Measured and projected beam backgrounds in the Belle II experiment at the SuperKEKB collider

Abstract

The Belle II experiment at the SuperKEKB electron-positron collider aims to collect an unprecedented data set of $50{\mathrm{ab}}^{-1}$ to study $CP$-violation in the $B$-meson system and to search for Physics beyond the Standard Model. SuperKEKB is already the world's highest-luminosity collider. In order to collect the planned data set within approximately one decade, the target is to reach a peak luminosity of $6\times 10^{35}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ by further increasing the beam currents and reducing the beam size at the interaction point by squeezing the betatron function down to ${\beta }_{\mathrm{y}}^{\ast }=0.3\mathrm{mm}$. To ensure detector longevity and maintain good reconstruction performance, beam backgrounds must remain well controlled. We report on current background rates in Belle II and compare these against simulation. We find that a number of recent refinements have significantly improved the background simulation accuracy. Finally, we estimate the safety margins going forward. We predict that backgrounds should remain high but acceptable until a luminosity of at least $2.8\times 10^{35}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ is reached for ${\beta }_{\mathrm{y}}^{\ast }=0.6\mathrm{mm}$. At this point, the most vulnerable Belle II detectors, the Time-of-Propagation (TOP) particle identification system and the Central Drift Chamber (CDC), have predicted background hit rates from single-beam and luminosity backgrounds that add up to approximately half of the maximum acceptable rates.

28 pages, 17 figures, 9 tables (revised); in v3, we corrected Table 5 units for the measured fast neutron fluence by the TPC detectors from x10^9 n_eq/cm^2 to x10^8 n_eq/cm^2, which was a typo in the previous versions; also, we added the value of the luminosity for Tables 5 and 6

References in corpus (6)

• Beam-Beam Issues for Colliding Schemes with Large Piwinski Angle and Crabbed Waist
• Performance of the diamond-based beam-loss monitor system of Belle II
• Measurements of Beam Backgrounds in SuperKEKB Phase 2
• Effects of gamma irradiation on DEPFET pixel sensors for the Belle II experiment
• Application of recoil-imaging time projection chambers to directional neutron background measurements in the SuperKEKB accelerator tunnel
• A neural network for beam background decomposition in Belle II at SuperKEKB

Cited by in corpus (10)

• Development and Implementation of Advanced Beam Diagnostic and Abort Systems in SuperKEKB
• Application of recoil-imaging time projection chambers to directional neutron background measurements in the SuperKEKB accelerator tunnel
• Measurement of $CP$ asymmetries and branching-fraction ratios for $B^{\pm }\to D{K}^{\pm }$ and $D{\pi }^{\pm }$ with $D\to K_{\mathrm{S}}^0{K}^{\pm }{\pi }^{\mp }$ using Belle and Belle II data
• Measurement of the time-integrated CP asymmetry in $D^0\to K_S^0{K}_S^0$ decays using Belle and Belle II data
• A neural network for beam background decomposition in Belle II at SuperKEKB
• Underground Production of Electromagnetic Dark States by MeV-scale Electron Beams and Detection with CCDs
• Model-independent measurement of $D^0$-$\overline{D}{}^0$ mixing parameters in $D^0\to K_S^0{\pi }^{+}{\pi }^{-}$ decays at Belle and Belle II
• Muon identification with Deep Neural Network in the Belle II K-Long and Muon detector
• A Collimation System Baseline Design for the Electron Storage Ring at the Electron-Ion Collider
• Real-time graph neural networks on FPGAs for the Belle II electromagnetic calorimeter