loop 2 → 2 scattering amplitudes in QCD;
- the application of advanced numerical methods to compute intricate scattering amplitudes, leading, for example, to a complex two-loop electroweak amplitude relevant for dark-matter studies at the LHC;
- investigations into the structure of high-energy scattering, shedding light on the underlying dynamics that govern fundamental interactions in the high-energy limit.
In parallel, together with international collaborators we have performed dedicated studies of non-perturbative corrections, quantifying their impact in regimes where perturbation theory alone is insufficient.
This has allowed us to assess the limits of purely perturbative approaches and to understand how non-perturbative physics might affect the precision goals of current and future LHC analyses.
Complementing these theoretical advances, we have performed phenomenological studies addressing important open issues in collider physics. These include:
- the development of a theoretically well-defined and experimentally practical definition of a “quark jet,” a long-standing problem in precision jet physics;
- applications of our results to deepen our understanding of the Higgs boson, both within the Standard Model — for example through improved strategies to constrain its total width — and beyond it, by analysing the interplay of Standard Model and BSM effects in processes such as VBF.
Group members have also contributed important tools and resources to the wider community. Several of our results have already been used by other research groups for
precision phenomenology, demonstrating the impact and broad relevance of the work carried out within the project.
The results achieved in this project have collectively pushed the precision frontier of collider physics well beyond the previous state of the art.
The formulation of the nested soft-collinear (NSC) subtraction scheme constitutes a major methodological leap, providing a general, robust, and efficient framework for NNLO calculations that is versatile enough to also easily handle mixed QCD–Electroweak corrections.
Our first computations of such mixed corrections for LHC processes, together with improved theoretical descriptions of key Higgs production channels and a critical reassessment of standard treatments of heavy-quark final states, have contributed establishing new benchmarks for precision phenomenology.
On the formal theory side, the project has delivered several landmark achievements: the first complete two-loop 2 → 3 amplitude with full QCD colour structure; the first-ever three-loop 2 → 2 scattering amplitudes in QCD; and complex two-loop electroweak amplitudes obtained using advanced numerical methods.
These results significantly extend the class of processes for which precision predictions are achievable. Additional progress on the structure of high-energy scattering, the establishment of a theoretically sound and experimentally usable definition of quark jets, investigations on the structure of non-perturbative corrections
and refined strategies to probe Higgs properties within and beyond the Standard Model further underscore the project’s impact. Some of the tools and results developed here are already being adopted by other groups, highlighting their broad relevance and the extent to which this work has advanced the field beyond previous capabilities.
The formulation of the nested soft-collinear (NSC) subtraction scheme constitutes a major methodological leap, providing a general, robust, and efficient framework for NNLO calculations that is versatile enough to also easily handle mixed QCD–Electroweak corrections.
Our first computations of such mixed corrections for LHC processes, together with improved theoretical descriptions of key Higgs production channels and a critical reassessment of standard treatments of heavy-quark final states, have contributed establishing new benchmarks for precision phenomenology.
On the formal theory side, the project has delivered several landmark achievements: the first complete two-loop 2 → 3 amplitude with full QCD colour structure; the first-ever three-loop 2 → 2 scattering amplitudes in QCD; and complex two-loop electroweak amplitudes obtained using advanced numerical methods.
These results significantly extend the class of processes for which precision predictions are achievable. Additional progress on the structure of high-energy scattering, the establishment of a theoretically sound and experimentally usable definition of quark jets, investigations on the structure of non-perturbative corrections
and refined strategies to probe Higgs properties within and beyond the Standard Model further underscore the project’s impact. Some of the tools and results developed here are already being adopted by other groups, highlighting their broad relevance and the extent to which this work has advanced the field beyond previous capabilities.