Measurement of the production cross section of pairs of isolated photons in pp collisions at 13 TeV with the ATLAS detector

A measurement of prompt photon-pair production in proton-proton collisions at √s = 13 TeV is presented. The data were recorded by the ATLAS detector at the LHC with an integrated luminosity of 139 fb−1. Events with two photons in the well-instrumented region of the detector are selected. The photons are required to be isolated and have a transverse momentum of pT,γ1(2) > 40 (30) GeV for the leading (sub-leading) photon. The differential cross sections as functions of several observables for the diphoton system are measured and compared with theoretical predictions from state-of-the-art Monte Carlo and fixed-order calculations. The QCD predictions from next-to-next-to-leading-order calculations and multi-leg merged calculations are able to describe the measured integrated and differential cross sections within uncertainties, whereas lower-order calculations show significant deviations, demonstrating that higher-order perturbative QCD corrections are crucial for this process. The resummed predictions with parton showers additionally provide an excellent description of the low transverse-momentum regime of the diphoton system.publishedVersio

Search for charginos and neutralinos in final states with two boosted hadronically decaying bosons and missing transverse momentum in pp collisions at √s=13 TeV with the ATLAS detector

A search for charginos and neutralinos at the Large Hadron Collider using fully hadronic final states and missing transverse momentum is reported. Pair-produced charginos or neutralinos are explored, each decaying into a high-pT Standard Model weak boson. Fully hadronic final states are studied to exploit the advantage of the large branching ratio, and the efficient rejection of backgrounds by identifying the high-pT bosons using large-radius jets and jet substructure information. An integrated luminosity of 139  fb−1 of proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 13 TeV is used. No significant excess is found beyond the Standard Model expectation. Exclusion limits at the 95% confidence level are set on wino or higgsino production with various assumptions about the decay branching ratios and the type of lightest supersymmetric particle. A wino (higgsino) mass up to 1060 (900) GeV is excluded when the lightest supersymmetry particle mass is below 400 (240) GeV and the mass splitting is larger than 400 (450) GeV. The sensitivity to high-mass winos and higgsinos is significantly extended relative to previous LHC searches using other final states.publishedVersio

The ABC130 barrel module prototyping programme for the ATLAS strip tracker

For the Phase-II Upgrade of the ATLAS Detector, its Inner Detector, consisting of silicon pixel, silicon strip and transition radiation sub-detectors, will be replaced with an all new 100 % silicon tracker, composed of a pixel tracker at inner radii and a strip tracker at outer radii. The future ATLAS strip tracker will include 11,000 silicon sensor modules in the central region (barrel) and 7,000 modules in the forward region (end-caps), which are foreseen to be constructed over a period of 3.5 years. The construction of each module consists of a series of assembly and quality control steps, which were engineered to be identical for all production sites. In order to develop the tooling and procedures for assembly and testing of these modules, two series of major prototyping programs were conducted: an early program using readout chips designed using a 250 nm fabrication process (ABCN-25) and a subsequent program using a follow-up chip set made using 130 nm processing (ABC130 and HCC130 chips). This second generation of readout chips was used for an extensive prototyping program that produced around 100 barrel-type modules and contributed significantly to the development of the final module layout. This paper gives an overview of the components used in ABC130 barrel modules, their assembly procedure and findings resulting from their tests.Comment: 82 pages, 66 figure

A Search for Long-Lived Heavy Neutral Leptons in Events with Displaced Vertices Using the ATLAS Experiment

The existence of right-handed neutrinos with Majorana masses below the electroweak scale could give rise to the smallness of neutrino masses and matter-antimatter asymmetry in the universe. This analysis searches for long-lived right-handed neutrinos, called heavy neutral leptons (HNLs), produced through a $W$ boson decaying into a muon (electron) and a muon (electron) (anti-)neutrino that will mix with an HNL. The ATLAS reconstruction chain went through significant improvements in preparation for data taking in 2022. The improved techniques for reconstructing the displaced signature of the long-lived HNL are applied to 140.1 fb$^{-1}$ of data used in this analysis collected at $\sqrt{s}=13$ TeV in 2015--2018. The search results are interpreted in scenarios where the HNL mixes with one more Standard Model neutrino

Higgs differential cross-sections and CP Higgs couplings results with the ATLAS experiment

With the pp collision dataset collected at 13 TeV, detailed measurements of Higgs boson properties can be performed. The Higgs kinematics and CP properties can be measured with various production and decay modes and interpreted to constrain beyond-the-Standard-Model phenomena. This talk presents the measurements of Higgs boson differential and fiducial cross-sections as well as their combination and interpretations, and limits on the mixing of CP-even and CP-odd Higgs states are set by exploiting the properties of diverse final state

Machine learning techniques for cross-section measurements for the vector-boson-fusion production of the Higgs boson in the H→WW∗→eνμνH\rightarrow WW^\ast\rightarrow e\nu\mu\nu decay channel with the ATLAS detector.

This talk reports on the measurements of the fiducial and differential production cross section of Higgs bosons with an electron, a muon, and two energetic neutrinos from the decay of W bosons in the final state. The understanding of the fundamental properties of the Higgs boson is one of the main goals of the physics programme of the Large Hadron Collider. The analysis of 139 fb$^{-1}$ of proton–proton collision data at a centre-of-mass energy of $\sqrt(s)$ = 13 TeV recorded by the ATLAS experiment unlocks the study of the Higgs boson’s properties with unprecedented precision. While the first differential and fiducial production cross section measurements had been reported in the di-photon and four-lepton final states, the exploration of secondary production mechanisms in extreme kinematic regions has been heavily anticipated. Cutting-edge machine-learning-based methodologies are exploited for maximising the signal sensitivity while minimising the model-dependency of the results. The results are compared with state-of-the-art theoretical predictions. Furthermore, the measurements are used to constrain the presence of new phenomena in the framework of Effective Field Theories

Silicon strip detector system assembly for the ATLAS Inner Tracker upgrade at the LHC

During the past 18 months, I worked on the assembly of the strip detector system at Brookhaven National Laboratory for the Inner Tracker (ITk) detector for the planned upgrade of the ATLAS detector for the High Luminosity LHC. The ITk consists of two concentric silicon sub-detectors, the pixel and the strips detectors. The strip detector consists of 4 layers. Each layer consists of electrical modules glued on the local support structures, or $\it{staves}$. This report will discuss the work I did to assemble the first stave using the new-generation chips, ABC* and HCC*, and to perform the required metrology. I will also discuss the modifications to the engineering drawings for the staves to adhere to expected manufacturing standards

Machine learning techniques for cross-section measurements for the vector-boson fusion production of the Higgs boson in the H→WW∗→eνμν H\rightarrow WW^* \rightarrow e\nu\mu\nu\ decay channel with the ATLAS detector

This article reports on the measurements of the fiducial and differential production cross section of Higgs bosons with an electron, a muon, and two energetic neutrinos from the decay of $W$ bosons in the final state. The understanding of the fundamental properties of the Higgs boson is one of the main goals of the physics programme of the Large Hadron Collider. The analysis of 139 fb$^{-1}$ of proton--proton collision data at a centre-of-mass energy of $\sqrt s$ = 13 TeV recorded by the ATLAS experiment unlocks the study of the Higgs boson’s properties with unprecedented precision. While the first differential and fiducial production cross section measurements had been reported in the diphoton and four-lepton final states, the exploration of secondary production mechanisms in extreme kinematic regions has been heavily anticipated. Cutting-edge machine- learning-based methodologies are exploited for maximising the signal sensitivity while minimising the model- dependency of the results. The results are compared with state-of-the-art theoretical predictions. Furthermore, the measurements are used to constrain the presence of new phenomena in the framework of Effective Field Theories

Evidence for the Higgs Boson Decay to a Boson and a Photon at the LHC

The first evidence for the Higgs boson decay to a Z boson and a photon is presented, with a statistical significance of 3.4 standard deviations. The result is derived from a combined analysis of the searches performed by the ATLAS and CMS Collaborations with proton-proton collision datasets collected at the CERN Large Hadron Collider (LHC) from 2015 to 2018. These correspond to integrated luminosities of around 140 fb^−1 for each experiment, at a center-of-mass energy of 13 TeV. The measured signal yield is 2.2 +/- 0.7 times the standard model prediction, and agrees with the theoretical expectation within 1.9 standard deviations

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