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Precision Multi-Scale Predictions for the LHC: Higgs, Jets and Supersymmetry

Periodic Reporting for period 4 - MULTISCALE (Precision Multi-Scale Predictions for the LHC: Higgs, Jets and Supersymmetry)

Reporting period: 2021-03-01 to 2021-08-31

The Large Hadron Collider (LHC) is leading the search for new physics at the shortest distance scales. The LHC has discovered the Higgs boson, and precise measurements of its properties or of other scattering processes may lead to the discovery of new physics (beyond the Standard Model). To identify small deviations in the data due to new physics requires precise theory predictions, which is the aim of this project. This project focusses on the effect of the strong force, that is the dominant source of uncertainty. In particular, we address the fact that many measurements at the LHC are sensitive to several different length scales, which we can disentangle using effective field theories. The overall objectives are: the development of new effective field theories to account for these multiple scales in scattering processes at the LHC, and their application to a wide range of processes and measurements. Jets feature prominently in this research, as these sprays of collimated hadrons are produced copiously at the LHC and naturally lead to additional scales.
We have developed an effective field theory framework to account for multiple scales in scattering processes, addressing additional scales that are due to: performing multiple measurements on the same final state, hierarchies between jet energies or angles, the size of the jet, jet grooming techniques that are used to remove contamination, etc. Highlighting two important applications we studied: Higgs plus jet production at the LHC, and the groomed momentum sharing variable. The latter has been measured extensively, as it probes fundamental properties of quarks and gluons. Our calculations take their theoretical description to the next level. There were also surprises during this project: for example, we discovered that a novel jet definition leads to considerable simplifications in (and thereby much more precise) theoretical calculations. We proposed using this as a new way of measuring the three-dimensional structure of the proton using jets, that could play an important role at the future Electron-Ion Collider.
Monte Carlo parton showers are computer programs that provide a complete description of the final state produced by the scattering, but at limited accuracy (often leading logarithmic accuracy). On the other hand, fixed-order and resummed calculations are much more precise but provide little detail on the final state. In this project we showed that next-to-leading logarithmic accuracy and beyond is achievable for measurements that probe a fair amount of detail of the final state, through the development of new effective field theory techniques and their application to several interesting measurements and processes.
Our predictions for the momentum sharing variable compared to data from the ATLAS experiment.