## Projects

Doktoral School “Particles and Interactions”, FWF-W1252-N27

Participants: André H. Hoang, Massimiliano Procura

The doctoral school covers all aspects of particle physics conducted in Vienna. Research covered at the particle physics group at the University of Vienna covers Standard Model phenomenology focusing on predictions of collider physics observables at the LHC and at future colliders and high precision.

Stand-Alone Project “Heavy Quark Masses from Jets using Effective Field Theory”, FWF-P28535-N27

Participants: Andre H. Hoang, Bahman Dehnadi, Christopher Lepenik, Adithia Pathak

Currently the most precise experimental measurements of the top quark mass are obtained from determinations of the top mass parameter in the Monte-Carlo event generator used for the experimental analyses based on kinematic fits to top quark events. However, the exact relation of the Monte-Carlo top mass parameter to a quantum-field-theoretically well-defined top quark mass is unknown. Thus the interpretation of these measurements is limited, because the results contain an intrinsic ambiguity, which has not even been quantified reliably up to now. The proposed project has the aim to remedy the situation by reaching two goals not achieved in the literature before. The first goal is a high precision theoretical calculation of so-called event shape distributions such as “thrust” for massive quark production at the observable particle level. Event shape distributions have a strong sensitivity to the quark mass and the precise calculation requires the implementation of quark mass definitions consistent at the quantum level. The required computations are carried out using effective field theory methods. These allow for a systematic summation of so-called large logarithmic corrections in perturbation theory as well as a consistent account of so-called hadronization corrections which describe the transition from quarks and gluons to the experimentally observable particles. The second goal is to analyze systematically the relation between the Monte-Carlo top quark mass parameter and field-theoretical top quark mass definitions by comparing the theory calculations of the electron-positron eventshape distributions with the corresponding Monte-Carlo results. This analysis will be carried out using numerical fits and will provide the information how the Monte-Carlo top quark mass parameter can be converted to a well-defined field-theoretic top quark mass definition.

Stand-Alone Project “Infrared Parton Shower Dynamics and the Top Quark Mass" FWF-P32383-N27

Participants: Andre H. Hoang, Simon Plätzer, Daniel Samitz

It is the aim of the project to analyse and quantify the theoretical scheme dependence of the top quark mass in Monte-Carlo event generators (MMC's) for observables that enter the direct reconstruction method. In a recent work on eventshapes it was found that the infrared cutoff of the parton shower evolution, which is the perturbative component in MMC’s, is the essential property that determines this scheme dependence. In the proposed project the parton shower algorithms shall be solved analytically for the observables mentioned above and compared with theoretical factorization computations in which the same infrared cutoff is accounted for. In this way the perturbative component of the scheme dependence of the top quark mass in MMC’s can be quantified coherently for observables that enter the direct reconstruction method. In addition, also non-perturbative corrections and effects of the finite top quark lifetime shall be analyzed in a systematic manner.

COST-Action "Uravelling New Physics at the LHC through the Precision Frontier”, CA16201

Local Participants: Andre H. Hoang, Simon Plätzer

The aim of this Action is to shift the precision frontier in theoretical predictions for high energy collisions a new level of accuracy and to create new resources of networking and innovation, with the quest for discovery of New Physics effects as the main motivation. It is designed to work through long-standing challenges on the basis of the most encouraging advances in quantum field theory and related areas of pure mathematics and computer science by uniting the leaders of the field in a coherent effort.

COST-Action "Vector Boson Scattering Coordination and Action Network", CA16108

Local participants: Simon Plätzer

The main goal of the VBSCan project is to investigate the Vector Boson Scattering (VBS) process and its implications for the Standard Model, by coordinating existing theoretical and experimental efforts in the area and by best exploiting hadron colliders data, thereby laying the groundwork for long-term studies of the subject and creating a solidly interconnected community of VBS experts.

Website: https://vbscanaction.web.cern.ch/ , https://www.cost.eu/actions/CA16108/

MCnet3 Initial Training Network Monte-Carlo Net

Local participants: Simon Plätzer

MCnet is a European Union funded Marie Curie Initial Training Network dedicated to developing and supporting general-purpose Monte Carlo event generators throughout the LHC era and beyond, and providing training of a wide selection of its user base, particularly through funded short-term 'residencies' and Annual Schools.

DACH Project “Short-distance constraints on the hadronic light-by-light”, FWF-I3845-N27

Participants: Massimiliano Procura, Jan Luedtke

The anomalous magnetic moment (AMM) of the muon is one of the most accurately measured quantities in particle physics and one of the very few to exhibit a significant discrepancy with respect to its Standard Model (SM) determination. The origin of this discrepancy is unknown and has traditionally been considered a harbinger of New Physics. In order to clarify this issue, two new experiments have been designed with the concrete goal to improve in the next few years the already astonishing accuracy of 0.54 parts per million reached by previous measurements.

This strongly calls for improved theory predictions.

SM uncertainties are dominated by virtual low-energy strong interaction effects that cannot be computed using perturbative methods. In particular, the hadronic light-by-light (HLbL) contribution is emerging as a potential roadblock. In order to evaluate it, we had so far to rely on hadronic models, which introduce sources of systematic errors that are impossible to quantify. In a recent theory breakthrough, we have set up a novel dispersive formalism for the first data-driven determination of the HLbL and produced first numerical estimates of HLbL within this approach. One of the most pressing open issues is that short-distance constraints on HLbL scattering are not yet incorporated into our formalism. The proposed project will fill this gap using perturbative and non-perturbative quantum field theory techniques. The importance of these constraints in the determination of the muon AMM will be assessed via a thorough numerical analysis. We will provide a theoretical guidance for controlled interpolations across all relevant kinematic regimes in terms of consistent dispersion relations for HLbL and will systematically study the role played in this context by hadronic resonances.

The proposed research represents a crucial and timely step forward for the completion of our data-driven determination of HLbL with controlled uncertainties, with the aim of being sufficiently accurate to make forthcoming measurements of the muon AMM a decisively stringent test of the SM.

Research in Teams Project @ ESI "Higher-order Corrections to Parton Branching at Amplitude Level" 13.1.-14.2.2020

Local participants: Simon Plätzer

Parton branching algorithms at amplitude level are vital to achieve resummation of logarithmically enhanced corrections to general particle collider observables. Specifically the so-called non-global nature of most observables sets the level of complexity and requires approaches beyond simple probabilistic reasoning to include quantum mechanical interference effects. Following our initial steps to explore amplitude-level evolution in the space of colour structures and related Monte Carlo methods, we now will consolidate this research program and perform first steps to include corrections at the next order in perturbation theory. This will go hand in hand with exploring formal aspects of the approach and an ongoing effort to put it into relation with effective field theory methods.