Measurements of Beam Backgrounds in SuperKEKB Phase 2

TL;DR

This paper reports on the initial measurements of beam-induced backgrounds in SuperKEKB Phase 2, comparing them with simulations, and assesses the safety for installing the Belle II vertex detector based on these findings.

Contribution

First background measurements in SuperKEKB Phase 2 using BEAST II and Belle II detectors, with revised simulations to improve understanding of beam backgrounds.

Findings

HER backgrounds are an order of magnitude higher than simulations.

LER backgrounds agree reasonably well with simulations.

Background levels are deemed safe for vertex detector installation.

Abstract

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.