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| Home > Publications database > Precision Measurement of Strange Quark Asymmetry and Monolithic Pixel Sensors for Vertexing at the FCC-ee |
| Book/Dissertation / PhD Thesis | PUBDB-2025-03896 |
;
2025
Verlag Deutsches Elektronen-Synchrotron DESY
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2025-03896
Report No.: DESY-THESIS-2025-017
Abstract: Future e+e− colliders, such as the FCC-ee, aim to test the Standard Model to the ultimateprecision. This requires synergistic progress in detector technology and data analysis techniques.This thesis comprehensively presents the interlinked efforts targeting these goals.Jet flavour identification algorithms play a crucial role in maximally exploiting the physicspotential of the FCC-ee, particularly in the Higgs and electroweak sectors. The DeepJet-Transformer algorithm, exploiting a transformer-based neural network that is substantiallyfaster to train than state-of-the-art graph neural networks, combines particle-flow reconstruc-tion with advanced vertex reconstruction and hadron particle identification. Beyond an excellentb- and c-jet discrimination, an s-jet tagging efficiency of 40% can be achieved with a 10% ud-jetbackground efficiency. The impact of K±/π± discrimination with varying efficiencies and inclu-sion of reconstructed V0s for strange tagging performance is shown. Similarly, the importanceof charged jet constituents and reconstructed secondary vertices for bottom and charm taggingis presented. The bottom and charm tagging efficiencies were largely uniform over the entiremomentum and polar angle range of interest. However, the strange tagging efficiency showed adependence on the K± and V0 multiplicities in different momentum regimes.A 5σ discovery significance can be achieved while isolating Z → s¯s events from the exclusivedecays of the Z boson with an integrated luminosity of 60 nb−1 of e+e− collisions at √s =91.2 GeV, corresponding to less than a second of the FCC-ee run plan at the Z boson resonance.The improved strange tagging performance opens new avenues for extracting quark-specificasymmetries, such as the forward-backward asymmetry (AFB). Using advanced neural network-based strange tagging and quark-antiquark jet separation with jet charge, the AFB measurementin the strange decay channel of the Z boson is possible with an absolute statistical precisionof 2.6 · 10−6 at the FCC-ee, considering a luminosity of 125 ab−1 at the Z resonance. Thecorresponding sin2 θW value can be measured with an absolute statistical precision of 4.6 · 10−6.To support these improvements, ultra-thin, high-resolution, and robust vertex detectors areessential. Recent advancements in the use of ultra-light monolithic active pixel sensors (MAPS),developed in the 65 nm imaging process, for the ALICE ITS3 project by an international consor-tium of the ALICE collaboration and the CERN EP R&D project envision drastic improvementsin vertexing performance. Process modification is introduced by adding a deep low-dose n-typelayer, which aids in the depletion of full sensor volume by extending the depletion region later-ally while keeping a low sensor capacitance. It leads to faster charge collection and reduction incharge shared with neighbouring pixels, thus improving the detection efficiency.Pixel response calibration and energy resolution measurements performed with the 55Fe ra-dioactive source in the controlled laboratory environment are presented. Four variants of pixelgeometry were tested with a small-scale analogue prototype, APTS, all showing over 99% chargecollection efficiency for a small reverse bias voltage of 1.2 V. The impact of geometry variationon detection efficiency, spatial resolution, and radiation tolerance is highlighted. In-pixel effi-ciency studies show that the loss of efficiency at high thresholds is primarily concentrated at theedges and corners of the pixels. APTS shows sub-3 μm spatial resolution and > 99% detectionefficiency, even under moderate irradiation, satisfying the stringent requirements of ALICE andfuture collider environments.Together, these developments highlight the integrated advancements in detector design, re-construction algorithms, and physics potential, strengthening the next generation of precisionmeasurements at the FCC-ee and beyond.
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