Search

Machine learning has become a popular instrument for the identification of dark matter candidates at particle collider experiments. They enable the processing of large datasets and are therefore suitable to operate directly on raw data coming from the detector, instead of reconstructed objects. Here, we investigate patterns of raw pixel hits recorded by the Belle II pixel detector, that is operational since 2019 and presently features 4 M pixels and trigger rates up to 5 kHz. In particular, we focus on unsupervised techniques that operate without the need for a theoretical model. These model-agnostic approaches allow for an unbiased exploration of data, while filtering out anomalous detector signatures that could hint at new physics scenarios. We present the identification of hypothetical magnetic monopoles against Belle II beam background using Self-Organizing Kohonen Maps and Autoencoders. The two unsupervised algorithms are compared to a convolutional Multilayer Perceptron and a superior signal efficiency is found at high background rejection levels. Our results strengthen the case for using unsupervised machine learning techniques to complement traditional search strategies at particle colliders and pave the way to potential online applications of the algorithms in the near future.

We present an FPGA-based online data reduction system for the pixel detector of the future Belle II experiment. The occupancy of the pixel detector is estimated at 3 %. This corresponds to a data output rate of more than 20 GB/s after zero suppression, dominated by background. The Online Selection Nodes (ONSEN) system aims to reduce the background data by a factor of 30. It consists of 33 MicroTCA cards, each equipped with a Xilinx Virtex-5 FPGA and 4 GiB DDR2 RAM. These cards are hosted by 9 AdvancedTCA carrier boards. The ONSEN system buffers the entire output data from the pixel detector for up to 5 seconds. During this time, the Belle II high-level trigger PC farm performs an online event reconstruction, using data from the other Belle II subdetectors. It extrapolates reconstructed tracks to the layers of the pixel detector and defines regions of interest around the intercepts. Based on this information, the ONSEN system discards all pixels not inside a region of interest before sending the remaining hits to the event builder system. During a beam test with one layer of the pixel detector and four layers of the surrounding silicon strip detector, including a scaled-down version of the high-level trigger and data acquisition system, the pixel data reduction using regions of interest was exercised. We investigated the data produced in more than 20 million events and verified that the ONSEN system behaved correctly, forwarding all pixels inside regions of interest and discarding the rest., Comment: 7 pages, 10 figures

Monte-Carlo simulations for a resonance scan of the charmonium-like state X(3872) at Panda are performed. Final state radiation hadronic background reactions are taken into account. The signal reconstruction uses a realistic pattern recognition (track finder and track fitter) and electron/pion discrimination., Comment: Conference Talk presented at MENU10, 12th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, 05/31-06/04, 2010, College of William and Mary, Williamsburg, Virginia

Monte-Carlo simulations for a resonance scan of the charmonium-like state X(3872) at Panda are performed. Final state radiation hadronic background reactions are taken into account. The signal reconstruction uses a realistic pattern recognition (track finder and track fitter) and electron/pion discrimination., Comment: Conference Talk presented at MENU10, 12th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, 05/31-06/04, 2010, College of William and Mary, Williamsburg, Virginia

Die vorliegende Arbeit umfasst zwei Teilgebiete der elementaren Reaktionen am HADES-Detektor der GSI. Ein Aspekt sind die apparativen Entwicklungen zur Optimierung des Pionenstrahls. Im zweiten Teil werden Ergebnisse eines Proton-Proton Experiments zur Bestimmung des elektromagnetischen Übergangsformfaktors des Eta-Mesons dargestellt. Der für die Pion-Kern Experimente benötigte Pionenstrahl wird durch den Beschuss eines Beryllium-Targets mit einem Schwerionenstrahl erzeugt. Nachteil eines Sekundärstrahls ist die schlechte Emittanz, da die im Produktionstarget erzeugten Pionen eine breite Impulsverteilung besitzen und über eine breite Winkelverteilung emittiert werden. Daneben ist die Intensität des Strahls um Größenordnungen kleiner als die eines Primärstrahls. Die Qualität des sekundären Pionenstrahls am HADES-Target ist das wichtigste Kriterium für die Durchführbarkeit eines Produktionsexperiments. Pionen außerhalb des vorgesehenen Strahlflecks führen zu Sekundärreaktionen mit dem das Target umgebende Material (Halterung und Strahlrohr). Der dadurch entstehende Untergrund kann ein Experiment unmöglich machen. Um die Größe und die Eigenschaften des Strahlfokus zu untersuchen, wurden im Rahmen dieser Arbeit zwei Detektoren aus szintillierenden Fasern aufgebaut und in verschiedenen Strahlzeiten eingesetzt. Die Signal-Auslese erfolgte mit einer neu entwickelten Elektronik. Die Ergebnisse zeigten eine achromatische Fokussierung der Pionen, die zu einer inakzeptablen Strahlbreite am HADES-Target führte. In Simulationsrechnungen konnte gezeigt werden, dass ein zusätzlicher Quadrupol in der Strahlführung direkt vor dem HADES-Target die Qualität des Strahlflecks verbessert. Ein Testexperiment mit diesem zusätzlichen Quadrupol wurde durchgeführt und wird zur Zeit ausgewertet. Erste Ergebnisse zeigen bereits deutliche Verbesserungen. Insbesondere konnte die Impulsabhängigkeit der Fokussierung stark reduziert werden. Die mit indirekten Methoden bestimmte Halbwertsbreite d, The measurement of hadron properties in hot and dense nuclear matter is the main topic of the dilepton spectrometer HADES at GSI. The physics program covers collisions between heavy ions as well as elementary reactions with proton or pion beams. For an interpretation of the measured dilepton spectra in heavy ion collisions systematic studies of the dilepton decays in elementary processes are needed. One of the hot topics is the study of the properties of eta and omega mesons. The measurement of the transition form factor in Dalitz decays provides access to the inner hadronic structure. Furthermore, the measurement of the omega meson in pion induced reactions on heavy nuclei might reveal a possible in-medium modification as observed before in photonuclear reactions. This thesis contains two tasks. The first part focuses on the development and optimization of the pion beam facility for the HADES experiment. The second part describes the measurement of the electromagnetic transition form factor of the eta meson in proton-proton reactions. To investigate pion-nucleon reaction, a secondary pion beam is required. The pions are produced by a heavy ion beam impinging on a beryllium target. Since the pions are emitted in a wide momentum and angle range, the intensity of the secondary beam is orders of magnitude lower. The final size of the beam spot at the HADES target location is a crucial parameter. Pions hitting the material outside the target area (holding and beam pipe) will cause a tremendous background, which limits the feasibility of the experiment. In order to determine the profile of the beam focus, two scintillating fiber detectors have been built as part of this thesis and are read out with recently developed electronics. The measured size of the beam focus appeared to be not acceptable, which can be attributed to the achromatic magnetic focusing in the beam line. Simulations have shown, that an additional quadrupole magnet directly in front of HADES woul