Measurements of Beam Backgrounds in SuperKEKB Phase 2

Abstract

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 Su-perKEKB 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.

The design, construction, operation and performance of the Belle II silicon vertex detector

“…For the Phase 2 commissioning, a special VXD which had one PXD or SVD ladder per layer, with a detector configuration shown in figure 86 was designed to study the detector performance in each layer. The FANGS, CLAWS, and PLUME sensors constituted the BEAST II system for background measurement [42]. For the ease of the installation of those four SVD ladders, an "SVD cartridge" accommodating the four ladders, mechanical supports, and cooling services was developed, as shown in figure 87.…”

“…The remaining volume of the VXD was instrumented with the BEAST II detector system, designed to measure the radiation field in the inner detector region. The full description of the system and the results of the studies are available in [42]. After the VXD installation and the start of Phase 3, many studies were conducted during every data taking period by machine and detector groups to carefully evaluate background levels, in order to separate background components and to optimize collimators settings, to lower the backgrounds in the interaction region preserving the beam lifetime and injection efficiency [50].…”