Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Studies of tt¯H(ττ) event topologies and kinematics with the ATLAS experiment

The tt¯H process grants one of the few opportunities to access the Yukawa coupling between the Higgs field and top quarks on tree level. This thesis examines possibilities for more elabor ate analysis methods going beyond the cut and count approach in the tt¯H(ττ) channel, with a focus on the semileptonic decay of the top quarks and the tau pair. Experience has shown that in the H → ττ channel the shape of the reconstructed invariant mass can be a powerful tool for signal extraction. In the channel considered reconstructing the invariant ττ mass be comes especially challenging as an ambiguity due to two charged leptons in the final state only one of which is a product of the H → ττ decay- arises. Moreover, the missing momenta of the neutrinos cannot be reconstructed easily. To address these problems, the specific topo logies and kinematics of tt¯H(ττ) events are studied. As simple, topological considerations are found to be insufficient for the intended purpose, a likelihood based kinematic fit to fully reconstruct the tt¯ system is employed in order to associate the two leptons correctly with their respective decay systems. Finally, a new likelihood, based on the transverse masses of the leptonically decaying top quark and W boson, is defined and its potential is investigated

Search for heavy resonances in the HZZ channel with ATLAS

This talk presents a search for an additional heavy Higgs boson decaying to a pair of SM Z bosons, covering heavy Higgs boson masses in the range between 200 GeV and 2 TeV. To maximize the sensitivity the search combines the two fully leptonic decay channels of the ZZ pair – ZZ→ 4l and ZZ → llvv, where l stands for a charged light lepton. The 4l channel profits from the very good resolution of the invariant mass of the 4 leptons, but its branching fraction is low. In contrast, events in the llvv channel are more abundant, but the final state is not fully reconstructable. For the llvv analysis, the transverse mass calculated from the transverse momentum of the charged lepton pair and the missing transverse energy, is used as an observable. In both channels, kinematic selections are applied and separate categories for the gluon–gluon fusion (ggF) and vector-boson fusion (VBF) production mode of the additional Higgs boson are defined. The results are interpreted as limits on the production cross section for an additional heavy Higgs boson under the narrow and large width assumption

Di-Higgs with missing transverse momentum at FCC-hh

The determination of the Higgs self-coupling from di-Higgs events with very high precision is one of the clearest benchmarks for the FCC-hh. Its potential has been well established already in several final states. In this talk studies into final states of the di-Higgs system which involve neutrinos are presented. The benefit of studying yet another di-Higgs final state is two-fold: First, any additional events included will add further precision to the measurement. Second, specifically neutrino channels will help to shed light on an experimental aspect for the FCC-hh which has not been well investigated yet: a robust reconstruction of the missing transverse momentum (ETMiss) is crucial for such analyses. It is is clear that ETMiss reconstruction at the FCC-hh will be extremely challenging due to the high pile-up environment, with average interactions per bunch crossing of the order of 1000. In particular, bbWW, bbττ and bbZZ signals are analysed in the final state with 2 light charged leptons in addition to ETMiss, using cut-based as well as multi-variate techniques. Their expected sensitivity is extracted, and the impact of different scenarios for systematic uncertainties, such as the worsening of the ETMiss resolution, is assessed

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 $\kappa_{\lambda}$

(Prospects of) 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 dataset of 30 which is envisioned to be collected during the FCC-hh runtime the Higgs self-coupling will be determined down to the percent level.This talk presents recent studies of di-Higgs measurements in various final states (e.g. , , ) and their combination. More challenging final states, such asare explored for their potential at the FCC-hh for the first time. Updates to the parametrization of detector scenarios for a project so far ahead in the future are discussed as well

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 $\kappa_{\lambda}$

Key4hep Software Stack for Detector Studies

Detector optimisation and physics performance studies are an integral part of the development of future collider experiments. The Key4hep project aims to design a common set of software tools for future, or even present, High Energy Physics projects. Based on the iLCSoft and FCCSW frameworks an integrated solution for detector simulation, reconstruction and analyses is being developed. This presentation will give a short overview of the major ingredients of the Key4hep turnkey software; the common event datamodel – EDM4hep, the Gaudi based event processing framework, some dedicated packages developed on top of these, and the use of the Spack package manager, which enables installations on a variety of platforms. A first example highlights the synergies that were already achieved by the collaboration of the different collider communities (CEPC, CLIC, FCC, ILC). It shows the seamless integration of fast simulation with Delphes and the LCFIplus vertexing processor from iLCSoft. This combination is possible due to EDM4hep, the processor wrapper (k4MarlinWrapper), and the k4SimDelphes framework integration in Key4hep. As a second example, the multi-threaded execution of the iLCSoft processors in the Gaudi framework making use of the k4MarlinWrapper will be showcased. The third example will demonstrate the execution of a multi-threaded simulation of a drift chamber in the CEPC experiment. The chosen application will take information of primary ionizations generated by an incident charged particle as input and will create the waveform as the final output, which represents the signal collected by the chamber’s signal wire. The simulation algorithm is based on a neural network model which can be used to simulate the effects from the ionization electron’s drift and avalanches. Using these examples we review some of the challenges and issues that we encountered as well as how they were addressed in the end. We close with a brief discussion of the next steps for the Key4hep project

Key4hep Software Stack for Detector Studies

Abstract Detector optimisation and physics performance studies are an integral part of the development of future collider experiments. The Key4hep project aims to design a common set of software tools for future, or even present, High Energy Physics projects. Based on the iLCSoft and FCCSW frameworks an integrated solution for detector simulation, reconstruction and analyses is being developed. This presentation will give a short overview of the major ingredients of the Key4hep turnkey software; the common event datamodel – EDM4hep, the Gaudi based event processing framework, some dedicated packages developed on top of these, and the use of the Spack package manager, which enables installations on a variety of platforms. A first example highlights the synergies that were already achieved by the collaboration of the different collider communities (CEPC, CLIC, FCC, ILC). It shows the seamless integration of fast simulation with Delphes and the LCFIplus vertexing processor from iLCSoft. This combination is possible due to EDM4hep, the processor wrapper (k4MarlinWrapper), and the k4SimDelphes framework integration in Key4hep. As a second example, the multi-threaded execution of the iLCSoft processors in the Gaudi framework making use of the k4MarlinWrapper will be showcased. The third example will demonstrate the execution of a multi-threaded simulation of a drift chamber in the CEPC experiment. The chosen application will take information of primary ionizations generated by an incident charged particle as input and will create the waveform as the final output, which represents the signal collected by the chamber’s signal wire. The simulation algorithm is based on a neural network model which can be used to simulate the effects from the ionization electron’s drift and avalanches. Using these examples we review some of the challenges and issues that we encountered as well as how they were addressed in the end. We close with a brief discussion of the next steps for the Key4hep project.</jats:p

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

International audienceThis 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