Study of exclusive one-pion and one-eta production using hadron and dielectron channels in pp reactions at kinetic beam energies of 1.25 GeV and 2.2 GeV with HADES

We present measurements of exclusive ensuremathπ+,0 and η production in pp reactions at 1.25GeV and 2.2GeV beam kinetic energy in hadron and dielectron channels. In the case of π+ and π0 , high-statistics invariant-mass and angular distributions are obtained within the HADES acceptance as well as acceptance-corrected distributions, which are compared to a resonance model. The sensitivity of the data to the yield and production angular distribution of Δ (1232) and higher-lying baryon resonances is shown, and an improved parameterization is proposed. The extracted cross-sections are of special interest in the case of pp → pp η , since controversial data exist at 2.0GeV; we find \ensuremathσ=0.142±0.022 mb. Using the dielectron channels, the π0 and η Dalitz decay signals are reconstructed with yields fully consistent with the hadronic channels. The electron invariant masses and acceptance-corrected helicity angle distributions are found in good agreement with model predictions

Status of the BELLE II Pixel Detector

The Belle II experiment at the super KEK B-factory (SuperKEKB) in Tsukuba, Japan, has been collecting $e^+e^−$ collision data since March 2019. Operating at a record-breaking luminosity of up to $4.7×10^{34} cm^{−2}s^{−1}$, data corresponding to $424 fb^{−1}$ has since been recorded. The Belle II VerteX Detector (VXD) is central to the Belle II detector and its physics program and plays a crucial role in reconstructing precise primary and decay vertices. It consists of the outer 4-layer Silicon Vertex Detector (SVD) using double sided silicon strips and the inner two-layer PiXel Detector (PXD) based on the Depleted P-channel Field Effect Transistor (DePFET) technology. The PXD DePFET structure combines signal generation and amplification within pixels with a minimum pitch of $(50×55) μm^2$. A high gain and a high signal-to-noise ratio allow thinning the pixels to $75 μm$ while retaining a high pixel hit efficiency of about $99$. As a consequence, also the material budget of the full detector is kept low at $≈0.21$$\frac{X}{X_0}$ per layer in the acceptance region. This also includes contributions from the control, Analog-to-Digital Converter (ADC), and data processing Application Specific Integrated Circuits (ASICs) as well as from cooling and support structures. This article will present the experience gained from four years of operating PXD; the first full scale detector employing the DePFET technology in High Energy Physics. Overall, the PXD has met the expectations. Operating in the intense SuperKEKB environment poses many challenges that will also be discussed. The current PXD system remains incomplete with only 20 out of 40 modules having been installed. A full replacement has been constructed and is currently in its final testing stage before it will be installed into Belle II during the ongoing long shutdown that will last throughout 2023

Optimization of the pion beam for the HADES detector and determination of the eta form factor in proton-proton reactions at 2.2 GeV

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 der vertikalen Verteilung am Target war 14mm. Mit den erreichten Optimierungen ist die Qualität der Fokussierung jetzt ausreichend, um damit ein Experiment pi-+A erfolgreich durchführen zu können. Die Intensität des Pionenstrahls liegt mit ca. 10^6 Pionen/Puls um etwa eine Größenordnung unter der typischen in HADES-Experimenten genutzten Primärstrahlintensität. Um die elektromagnetische Struktur von Hadronen in Experimenten mit HADES aufzuklären, wurde in einem ersten Schritt der Eta-Dalitz-Zerfall in der p+p Reaktionen bei 2.2 GeV kinetischer Energie mit dem Ziel untersucht, den elektromagnetischen Übergangsformfaktor des Eta-Mesons zu bestimmen. Hierbei kam die im Rahmen dieser Arbeit neu entwickelte Analyse-Methode der Ereignis Hypothese zum Einsatz. Im Gegensatz zu den inklusiven Analysen der Schwerionenexperimente werden dabei die gemessenen Teilchenspuren nicht separat betrachtet, sondern die Gesamtheit des Ereignisses berücksichtigt. Aus den gefundenen Teilchenspuren wird dabei eine Hypothese des Ereignisses erstellt und den Spuren entsprechend eine hypothetische Teilchenidentität zugewiesen. Verschiedene Algorithmen testen die Hypothese anhand von Detektorresponse und kinematischen Bedingungen und bestätigen oder verwerfen sie. In einem letzten Schritt wird eine kinematische Anpassung an die Hypothese durchgeführt, wobei nur die mit dem besten chi^2 übernommen wird. Nach Untergrundabzug und Faltung mit der aus einer Simulation berechneten Akzeptanzkorrektur entspricht das invariante Massenspektrum innerhalb der Fehler den Erwartungen aus Vektormesonen-Dominanz-Rechnungen. Das Verhältnis zwischen gemessenen Daten und der entsprechenden QED-Rechnung ergibt den elektromagnetischen Übergangsformfaktor des Eta-Mesons. Aus einer Anpassung an die Daten lässt sich eine Steigung des Formfaktors von b=2.2 (+1.2-1.4) GeV^-2 bestimmen. Dies entspricht einer Pol-Masse von lambda=680 (+460-130) MeVc^2. Die großen Fehler sind durch die Sensitivität der Steigung auf die Bereiche großer invarianter Massen und kleiner Statistik bedingt. Das Ergebnis ist innerhalb der Fehler konsistent mit den in Eta -> e+ e- Gamma und Eta -> mu+ mu- Gamma Zerfällen gemessenen Formfaktoren früherer Experimente.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 would solve this problem and improve the beam quality. A test experiment including this new quadrupole has been performed and the analysis is still in progress. Preliminary results show a significant reduction of the momentum dependency of the focus. The size of the actual beam spot has been deduced to 14mm by using an indirect tracking approach. Presently, the quality of the beam focus appears sufficient to do a pi-+A experiment. The pion beam intensity of roughly 10^6 pions/spill is one order of magnitude below the intensity typically used in the HADES proton experiments. For deducing the electromagnetic structure of hadrons, a first step has been done by analyzing the eta Dalitz decay in p+p reactions at 2.2GeV kinetic energy to determine the electromagnetic transition form factor of the eta meson. A new analysis approach has been developed, the so called event hypothesis. In contrast to the inclusive reconstruction in heavy ion experiments, the individual particle tracks are not analyzed separately, but the whole event is taken into account. For each event a hypothesis is created, which assigns hypothetical particle identities to the measured tracks. These assumptions are evaluated by several algorithms based on detector response or kinematic constraints. In a final step kinematic fitting is applied to the hypotheses, only the one with the best chi^2 is propagated further. After subtracting the background and applying acceptance corrections (taken from simulation) the invariant mass spectra are compatible with the one obtained by vector meson dominance calculations within the error bars. Taking the ratio of the measured spectra and the corresponding QED calculations provides the electromagnetic transition form factor of the eta meson. A fit to the data leads to a form factor slope of b=2.2 (+1.2-1.4) GeV^-2. This corresponds to a pole mass of lambda=680 (+460-130) MeVc^2. The large errors are dominated by the events at large invariant masses which have the largest statistical errors. Within the error bars the result is consistent with form factor determinations given in literature for eta -> e+ e- gamma and eta -> mu+ mu- gamma decays

Comparison of Supervised and Unsupervised Anomaly Detection in Belle II Pixel Detector Data

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

Cyclin-Dependent Kinase-Associated Proteins Cks1 and Cks2 Are Essential during Early Embryogenesis and for Cell Cycle Progression in Somatic Cells ▿

Cks proteins associate with cyclin-dependent kinases and have therefore been assumed to play a direct role in cell cycle regulation. Mammals have two paralogs, Cks1 and Cks2, and individually deleting the gene encoding either in the mouse has previously been shown not to impact viability. In this study we show that simultaneously disrupting CKS1 and CKS2 leads to embryonic lethality, with embryos dying at or before the morula stage after only two to four cell division cycles. RNA interference (RNAi)-mediated silencing of CKS genes in mouse embryonic fibroblasts (MEFs) or HeLa cells causes cessation of proliferation. In MEFs CKS silencing leads to cell cycle arrest in G2, followed by rereplication and polyploidy. This phenotype can be attributed to impaired transcription of the CCNB1, CCNA2, and CDK1 genes, encoding cyclin B1, cyclin A, and Cdk1, respectively. Restoration of cyclin B1 expression rescues the cell cycle arrest phenotype conferred by RNAi-mediated Cks protein depletion. Consistent with a direct role in transcription, Cks2 is recruited to chromatin in general and to the promoter regions and open reading frames of genes requiring Cks function with a cell cycle periodicity that correlates with their transcription

Track finding at Belle II

International audienceThis paper describes the track-finding algorithm that is used for event reconstruction in the Belle II experiment operating at the SuperKEKB B-factory in Tsukuba, Japan. The algorithm is designed to balance the requirements of a high efficiency to find charged particles with a good track parameter resolution, a low rate of spurious tracks, and a reasonable demand on CPU resources. The software is implemented in a flexible, modular manner and employs a diverse selection of global and local track-finding algorithms to achieve an optimal performance

Data quality monitors of vertex detectors at the start of the Belle II experiment

The Belle II experiment features a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric e+e− collider at KEK in Tsukuba, Japan. The accelerator completed its first phase of commissioning in 2016, and the Belle II detector saw its first electron-positron collisions in April 2018. Belle II features a newly designed silicon vertex detector based on double-sided strip layers and DEPFET pixel layers. A subset of the vertex detector was operated in 2018 to determine background conditions (Phase 2 operation). The collaboration completed full detector installation in January 2019, and the experiment started full data taking. This paper will report on the final arrangement of the silicon vertex detector part of Belle II with a focus on online monitoring of detector conditions and data quality, on the design and use of diagnostic and reference plots, and on integration with the software framework of Belle II. Data quality monitoring plots will be discussed with a focus on simulation and acquired cosmic and collision data