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

TL;DR

This paper assesses beam backgrounds in the Belle II experiment at SuperKEKB, comparing measurements with simulations, improving background modeling, and estimating safe operational luminosity limits to protect detector components.

Contribution

It provides refined background simulations and safety margin estimates for Belle II at SuperKEKB, enhancing understanding of background impacts on detector longevity.

Findings

Background rates are well modeled and match measurements after recent refinements.

Background levels remain acceptable up to a luminosity of 2.8×10^{35} cm^{-2}s^{-1}.

Key detectors face about half of maximum acceptable background hit rates at this luminosity.

Abstract

The Belle II experiment at the SuperKEKB electron-positron collider aims to collect an unprecedented data set of $50~{\rm ab}^{-1}$ 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 $\rm 6 \times 10^{35}~cm^{-2}s^{-1}$ by further increasing the beam currents and reducing the beam size at the interaction point by squeezing the betatron function down to $\beta^{*}_{\rm y}=\rm 0.3~mm$. 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 $\rm 2.8 \times 10^{35}~cm^{-2}s^{-1}$ is reached for $\beta^{*}_{\rm y}=\rm 0.6~mm$. 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.