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Effective Theories for Strong Interactions: Precision Tools to Meet Experiment

Final Report Summary - EFTSTRONG (Effective Theories for Strong Interactions: Precision Tools to Meet Experiment)

The goal of my research is to boost the precision of theoretical predictions that allow us to exploit the potential of both high-energy and high-intensity particle physics experiments. My main focus is on Standard Model (SM) analytic calculations, with a particular emphasis on strong interactions.
In the context of low-energy tests of the SM, the persisting discrepancy between the experimental value and the theory prediction for the anomalous magnetic moment of the muon provides an intriguing puzzle and a benchmark for New Physics scenarios. The significance of this deviation crucially depends on the theory uncertainties where the dominant role is played by virtual low-energy strong interaction effects, with the hadronic light-by-light (HLbL) contribution emerging as a roadblock. During my Marie Curie fellowship, I developed with my collaborators a novel formalism to reconstruct the HLbL amplitude by exploiting the general principles of unitarity, analyticity, gauge invariance and crossing symmetry. This framework based on dispersion theory allows us to rigorously define contributions to HLbL and link them to experimentally accessible quantities (form factors and cross sections). Our results pave the way for the first data-driven determination of the HLbL contribution to the muon anomalous magnetic moment with controlled theory uncertainties. I published several papers about details of the formalism and first numerical results, which attracted great attention in the community and led to several invitations to give talks at international conferences and seminars at research institutions.

With regard to the physics of hadronic jets at the Large Hadron Collider (LHC), a key open issue to enhance the reach of New Physics searches is how best to discriminate quark-initiated from gluon-initiated jets, based on jet substructure. During my Marie Curie fellowship, I studied the possibility that an analytic understanding of the fragmentation of a parton into a subset of final hadrons with arbitrary multiplicities could help us gain a deeper insight into jet substructure and quark/gluon discrimination. In a published paper, I studied a broad class of such observables which includes and generalizes jet substructure proxies commonly used in experimental analyses at the LHC. Building on this study, I started to collaborate with my Marie Curie supervisor on the quark/gluon discrimination power of a novel characterization of jet substructure which exploits features tied to both showering and fragmentation.
Based on a theory framework that I have recently developed, I have also worked on the numerical study of doubly-differential cross sections for the simultaneous measurement of two electron-positron event shape observables (angularities) that both require the resummation of large logarithmic terms in their perturbative expansions. I am presently completing a paper containing many novel aspects of the formalism and a detailed numerical analysis.

The theoretical background and the tools developed for my Marie Curie project allowed me to start a collaboration with the Particle Physics Group at the University of Vienna, which I joined after my fellowship period as a University Assistant (6-year position) with the opportunity to get my Habilitation. During my stay at CERN, I co-supervised a Master student from the University of Vienna, who recently obtained his degree with top marks for his thesis work on electroweak corrections to boosted top production. We are now in the process of finalizing a paper on this subject.
Building on my expertise, I worked also on physics beyond the SM and published a paper on effective field theories for Two-Higgs-Doublet Models with a former Marie Curie fellow at CERN.

I have attended many seminars, lectures, training courses and workshops at CERN, discussed with many leading researchers in my field and colleagues at CERN, followed rapid and interesting developments concerning LHC physics, both on the experimental and theoretical side. I started new collaborations and had the opportunity to directly interact with LHC experimentalists working on jet substructure in the group of Prof. Wulz (CERN and Vienna).

I co-organized the Vienna Central European Seminar 2016, in the context of the exhibition “The beginning of everything. About galaxies, quarks and collisions” at the Museum of Natural History in Vienna, whose primary goal was to communicate most recent scientific knowledge of particle physics and cosmology to a wide audience in a readily comprehensible manner. I helped arrange visits and guided tours for visitors of the Theory Department at CERN and collaborated with the Education, Communications and Outreach group for an interactive exhibition at CERN involving theory topics.
Thanks to my Marie Curie funds, I was able to visit my collaborators in Europe and in the USA, and to present the results of my research at several conferences and workshops.

Staying at CERN, I had the opportunity to experience on a daily basis how fundamental research in theoretical physics is able to constantly push the boundaries of experimental physics with important spin-off effects. Applications range from new high-performance distributed computing infrastructures for a near real-time access to a huge amount of data (as the one generated by the LHC) to magnets designed for LHC upgrades that will improve CAT scan procedure, imaging and cancer treatments.