Beam background expectations for Belle II at SuperKEKB

TL;DR

This paper estimates the evolution of beam backgrounds at Belle II at SuperKEKB, analyzing challenges and mitigation strategies to support high-luminosity data collection over the next decade.

Contribution

It provides detailed predictions of background levels at various luminosities and discusses the impact of planned interaction region redesigns on background mitigation.

Findings

Backgrounds remain manageable up to a luminosity of 2.8×10^{35} cm^{-2}s^{-1}.

Predictions beyond this luminosity are highly uncertain due to planned redesigns.

Effective background mitigation strategies are essential for achieving maximum luminosity.

Abstract

The Belle II experiment at the SuperKEKB electron-positron collider aims to collect an unprecedented data set of $\rm 50~{\rm ab}^{-1}$ to study $CP$-violation in the $B$-meson system and to search for Physics beyond the Standard Model (BSM). 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 $\rm 6.3 \times 10^{35}~cm^{-2}s^{-1}$ by further increasing the beam currents and reducing the beam-size at the interaction point by squeezing the betatron function down to $\beta^{*}_{\rm y}=\rm 0.3~mm$. Beam backgrounds are a key challenge in this context. We estimate the expected background evolution in the next ten years and discuss potential challenges and background mitigation strategies. We find that backgrounds will remain high but acceptable until a luminosity of at least $\rm 2.8\times 10^{35}~cm^{-2}s^{-1}$ is reached at $\beta^{*}_{\rm y}=\rm 0.6~mm$. Beyond this luminosity, predictions are highly uncertain, owing to a planned redesign of the interaction region. Improved background estimates with reduced uncertainties for the final, maximum-luminosity operation will require completion of this redesign.