arXiv subjects

Birgit Stapf

Publications and source records attributed to Birgit Stapf.

4 recordsLinked to original sources

arXiv2026

Modern jet flavour tagging in hadronic Z decays with archived ALEPH data

We present a reanalysis of archived data from the ALEPH experiment at LEP in the $\mathrm{Z \to q\bar{q}}$ final state. We apply modern jet flavour tagging techniques to improve the separation between the different hadronic decay channels of the Z boson, achieving up to one order of magnitude improvement in misidentification rate for b- and c-quark jets compared to the legacy algorithms used for the most recent ALEPH results, for the same identification efficiency. We also present the first implementation of strange quark jet tagging with LEP data, which allows for the selection of a $\mathrm{Z \to s\bar{s}}$ enriched event sample. These improvements in the flavour tagging performance are achieved by leveraging the lifetime, particle identification, and secondary vertex information, as well as modern classifiers based on a deep learning approach. We also demonstrate the calibration of the tagger in data using a tag-and-probe method, obtaining good data to simulation agreement for all quark flavours. These results pave the way for improved measurements of electroweak precision observables with LEP archived data, and can serve as a guidance for the development of detectors and algorithms for future electron-positron colliders.

hep-ex↗

arXiv2026

Differential measurements of $\bar{t}tZ$ and $\bar{t}t\bar{t}t$ at large $Q^2$ at FCC-hh

This contribution presents studies of differential top-quark measurements at the Future Circular Collider in its proton-proton stage (FCC-hh). The analyses target the leptonic final states of the $\bar{t}tZ$ and $\bar{t}t\bar{t}t$ production processes. Particular focus is given to the high-$Q^2$ regime, where sensitivity to new physics, as encoded in an Effective Field Theory framework, is enhanced. The dataset corresponds to an integrated luminosity of 30 $\text{ab}^{-1}$ at $\sqrt{s} = 84$ TeV. The reconstructed transverse momentum $\text{p}_\text{T}(Z_{\ell\ell})$ distribution is shown to be measurable up to approximately 2 TeV with a precision of 20% in the high energy region ($\text{p}_\text{T}(Z_{\ell\ell})$ >1.8 TeV). The scalar transverse momentum $\text{H}_\text{T}$ of the four-top production reaches 3.5 TeV with a precision of 35%. Additionally, a dedicated study of the lepton reconstruction efficiency shows that redefining the isolation variable to account for the highly boosted objects at FCC-hh improves the total signal yield in $\bar{t}tZ$ by a factor of 1.5.

hep-ex↗

arXiv2025

Early Career Researcher Input to the European Strategy for Particle Physics Update: White Paper

This document, written by early career researchers (ECRs) in particle physics, aims to represent the perspectives of the European ECR community and serves as input for the 2025--2026 update of the European Strategy for Particle Physics. With input from a community-wide survey, it highlights key challenges faced by ECRs -- career stability, funding access and long-term research opportunities -- while proposing policy recommendations and targeted initiatives. It underscores the importance of practices fostering diverse, equitable, inclusive and healthy workplaces, as well as of stronger ECR communities, and highlights how effective communication and interdisciplinary collaborations reinforce the societal relevance of particle physics and promote continued support for large-scale and long-term projects. Finally, the future of both collider and beyond-collider experiments is addressed, emphasising the critical role of ECRs in shaping future projects. The ECR contribution is formed of two parts: the ten-page executive summary submitted as input to the European Strategy for Particle Physics Update and, as backup document, this extended white paper providing additional context.

hep-ex↗

arXiv2023

Higgs self-coupling measurements at the FCC-hh

The hadron collider phase of the Future Circular Collider (FCC-hh) is a proton-proton collider operating at a center-of-mass energy of 100 TeV. It is one of the most ambitious projects planned for the rest of this century and offers ample opportunities in the hunt for new physics, both through its direct detection reach as well as through indirect evidence from precision measurements. Extracting a precision measurement of the Higgs self-coupling from the Higgs pair production cross-section will play a key role in our understanding of electroweak symmetry breaking, as the self-coupling gives insight into the nature of the Higgs potential. With the large data set of in total 30 $\text{ab}^{-1}$ which is envisioned to be collected during the FCC-hh runtime the Higgs self-coupling will be determined down to the percent level. This paper presents prospect studies for Higgs self-coupling measurements in the $b\bar{b}\gamma\gamma$ and $b\bar{b}\ell\ell + E_{\text{T}}^{\text{miss}}$ final states, with the combined, expected precision on the Higgs self-coupling modifier $\kappa_{\lambda}$ reaching 3.2-5.7% at 68% confidence level, assuming all other Higgs couplings follow their Standard Model expectations and depending on the systematic uncertainties assumed. This high precision is mostly driven by the $b\bar{b}\gamma\gamma$ final state analysis, while the $b\bar{b}\ell\ell + E_{\text{T}}^{\text{miss}}$ final state - newly studied for its FCC-hh prospects in this document - on its own reaches a maximum precision of roughly 20% on $\kappa_{\lambda}$.

hep-ex↗