Real-time Graph Building on FPGAs for Machine Learning Trigger Applications in Particle Physics

We present a design methodology that enables the semi-automatic generation of a hardware-accelerated graph building architectures for locally constrained graphs based on formally described detector definitions. In addition, we define a similarity measure in order to compare our locally constrained graph building approaches with commonly used k-nearest neighbour building approaches. To demonstrate the feasibility of our solution for particle physics applications, we implemented a real-time graph building approach in a case study for the Belle~II central drift chamber using Field-Programmable Gate Arrays~(FPGAs). Our presented solution adheres to all throughput and latency constraints currently present in the hardware-based trigger of the Belle~II experiment. We achieve constant time complexity at the expense of linear space complexity and thus prove that our automated methodology generates online graph building designs suitable for a wide range of particle physics applications. By enabling an hardware-accelerated pre-processing of graphs, we enable the deployment of novel Graph Neural Networks~(GNNs) in first level triggers of particle physics experiments.Comment: 18 page

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

Measurement of the B0s→μ+μ− Branching Fraction and Effective Lifetime and Search for B0→μ+μ− Decays

A search for the rare decays Bs0→μ+μ- and B0→μ+μ- is performed at the LHCb experiment using data collected in pp collisions corresponding to a total integrated luminosity of 4.4  fb-1. An excess of Bs0→μ+μ- decays is observed with a significance of 7.8 standard deviations, representing the first observation of this decay in a single experiment. The branching fraction is measured to be B(Bs0→μ+μ-)=(3.0±0.6-0.2+0.3)×10-9, where the first uncertainty is statistical and the second systematic. The first measurement of the Bs0→μ+μ- effective lifetime, τ(Bs0→μ+μ-)=2.04±0.44±0.05  ps, is reported. No significant excess of B0→μ+μ- decays is found, and a 95% confidence level upper limit, B(B0→μ+μ-)<3.4×10-10, is determined. All results are in agreement with the standard model expectations.A search for the rare decays $B^0_s\to\mu^+\mu^-$ and $B^0\to\mu^+\mu^-$ is performed at the LHCb experiment using data collected in $pp$ collisions corresponding to a total integrated luminosity of 4.4 fb$^{-1}$. An excess of $B^0_s\to\mu^+\mu^-$ decays is observed with a significance of 7.8 standard deviations, representing the first observation of this decay in a single experiment. The branching fraction is measured to be ${\cal B}(B^0_s\to\mu^+\mu^-)=\left(3.0\pm 0.6^{+0.3}_{-0.2}\right)\times 10^{-9}$, where the first uncertainty is statistical and the second systematic. The first measurement of the $B^0_s\to\mu^+\mu^-$ effective lifetime, $\tau(B^0_s\to\mu^+\mu^-)=2.04\pm 0.44\pm 0.05$ ps, is reported. No significant excess of $B^0\to\mu^+\mu^-$ decays is found and a 95 % confidence level upper limit, ${\cal B}(B^0\to\mu^+\mu^-)<3.4\times 10^{-10}$, is determined. All results are in agreement with the Standard Model expectations

Prospects for |Vub_{ub}| measurements at LHCb

Precise measurements of the quark coupling strength, $|V_{ub}|$, the least well known CKM element, allow a strong test of the unitarity of the CKM matrix. Previous measurements using exclusive and inclusive methods have resulted in values of $|V_{ub}|$ which are 3$\sigma$ apart. The first measurement of $|V_{ub}|$ at LHCb confirmed this tension using the $\Lambda^{0}_{b} \rightarrow p \mu^{-} \overline{\nu}$ decay. This strengthens the motivation to measure this quantity using different channels, one of them being purely leptonic decay $B^{+} \rightarrow \mu^{+} \mu^{-} \mu^{+} \nu$, which benefits from LHCb's very good muon identification ability. This mode had never been observed before. With recent lattice QCD calculations, LHCb can also constrain $|V_{ub}|$ using the $B^{0}_s \rightarrow K^{+} \mu^{-} \overline{\nu}$ decay where the theoretical uncertainty is significantly smaller

Search for the rare fully leptonic decay B+ → μ+ μ− μ+ ν at LHCb

This thesis presents a first search for the fully leptonic decay B+ → μ+ μ− μ+ ν in any experiment. This search is performed using proton-proton collision data at LHCb corresponding to an integrated luminosity of 4.7 fb^{−1} . The search is carried out in the region where the minimum of the two μ+ μ− mass combinations is below 980 MeV/c^{2}. The measurement of the branching fraction of this decay is even more interesting given that the recent theoretical prediction [1] of the branching fraction for B+ → μ+ μ− μ+ ν of 1.3 × 10^{−7} is high. Moreover, this decay is sensitive to the magnitude of the coupling strength between b and u quarks, which is of great interest given that there are some tensions in measurements of this magnitude. The data are consistent with the background only hypothesis and a limit of 1.4×10^{−8} at 95% confidence level is set on the branching fraction in the stated kinematic region. This is therefore not consistent with the theoretical prediction made in Ref. [1]. This thesis also presents a study of the response of the detector if three muons pass through it. This study shows that correlations induced by a trimuon system in the detector are substantial and they need to be addressed properly.Open Acces

Measurements of Beam Backgrounds in SuperKEKB Phase 2

The high design luminosity of the SuperKEKB electron-positron collider will result in challenging levels of beam-induced backgro unds in the interaction region. Understanding and mitigating these backgrounds is critical to the success of the Belle~II experi ment. We report on the first background measurements performed after roll-in of the Belle II detector, a period known as SuperKE KB Phase 2, utilizing both the BEAST II system of dedicated background detectors and the Belle II detector itself. We also repor t on first revisions to the background simulation made in response to our findings. Backgrounds measured include contributions f rom synchrotron radiation, beam-gas, Touschek, and injection backgrounds. At the end of Phase 2, single-beam backgrounds origina ting from the 4 GeV positron Low Energy Ring (LER) agree reasonably well with simulation, while backgrounds from the 7 GeV elect ron High Energy Ring (HER) are approximately one order of magnitude higher than simulation. We extrapolate these backgrounds for ward and conclude it is safe to install the Belle II vertex detector