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Abstract The high design luminosity of the SuperKEKB electron–positron collider is expected to result in challenging levels of beam-induced backgrounds in the interaction region. Properly simulating and mitigating these backgrounds is critical to the success of the Belle II experiment. We report on measurements performed with a suite of dedicated beam background detectors, collectively known as BEAST II, during the so-called Phase 1 commissioning run of SuperKEKB in 2016, which involved operation of both the high energy ring (HER) of 7 GeV electrons as well as the low energy ring (LER) of 4 GeV positrons. We describe the BEAST II detector systems, the simulation of beam backgrounds, and the measurements performed. The measurements include standard ones of dose rates versus accelerator conditions, and more novel investigations, such as bunch-by-bunch measurements of injection backgrounds and measurements sensitive to the energy spectrum and angular distribution of fast neutrons. We observe beam–gas, Touschek, beam–dust, and injection backgrounds. As there is no final focus of the beams in Phase 1, we do not observe significant synchrotron radiation, as expected. Measured LER beam–gas backgrounds and Touschek backgrounds in both rings are slightly elevated, on average three times larger than the levels predicted by simulation. HER beam–gas backgrounds are on average two orders of magnitude larger than predicted. Systematic uncertainties and channel-to-channel variations are large, so that these excesses constitute only 1–2 sigma level effects. Neutron background rates are higher than predicted and should be studied further. We will measure the remaining beam background processes, due to colliding beams, in the imminent commissioning Phase 2. These backgrounds are expected to be the most critical for Belle II, to the point of necessitating replacement of detector components during the Phase 3 (full-luminosity) operation of SuperKEB. [ABSTRACT FROM AUTHOR]

A high-level beam background is a crucial challenge to future upgrades to the BEPCII collider. We report on the first separate measurement of main beam background components at BEPCII. The separation measurement enables the beam background extrapolation towards the beam parameters assumed for the upgraded BEPCII. The measured rates of each background component are also compared with a brand-new simulation based on SAD and Geant4. The discrepancy between experiment and simulation remains 1 ∼ 2 magnitudes after calibration with beam lifetime, which should be further investigated. [ABSTRACT FROM AUTHOR]

We describe a neural network for predicting the background hit rate in the Belle II detector produced by the SuperKEKB electron–positron collider. The neural network, BGNet, learns to predict the individual contributions of different physical background sources, such as beam-gas scattering or continuous top-up injections into the collider, to Belle II sub-detector rates. The samples for learning are archived 1 Hz time series of diagnostic variables from the SuperKEKB collider subsystems and measured hit rates of Belle II used as regression targets. We test the learned model by predicting detector hit rates on archived data from different run periods not used during training. We show that a feature attribution method can help interpret the source of changes in the background level over time. [ABSTRACT FROM AUTHOR]

Gaseous time projection chambers (TPCs) with high readout segmentation are capable of reconstructing detailed 3D ionization distributions of nuclear recoils resulting from elastic neutron scattering. Using a system of six compact TPCs with pixel ASIC readout, filled with a 70:30 mixture of He : CO 2 gas, we analyze the first directional measurements of beam-induced neutron backgrounds in the tunnel regions surrounding the Belle II detector at the SuperKEKB e + e − collider. With the use of 3D recoil tracking, we show that these TPCs are capable of maintaining nearly 100% nuclear recoil purity to reconstructed ionization energies (E reco) as low as 5 keV˙ee. Using a large sample of Monte-Carlo (MC)-simulated 4 He , 12 C , and 16 O recoil tracks, we find consistency between predicted and measured recoil energy spectra in five of the six TPCs, providing useful validation of the neutron production mechanisms modeled in simulation. Restricting this sample to 4 He recoil tracks with E reco > 40 keV˙ee , we further demonstrate axial angular resolutions within 8 ° and we introduce a procedure that under suitable conditions, correctly assigns the vector direction to 91% of these simulated 4 He recoils. Applying this procedure to assign vector directions to measured 4 He recoil tracks, we observe consistency between the angular distributions of observed and simulated recoils, providing first experimental evidence of localized neutron "hotspots" in the accelerator tunnel. Observed rates of nuclear recoils in these TPCs suggest that simulation overestimates the neutron flux from these hotspots. Despite this, we estimate these hotspots to produce the majority of neutron backgrounds in the accelerator tunnel at SuperKEKB's target luminosity of 6.3 × 10 35 cm − 2 s − 1 , making them important regions to continue to monitor. [ABSTRACT FROM AUTHOR]

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

The Belle II experiment aims to accumulate 50 ab−1 of e + e − collision data at the SuperKEKB asymmetric energy collider (Tsukuba, Japan). The first physics data using all Belle II detectors were taken in spring 2019. In the vast physics program of the Belle II experiment, the vertex detector plays a crucial role for the determination of the B -meson decay vertices. It consists of two inner layers of pixelated silicon detectors and four outer layers of double-sided silicon strip detectors (SVD). To achieve a design luminosity of 8 × 1 0 35 cm − 2 s − 1 , 40 times higher than the recorded luminosity of its predecessor, the SuperKEKB collider squeezes the beams to a vertical size of 50 nm ("nano-beam scheme") and doubles the beam currents. Therefore, the detectors are required to tolerate intense beam induced background due to the very high luminosity. During the 2019 spring run we measured the occupancy rate in the SVD to estimate the level of the beam induced background. With the low initial luminosity, the observed beam induced background mostly originated from Touschek processes and beam-gas scattering within individual beams. Since these different background contributions depend differently on accelerator conditions, such as the beam current, beam size and pressure, they can be disentangled. We estimate the background rate of each contribution and compare them with simulated ones. The results enable us to predict the background levels at increased beam currents and luminosity in the coming years. They also hint at background mitigation measures for running at higher luminosity. In this proceeding we present the results of our study of the beam induced background in the SVD and the prospects for future operation. [ABSTRACT FROM AUTHOR]