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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HiggsEFTatNLO (Precise Higgs Properties in the LHC Era)

Teaser

Summary

The most recent update of the European Strategy for Particle Physics has identified the study of the Higgs boson at the LHC in order to test the validity of the Standard Model (SM) as a top priority. The SM offers only a parametrisation for Electroweak (EW) symmetry breaking (EWSB), the mechanism by which elementary particles acquire their mass, with no underlying explanation for its origin. This remains a key, unanswered question at the frontier of particle physics. The road towards understanding EWSB and its relation to new physics lies in the precise study of the interactions of the SMâ€™s most massive particles: the Higgs, EW gauge bosons and the top quark. The most general, model-independent approach is to interpret measurements using Effective Field Theory (EFT). This supplements the SM with all possible new interactions consistent with its symmetries, in a controlled energy expansion with a well-defined matching procedure that maps models of heavy NP to the operator Wilson coefficients.

The proposal develops a strategy to incorporate recent developments in higher order calculations for effective field theory (EFT) into global fits to high energy collider data with a particular focus on, though not limited to, Higgs physics at the LHC. I investigate the incorporation of Quantum Chromo Dynamics (QCD) corrections interfaced with parton shower and hadronisation along with the effects of renormalisation group mixing of the Wilson coefficients. I will deliver a software package for efficient translation between different EFT descriptions, providing both a connection to other high energy physics tools for EFT, a framework for the analysis and interpretation of the global fits and an implementation of operator mixing within this framework. These elements will combine to extract maximum information from existing collider data in terms of limits on EFT coefficients and their interpretations in the context of UV models, paving the way for the study of EFT at the LHC and future colliders.

Work performed

The project constituted the development of a state-of-the art model implementation for Monte Carlo predictions in the Standard Model EFT (SMEFT). This is the first tool to be compatible with general purpose event generation software, enabling predictions at Next-to-Leading order in QCD for processes that probe the Higgs, top and EW gauge sectors. Its development led to four projects exploiting the simulation capacities and developing understanding of SMEFT phenomenology through analytical calculations, machine learning methods and the study of new collider processes. Each project led to a peer-reviewed journal publication.

One presented novel results and statistical sensitivity analyses for precision predictions in EW Higgs production processes, and another for single top production in association with a Higgs or Z-bosons. The third presents an analysis of high energy top quark scattering involving Higgs and EW gauge bosons. It lead to new understanding of the connection between unitarity-violation and the effects from higher dimensional operators. New collider processes were studied to determine the sensitivity to the SMEFT parameters and quantify the transferral of the high-energy behaviour to the full process. The fourth studied top pair production process in association with a pair of b quarks as a probe, through SMEFT, of new physics strongly coupled to third generation quarks. This is a well motivated scenario for new physics and we determined sensitivity to previously unconstrained directions in parameter space. Novel machine learning methods were developed for multi-dimensional parametric sensitivity that are being exploited in an experimental analysis by the CMS collaboration.

The Rosetta software for translation between operator bases in SMEFT has been officially released, hosted on a high energy physics software repository. Tthis code has been significantly developed during the reporting period, adding a command-line interface and 4 new modules, quantifying the compatibility of SMEFT parameter space points to experimental data.

The results have been disseminated through 5 contributions to white papers and monographs, and presented at over 15 international conferences, workshops and seminars. During the reporting period, a 4-day, specialist international workshop \'Higgs and Effective Field Theory 2019\' was organised at the host institution, with the fellow acting as chair of the organisation committee.

The fellow also participated in the outreach activities at the Host institution. In particular, he was involved 2018 and 2019 editions of the \'Printemps des sciences\' activity delivering a public engagement talk entitled \'Le Monde des Particules\' and leading an LHC data analysis workshop for High school students.

Final results

The Monte Carlo simulation tool, SMEFTatNLO, constitutes a significant development in precision simulations of model-independent new physics effects at colliders. SMEFT is a major element of the LHC physics programme. It is especially important given the lack of direct evidence for new physics at the LHC so far. The too provides the necessary precision for SMEFT simulations for the duration of the LHC lifetime. It impact on the community is evidenced by numerous invitations for the fellow to present progress in international workshops & conferences including events organised by experimentalists such as CMS week and the LHC Higgs Cross Section and Top Working groups. It is being validated by LHC experimental collaborations to use in future LHC measurements. The functionality provided by Rosetta is also an important part of the SMEFT analyser\'s toolkit and its modules will be exploited to analyse results of forthcoming global fits to SMEFT parameters.

The projects that arose during the development of SMEFTatNLO presented state-of-the art, NLO predictions for LHC relevant processes, essential in the pursuit of global constraints in the SMEFT parameter space. These have since been incorporated into global fits to top quark measurements at the LHC. Additionally, a new found understanding of the high energy behaviour of top, Higgs and EW gauge boson scattering was obtained. Finally it also led to novel machine learning techniques that can be exploited for optimal sensitivity.

The societal implications lie in the improvement of our understanding of the fundamental interactions of the SM. Particle physics research also brings benefits in the policy making sector, that can base its funding decisions on well-communicated, high quality research outcomes. By gaining the required precision, the LHC project will be able to obtain the best possible global understanding of EWSB and potentially find hints for new physics or further directions to improve our knowledge. This has far-reaching implications in terms of the future of a field that is deciding on the next big collider project. The research outcomes will enable sensitivity studies that bolster the science cases while preserving the long-term legacy of the LHC via a global SMEFT likelihood to take forwards into the future.