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Rare decays and Charged Lepton Flavour Violation at the ATLAS Experiment
No full textBeing heavily suppressed in the Standard Model (SM), any evidence of charged lepton violation occurring at the Large Hadron Collider (LHC) would be an unambiguous indication of New Physics. Similarly, flavour changing neutral current processes are also heavily suppressed in the SM due to the GIM mechanism. Precision measurements of such processes can therefore serve as powerful tests of both the SM and Beyond the Standard Model theories. This talk will provide an overview of searches for, and measurements of, these rare processes with the ATLAS Experiment at the LHC, with a focus on recent results. As well as the analysis of B-meson decays, searches for the rare decays of the top-quark and Higgs boson will also be discussed
The Present and Future of the CMS Muon System
No full textThe CMS muon system is indispensable for identifying and studying key particles and processes at the LHC. A high-performance muon system not only underpins the discovery potential of CMS but also ensures the precision and reliability required for advancing our understanding of the fundamental laws of nature. The legacy CMS muon detector system, consists of drift tube (DT) chambers in the barrel and cathode strip chambers (CSC) in the end-cap regions, complemented by Resistive Plate Chambers (RPC) in both the barrel and end-cap. During the long shutdown (LS) 2 period, Gas Electron Multiplier (GEM) chambers were added in the first station of forward regions to enhance the redundancy of the muon system while maintaining the precision of muon momentum resolution at the Level-1 trigger. Despite increased luminosity and more challenging detector conditions, the CMS Muon system has demonstrated the expected performance in Run 3 across a wide pseudorapidity range. In order to ensure the performance, reliability, and safety of the CMS muon system at the challenging conditions of increased instantaneous luminosity and higher pileup expected during the high-luminosity LHC (HL-LHC), simultaneous background and aging studies are being actively conducted. These studies are crucial for understanding the long-term behavior of all muon sub-detectors and therefore planning the detector upgrade strategy during the next Long Shutdown (LS3). This presentation provides a summary of the results from recent background and aging studies of the CMS muon system and a brief outlook to the future upgrades
Searches for new phenomena in hadronic final states using the ATLAS detector
No full textMany theories beyond the Standard Model predict new phenomena giving rise to multijet final states. These jets can originate from the decay of heavy resonances into Standard Model quarks or gluons, or from more complex decay chains involving dark-sector particles. Also of interest are resonant and non-resonant hadronic final states where jets stem from a dark sector, leading to a diverse phenomenology depending on the interactions between dark and visible particles. This contribution summarises recent ATLAS results based on 13 TeV Run~2 data, highlighting three complementary analyses targeting boosted light resonances, flavour-violating dark matter, and semi-visible jets
MUSIC: A Multi-Purpose Detector Concept for Physics at the 10 TeV Muon Collider
No full textThis work presents a proof of concept for MUSIC, a multi-purpose detector conceived for high-precision and ultra-high-energy physics studies in the challenging environment of TeV muon-antimuon collisions. The detector features a central tracking system, electromagnetic and hadronic calorimeters, and dedicated muon detectors. This paper outlines the main design elements of each subdetector, with an emphasis on the effects of machine-induced backgrounds and the reconstruction strategies employed for key physics objects. Performance results for electrons, photons, muons, and jets are reported, and studies of jet flavour identification are discussed
Axion couplings in Orbifold GUTs
No full textWe consider the coupling of axions to gauge bosons in higher-dimensional Grand Unified Theories (GUT) inspired by string theory constructions with D-branes on orbifold singularities, the so-called orbifold GUTs. Due to their topological properties, axion couplings to gauge bosons are independent of the gauge symmetry reduction mechanism and the background geometry -- they only depend on the embedding of the SM into the UV gauge group. There are two kinds of axions in this class of theories: axions coming from gauge fields in the bulk and axions localised on the boundaries. The axion-photon coupling for the bulk axions coincide with the results from 4-dimensional GUTs, where axions which couple to photons necessarily couple also to QCD. The brane-localised axions can couple to photons independently of the QCD coupling, but if gauge couplings are approximately unified, the axions get large masses from unsuppressed instantons on the boundaries. This means that the ratio is below (or equal to) the QCD axion value for all axions in orbifold GUTs, as is the case for unified theories in 4d
The ATLAS Forward Proton Time-of-Flight Detector System
No full textThe Time-of-Flight (ToF) detectors in the ATLAS Forward Proton (AFP) system are designed to determine the primary vertex z-position of the pp -> pXp processes by comparing the arrival times measured in the ToF for the two intact protons in the final state. We present performance studies in terms of efficiencies and timing resolutions using high-statistics, low, and moderate pile-up data collected during the ATLAS Run 2 and Run 3 periods. In Run 2, low efficiencies of a few percent are observed, and the resolutions of the two ToF detectors measured individually are 21 ps and 28 ps. This results in an expected precision of 5.3 ± 0.6 mm for the vertex reconstruction resolution provided by the ToF. This value aligns with the results from subsequent statistical analysis of the distribution of differences between the vertex z-positions reconstructed by the ATLAS central detector and the ToF. This distribution comprises a background component from combinatorics due to non-negligible pile-up and a significantly narrower signal component (6.0 ± 2.0 mm) from events where protons from the same interaction are detected in the ToF. During LHC long shutdown 2, the ToF detector underwent major upgrades in electronics, optics, and mechanics, expected to provide a substantial improvement in detection efficiency. Preliminary results for efficiency and resolution studies based on Run 3 data taken so far will be presented
EFT+Higgs Experimental Outlook
No full textThe talk will summarize the current state of EFT and Higgs measurements at the LHC, with an overview of how these measurements generally proceed. The remainder of the talk will discuss possible directions for further improvement and highlight recent work where such developments have started to be explored or could have an impact
The ATLAS Experiment in 2025: Exploration, Innovation and Collaboration at CERN’s Large Hadron Collider
No full textA year-end brochure providing an overview of the ATLAS Experiment in 2025, highlighting major milestones in operations, physics results, upgrade progress, and collaboration activities
Jet Substructure Correction with Lund Jet Plane Reweighting
No full textWe overview a new technique to improve the modeling of boosted multi-prong jets. The technique is based on reclustering a multi-prong such that each prong is contained in a separate subjet. The substructure of each prong is then corrected via data-driven reweighting of splittings in the Lund Jet Plane. This is a generic technique that can be applied to jets with high numbers of prongs for which there is currently no known calibration procedure. This will enable future searches utilizing high prong jets