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ATLAS Collaboration(Aad, G. et al), Aikiot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Measurements of inclusive and differential cross-sections of t(t)over-barγ production in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 191–77pp.
Abstract: Inclusive and differential cross-sections are measured at particle level for the associated production of a top quark pair and a photon (t (t) over bar gamma). The analysis is performed using an integrated luminosity of 140 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13TeV collected by the ATLAS detector. The measurements are performed in the single-lepton and dilepton top quark pair decay channels focusing on t (t) over bar gamma topologies where the photon is radiated from an initial-state parton or one of the top quarks. The absolute and normalised differential cross-sections are measured for several variables characterising the photon, lepton and jet kinematics as well as the angular separation between those objects. The observables are found to be in good agreement with the Monte Carlo predictions. The photon transverse momentum differential distribution is used to set limits on effective field theory parameters related to the electroweak dipole moments of the top quark. The combined limits using the photon and the Z boson transverse momentum measured in t (t) over bar production in associations with a Z boson are also set.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2024). Measurements of the production cross-section for a Z boson in association with b- or c-jets in proton-proton collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 84(9), 984–40pp.
Abstract: This paper presents a measurement of the production cross-section of a Z boson in association with b- or c-jets, in proton-proton collisions at root s = 13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 140 fb(-1). Inclusive and differential cross-sections are measured for events containing a Z boson decaying into electrons or muons and produced in association with at least one b-jet, at least one c-jet, or at least two b-jets with transverse momentum p(T) > 20 GeV and rapidity vertical bar y vertical bar < 2.5. Predictions from several Monte Carlo generators based on next-to-leading-order matrix elements interfaced with a parton-shower simulation, with different choices of flavour schemes for initial-state partons, are compared with the measured cross-sections. The results are also compared with novel predictions, based on infrared and collinear safe jet flavour dressing algorithms. Selected Z+ >= 1 c-jet observables, optimized for sensitivity to intrinsic-charm, are compared with benchmark models with different intrinsic-charm fractions.
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Rizzo, G. et al, Lacasta, C., Marinas, C., Mazorra de Cos, J., & Vobbilisetti, V. (2025). The DMAPS upgrade of the Belle II Vertex Detector. Nucl. Instrum. Methods Phys. Res. A, 1072, 170164–5pp.
Abstract: The SuperKEKB collider will undergo a major upgrade at the end of the decade to reach the target luminosity of 6x1035 cm-2s-1, offering the opportunity to install a new fully pixelated vertex detector (VTX) for the Belle II experiment, based on depleted-MAPS sensors. The VTX will be more granular and robust against the expected higher level of machine background and more performant in terms of standalone track finding efficiency. The VTX baseline design includes five depleted-MAPS sensor layers, spanning radii from 14 mm to 140 mm, with a material budget ranging from 0.2% to 0.8% X/X0 per layer. All layers will be equipped with the same OBELIX sensor, designed in the TowerJazz 180 nm technology, with the pixel matrix derived from the TJ-Monopix2 sensor originally developed for the ATLAS experiment. The paper will describe the proposed VTX structure and review all project aspects: tests of the TJ-Monopix2 sensor, OBELIX-1 design status, ladder prototype fabrication and tests.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2024). Constraints on simplified dark matter models involving an s-channel mediator with the ATLAS detector in pp collisions at √s=13 TeV. Eur. Phys. J. C, 84(10), 1102–37pp.
Abstract: This paper reports a summary of searches for a fermionic dark matter candidate in the context of theoretical models characterised by a mediator particle exchange in the s-channel. The data sample considered consists of pp collisions delivered by the Large Hadron Collider during its Run 2 at a centre-of-mass energy of root s = 13 TeV and recorded by the ATLAS detector, corresponding to up to 140 fb(-1). The interpretations of the results are based on simplified models where the new mediator particles can be spin-0, with scalar or pseudo-scalar couplings to fermions, or spin-1, with vector or axial-vector couplings to fermions. Exclusion limits are obtained from various searches characterised by final states with resonant production of Standard Model particles, or production of Standard Model particles in association with large missing transverse momentum.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Precise measurements of W- and Z-boson transverse momentum spectra with the ATLAS detector using pp collisions at √s=5.02 TeV and 13 TeV. Eur. Phys. J. C, 84(10), 1126–60pp.
Abstract: This paper describes measurements of the transverse momentum spectra of W and Z bosons produced in proton-proton collisions at centre-of-mass energies of root s = 5.02 TeV and root s = 13 TeV with the ATLAS experiment at the Large Hadron Collider. Measurements are performed in the electron and muon channels, W -> l nu and Z -> ll (l = e or mu), and for W events further separated by charge. The data were collected in 2017 and 2018, in dedicated runs with reduced instantaneous luminosity, and correspond to 255 and 338 pb(-1) at root s = 5.02 TeV and 13 TeV, respectively. These conditions optimise the reconstruction of the W-boson transverse momentum. The distributions observed in the electron and muon channels are unfolded, combined, and compared to QCD calculations based on parton shower Monte Carlo event generators and analytical resummation. The description of the transverse momentum distributions by Monte Carlo event generators is imperfect and shows significant differences largely common to W-, W+ and Z production. The agreement is better at root s = 5.02 TeV, especvially for predictions that were tuned to Z production data at root s = 7 TeV. Higher-order, resummed predictions based on DYTURBO generally match the data best across the spectra. Distribution ratios are also presented and test the understanding of differences between the production processes.
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