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In recent years, machine-learning methods have become increasingly important for the experiments at the Large Hadron Collider (LHC). They are utilised in everything from trigger systems to reconstruction and data analysis. The recent UCluster method is a general model providing unsupervised clustering of particle physics data, that can be easily modified to provide solutions for a variety of different decision problems. In the current paper, we improve on the UCluster method by adding the option of training the model in a scalable and distributed fashion, and thereby extending its utility to learn from arbitrarily large data sets. UCluster combines a graph-based neural network called ABCnet with a clustering step, using a combined loss function in the training phase. The original code is publicly available in TensorFlow v1.14 and has previously been trained on a single GPU. It shows a clustering accuracy of 81% when applied to the problem of multi-class classification of simulated jet events. Our implementation adds the distributed training functionality by utilising the Horovod distributed training framework, which necessitated a migration of the code to TensorFlow v2. Together with using parquet files for splitting data up between different compute nodes, the distributed training makes the model scalable to any amount of input data, something that will be essential for use with real LHC data sets. We find that the model is well suited for distributed training, with the training time decreasing in direct relation to the number of GPU’s used. However, further improvements by a more exhaustive and possibly distributed hyper-parameter search is required in order to achieve the reported accuracy of the original UCluster method.

In this essay, we discuss the possibilities and associated challenges concerning beyond the Standard Model searches at FCC-ee, such as rare decays of heavy-flavoured particles and long-lived particles. The Standard Model contains several suppression mechanisms, which cause a given group of processes to happen rarely, resulting in rare decays. The interest in these decays lies in the fact that the physics beyond the Standard Model does not need to be affected by the same suppression mechanism and therefore can naturally manifest in these decays. Their interest is reinforced by the recent report of several measurements of b -flavoured rare decays, showing deviations with respect to the Standard Model predictions. We will show how the FCC-ee project has unique capabilities to address these scientific questions and will consider the related detector design challenges to meet. Another group of processes discussed are those that produce new particles with relatively long lifetimes that travel substantial distances inside the detectors before decaying. Models containing long-lived particles can give answers to many open questions of the Standard Model, such as the nature of dark matter, or the neutrino masses, among others, while providing an interesting experimental complement to mainstream searches. Long-lived particles often display unique experimental signatures, such as displaced tracks and vertices, “disappearing” tracks, or anomalously charged jets. Due to this, they are affected by very low background levels but in exchange, they often require dedicated reconstruction algorithms and triggers. The discovery of any of the discussed cases would have a critical impact in High Energy Physics, and FCC-ee could provide a unique experimental opportunity to explore them. Moreover, the searches proposed here could motivate an out-of-the-box optimization of the experimental conditions that could bring in innovative solutions, such as new, possibly very large tracking detectors; or cutting-edge reconstruction algorithms that would boost the FCC-ee reach for unusual final states.

This paper investigates the search for long-lived dark scalars from exotic Higgs boson decays at the Future Circular Collider in its \\(e^+e^-\\) stage, FCC-ee, considering an integrated luminosity of 10.8 \\(ab^-1\\) collected during the ZH run at a center-of-mass energy \\(s=240\\) GeV. The work considers \\(Zh\\) events where the \\(Z\\) boson decays leptonically and the Higgs boson \\(h\\) decays into two long-lived dark scalars \\(s\\) which further decay into bottom anti-bottom quark pairs. The analysis is performed using a parametrized simulation of the IDEA detector concept and targets dark scalar decays in the tracking volume, resulting in multiple displaced vertices in the final state. The sensitivity towards long-lived dark scalars at FCC-ee is estimated using an event selection requiring two opposite-charge, same-flavor leptons compatible with the \\(Z\\) boson, and at least two displaced vertices in the final state. The selection is seen to efficiently remove the Standard Model background, while retaining sensitivity for dark scalar masses between \\(m_s=20\\) GeV and \\(m_s=60\\) GeV and mean proper lifetimes \\(c\\) between approximately 10 mm and 10 m The results show that the search strategy has potential to probe Higgs to dark scalar branching ratios as low as \\(10^-4\\) for a mean proper lifetime \\(c 1\\) m. The results provide the first sensitivity estimate for exotic Higgs decays at FCC-ee with the IDEA detector concept, using the common FCC framework.

A bstract This paper investigates the search for long-lived dark scalars from exotic Higgs boson decays at the Future Circular Collider in its e + e − stage, FCC-ee, considering an integrated luminosity of 10 . 8 ab −1 collected during the ZH run at a center-of-mass energy s = 240 GeV. The work considers Zh events where the Z boson decays leptonically and the Higgs boson h decays into two long-lived dark scalars s which further decay into bottom anti-bottom quark pairs. The analysis is performed using a parametrized simulation of the IDEA detector concept and targets dark scalar decays in the tracking volume, resulting in multiple displaced vertices in the final state. The sensitivity towards long-lived dark scalars at FCC-ee is estimated using an event selection requiring two opposite-charge, same-flavor leptons compatible with the Z boson, and at least two displaced vertices in the final state. The selection is seen to efficiently remove the Standard Model background, while retaining sensitivity for dark scalar masses between m s = 20 GeV and m s = 60 GeV and mean proper lifetimes cτ between approximately 10 mm and 10 m. The results show that the search strategy has potential to probe Higgs to dark scalar branching ratios as low as 10 −4 for a mean proper lifetime cτ ≈ 1 m. The results provide the first sensitivity estimate for exotic Higgs decays at FCC-ee with the IDEA detector concept, using the common FCC framework.

Long-lived particles have significant enough lifetimes as to, when produced in collisions, leave a distinct signature in the detectors. Driven by increasingly higher energies, trigger and reconstruction algorithms at particle colliders are optimized for increasingly heavier particles, which in turn, tend to be short-lived. This makes searches for long-lived particles difficult, usually requiring dedicated methods and sometimes dedicated hardware top spot them. However, taking upon the challenge brings enormous potential, since new, long-lived particles feature in a variety of promising new physics models that could answer most of the open questions of the standard model, such as: neutrino masses, Dark Matter, or the matter-antimatter unbalance in the Universe. Currently, the international high energy physics community is planning future facilities post-LHC, and various particle colliders have been proposed. Crucial physics cases connected to long-lived particles will be accessible then, and in this presentation, three interesting examples are highlighted: Heavy Neutral Leptons, Hidden Sectors connected to Dark Matter, and exotic Higgs boson decays. This is followed by a small review of the preliminary studies assuming different future colliders, exploiting the complementary advantages that different colliding particles and accelerator types provide.

This study presents a novel method for the definition of signal regions in searches for new physics at collider experiments, specifically those conducted at CERNs Large Hadron Collider. By leveraging multi-dimensional histograms with precise arithmetic and utilizing the SparkDensityTree library, it is possible to identify high-density regions within the available phase space, potentially improving sensitivity to very small signals. Inspired by an ongoing search for dark mesons at the ATLAS experiment, CMS open data is used for this proof-of-concept intentionally targeting an already excluded signal. Several signal regions are defined based on density estimates of signal and background. These preliminary regions align well with the physical properties of the signal while effectively rejecting background events. While not explored in this work, this method is also scalable, which makes it ideal for large datasets such as those expected at the high-luminosity upgrade of the LHC. Finally, this method is flexible and can be easily extended, promising a boost to the signal region definition process for new physics searches at colliders.

In this essay we discuss the possibilities and associated challenges concerning beyond the Standard Model searches at FCC-ee, such as rare decays of heavy-flavoured particles and long-lived particles. The Standard Model contains several suppression mechanisms, which cause a given group of processes to happen rarely, resulting in rare decays. The interest in these decays lies in the fact that the physics beyond the Standard Model does not need to be affected by the same suppression mechanism and therefore can naturally manifest in these decays. Their interest is reinforced by the recent report of several measurements of \\(b\\)-flavoured rare decays, showing deviations with respect to the Standard Model predictions. We will show how the FCC-ee project has unique capabilities to address these scientific questions and will consider the related detector design challenges to meet. Another group of processes discussed are those that produce new particles with relatively long lifetimes, that travel substantial distances inside the detectors before decaying. Models containing long-lived particles can give answers to many open questions of the Standard Model, such as the nature of dark matter, or the neutrino masses, among others; while providing an interesting experimental complement to mainstream searches. Long-lived particles often display unique experimental signatures, such as displaced tracks and vertices, disappearing tracks, or anomalously charged jets. Due to this, they are affected by very low background levels but in exchange, they often require dedicated reconstruction algorithms and triggers.

This paper presents measurements of top-antitop quark pair (t¯t) production inassociation with additional b-jets. The analysis utilises 140 fb −1 of proton–proton collisiondata collected with the ATLAS detector at the Large Hadron Collider at a centre-of-massenergy of 13 TeV. Fiducial cross-sections are extracted in a final state featuring one electronand one muon, with at least three or four b-jets . Results are presented at the particle level forboth integrated cross-sections and normalised differential cross-sections, as functions of globalevent properties, jet kinematics, and b-jet pair properties. Observable quantities characterisingb-jets originating from the top quark decay and additional b-jets are also measured at theparticle level, after correcting for detector effects. The measured integrated fiducial cross-sections are consistent with t¯tb¯b predictions from various next-to-leading-order matrix elementcalculations matched to a parton shower within the uncertainties of the predictions. State-of-the-art theoretical predictions are compared with the differential measurements; none ofthem simultaneously describes all observables. Differences between any two predictions aresmaller than the measurement uncertainties for most observables.

A measurement of the top-quark pole mass m t pole is presented in tt <over bar> events with an additional jet, tt <over bar>+ 1-jet, produced in pp collisions at √s = 13TeV. The data sample, recorded with the ATLAS experiment during Run 2 of the LHC, corresponds to an integrated luminosity of 140 fb -1 . Events with one electron and one muon of opposite electric charge in the final state are selected to measure the tt <over bar>+ 1-jet differential cross-section as a function of the inverse of the invariant mass of the tt <over bar> + 1-jet system. Iterative Bayesian Unfolding is used to correct the data to enable comparison with fixed-order calculations at next-to-leading-order accuracy in the strong coupling. The process pp → tt <over bar> j (2 → 3), where top quarks are taken as stable particles, and the process pp → bb <over bar>l + νl - ν<over bar> j (2 → 7), which includes top-quark decays to the dilepton final state and off-shell effects, are considered. The top-quark mass is extracted using χ 2 fit of the unfolded normalized differential cross-section distribution. The results obtained with the 2 → 3 and 2 → 7 calculations are compatible within theoretical uncertainties, providing an important consistency check. The more precise determination is obtained for the 2 → 3 measurement: m t pole = 170.7 ± 0.3 (stat.) ± 1.4 (syst.) ± 0.3 (scale) ±0.2 (PDF ⊕ α S ) GeV, which is in good agreement with other top-quark mass results.

Jets with different radius parameters R are an important tool for probing quantum chromodynamics processes at different angular scales. Jets with small R = 0.2 are instrumental in measurements of the substructure of large-R jets resulting from collimated hadronic decays of energetic W, Z, and Higgs bosons, top quarks, and of potential new resonances. This paper presents measurements of the energy scale, resolution, and associated uncertainties of jets with radius parameters R = 0.2 and 0.6, obtained using the ATLAS detector. The results are based on 37 fb(-1) of proton-proton collision data from the Large Hadron Collider at a centre-of-mass energy of root s = 13 TeV. A new in situ method for measuring jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets (vertical bar eta vertical bar < 1.2) typically vary from 1% to about 5% as a function of vertical bar eta vertical bar at very low transverse momentum, p(T), of around 20 GeV for both R = 0.2 and 0.6 jets. The relative energy resolution ranges from (35 +/- 6)% at p(T) = 20 GeV to (6 +/- 0.5)% at p(T) = 300 GeV for central R = 0.2 jets, and is found to be slightly worse for R = 0.6 jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.