Seminare SS2023

Dienstag, 11.07.2023, 16:30 Uhr

Ilaria Brivio   (University of Bologna, Italy)

Hunting new physics footprints: EFT interpretations of LHC data



About a decade after the Higgs discovery, the LHC faces the possibility that beyond-​Standard-Model particles might not be within its energy reach. In this case, new physics would only leave small indirect signatures in the observed spectra, that can be very challenging to detect. Effective Field Theories (EFTs) represent ideal frameworks to organize searches for these effects and, in the past few years, this has motivated enormous progress in the understanding of their theoretical properties, and also in the development of tools and strategies for their use at the LHC. 
The talk will give a pedagogical introduction to EFT interpretations of LHC data and will discuss status and prospects of a broad “EFT program”, that aims at learning “agnostically” about new physics sectors by combining large numbers of measurements across different sectors of the SM.


Host: Johannes Erdmann




Dienstag, 04.07.2023, 16:30 Uhr Physik Hörsaal

Colomba Brancaccio (RTG / RWTH Aachen University)

Top- to-Higgs fragmentation and its implications in ttH production



The QCD Factorization Theorem is a crucial framework in the realm of hadron collision and production. It enables the separation of the non-perturbative aspects of hadrons, making it an essential tool. When dealing with heavy quarks in scattering processes, the factorization theorem becomes applicable in isolating the high energy component from the low energy one, with the latter still attainable within perturbation theory. This presentation specifically focuses on the computation and usefulness of perturbative fragmentation functions, with particular emphasis placed on the calculation of top to Higgs fragmentation. Lastly, a brief overview is provided regarding the significance of top-to-Higgs fragmentation in the context of high precision calculations for ttH production.


Host: Lukas Simon



Lukas Simon (RTG / RWTH Aachen University)

 HiStory - Higgs-Strahlung Theory



Associated Higgs production with a massive gauge boson, or short Higgs-Strahlung, allows for precise measurements of the Higgs boson's properties, such as its mass, production rate and couplings. Beyond its utility for precision physics in the Standard Model (SM), this process makes possible New Physics effects accessible. Thereby, the asymmetry between WH and ZH production can offer a major advantage over other searches for such effects. For analyzing this asymmetry, precise theoretical predictions of fully differential cross section at NNLO in QCD are key. To obtain them efficiently we have to unravel the challenges introduced by infrared divergences for higher order corrections.
I my talk, I will outline the concept of subtraction schemes that allow us to deal with these infrared divergences, so that we are able to design infrared-safe observables that are sensitive to signatures beyond the SM.


Host: Colomba Brancaccio




Dienstag, 20.06.2023, 16:30 Uhr

Jonanathan Hermann (RTG / RWTH Aachen University)

Precision Top-Quark Physics Beyond the Standard Model



In this talk, I will present several different studies on modelling effects in BSM top-quark physics, with each of them highlighting a different aspect of BSM analyses. Modelling effects in this context encompass the treatment of unstable particles, either in the full off-shell approach or the narrow-width approximation, as well as the inclusion NLO QCD corrections. After giving a brief explanation on all of these aspects, I will present an analysis of top-quark pair associated dark matter production at the LHC. By calculating signal-strength exclusion limits for this process, I will underline the importance of accurately modelling the dominant SM background processes tt and ttZ. For the second study we instead focus on a NP signal and asses the size of higher-order corrections and off-shell effects in ttH production in the Higgs characterization model. This model extends the SM Yukawa interactions such that the Higgs boson can be a scalar, pseudoscalar or CP-mixed particle. Finally, in the third analysis we forego the usage of an explicit NP model altogether and instead analyze modelling effects in SMEFT, focusing again on ttH production.



Danilo Meuser (RWTH Aachen University)

Measurement of the differential dileptonic ttbar cross section in a BSM phase space at CMS



Measurements of the ttbar production cross section yield important precision tests of the Standard Model (SM), while also probing scenarios for physics beyond the SM (BSM). This analysis aims to measure the ttbar cross section in a phase space where additional contributions from BSM scenarios could be present. It is based on the data set recorded by CMS in the years 2016 to 2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb-1. The BSM scenarios considered include supersymmetric and dark matter models, where, similarly to the dileptonic ttbar channel, two leptons, b jets and undetected particles are produced. Unlike previous measurements, where the differential cross sections were mainly measured as a function of kinematic variables of the leptons or top quarks, this analysis focuses on observables related to the neutrinos, like the missing transverse momentum and the angular distance between the missing transverse momentum and the nearest lepton, to separate BSM from SM ttbar events. In order to increase the sensitivity of the analysis multivariant techniques are used which improve the resolution of the missing transverse momentum in SM ttbar events. The final results of the differential cross section measurements are compared to monte carlo simulations and fixed order theory predictions.


Host: Danilo Meuser/Jonathan Hermann




Dienstag, 06.06.2023, 16:30 Uhr

Terry Generet (RTG, RWTH Aachen University)

Top-pair production in association with a B-hadron at NNLO



In this talk, I will present NNLO QCD predictions for several differential distributions of B-hadrons in top-pair events at the LHC. After discussing the first calculation of this kind, I will describe several improvements that have been made recently. First of all, a new set of B-hadron fragmentation functions has been obtained, which features reduced uncertainties. Additionally, the decay of the produced B-hadron to a muon or a J/ψ meson has been incorporated, allowing us to make predictions for distributions involving those decay products, which is what is typically measured by experiments. These improvements are then applied to a second calculation, which studies the effect of the NNLO corrections on several methods of measuring the top-quark mass.


Host: Felix Eschment




Dienstag, 23.05.2023, 16:30 Uhr Zoom

Abideh Jafari (DESY and Isfahan University of Technology

Still on top after about three decades



The top quark continues to be appealing to high energy physicists even though its discovery was 30 years ago. The heaviest known particle has still a number of aspects to scrutinize, while it might be “the portal” to new theories that address the shortcomings of the standard model (SM) of particle physics. The wealth of LHC data at the highest-ever-reached energies provides a handful of events where top quarks are produced with multiple other heavy (or light) particles. We are in an era where, for the first time, top quark interactions can be measured directly and without strong assumptions on the underlying theory. I will go through a number of measurements targeting top quark electroweak interactions, and discuss how we are putting different pieces together towards a more complete picture of the SM top sector. Alongside this effort, I will point out other corners of the SM where different new phenomena might be routed in the same theory, going beyond what we know. 


Host: Kerstin Borras




Dienstag, 25.04.2023, 16:30 Uhr

Markus Steidl (KIT, Karlsruhe)

KATRIN experiment - What is the absolute mass scale of neutrinos?



The neutrino mass remains unfinished business for the standard model as not only the mass generating mechanism is unknown but even the mass scale. Different methods are currently applied in experiments to reveal this experimental challenge. In this talk the method of measuring the neutrino mass via spectroscopy of the beta electrons from the tritium decay with the KARlsruhe TRitium Neutrino experiment (KATRIN) is explained and latest results presented. At the end of the talk we look a few years ahead and get an idea how KATRIN will adapt its configuration to be also sensitive to sterile keV neutrinos.


Host: Robert Harlander




Dienstag, 18.04.2023, 16:30 Uhr

Rene Poncelet (Cambridge University, UK)

Jet identification and flavoured jet algorithms



The study of jets is a staple of the research program at the LHC. As suitably defined sets of highly-energetic particles, they constitute a valuable tool to establish a link between Quantum Chromodynamics (QCD) of quarks and gluons and the realm of actual strongly-interacting particles, baryons and mesons. Besides the general importance of jets for collider phenomenology, there is a growing interest in studying jet substructure to disentangle various QCD effects governing jet dynamics. Specifically, "flavoured" jets allow access to the partonic structure of the hard scattering event, which in turn can be used for Standard Model measurements, Parton Distribution Function fits, New Physics searches and tuning/improvement of Monte Carlo simulations. In this talk, I discuss the challenges in defining flavoured jet cross sections in theory and experiment and the implications for phenomenology.


Host: Malgorzata Worek




Dienstag, 11.04.2023, 16:30 Uhr

Rikkert Frederix (Lund University, Sweden)

Matching and Merging: Combining Matrix Elements and Parton Showers



The presentation will focus on the FxFx merging technique, which has become a popular method for simulating multi-jet final states in particle physics. The merging of matrix-element calculations at different jet multiplicities is a crucial step in the prediction of the event rates for high-energy collisions. In the FxFx merging technique, matrix-element calculations for fixed numbers of jets are combined using a set of merging scales to obtain a smooth and accurate description of the full jet multiplicity spectrum. We will provide an accessible overview of the theoretical basis of this technique and its practical implementation in the event generator MadGraph5_aMC@NLO. Additionally, we will showcase some recent work on improving this method in the context of "electroweak jets”.


Host: Malgorzata Worek 




Dienstag, 04.04.2023, 16:30 Uhr

Javier Llorente (Simon Fraser University, Canada)

Multijet production at the LHC: Event shapes, NNLO predictions and α_s


Abstrakt: Event shapes are a class of observables whose first non-trivial contributions in perturbative QCD arise from the radiation of a third jet. Measurements of such variables at hadron colliders can serve as a precision test of perturbative QCD. This talk will review the status of such measurements in the ATLAS experiment, as well as the comparison of these data to high-precision next-to-next-to-leading order predictions for three-jet production, which have become recently available. Fits of these theoretical predictions to measurements of Transverse Energy-Energy Correlations and their azimuthal asymmetry are performed to determine the value of the strong coupling constant αs at different energy scales, thus testing the running of the QCD coupling with unprecedented precision beyond the TeV scale.


Host: Michal Czakon