Beam background expectations for Belle II at SuperKEKB

Abstract

The Belle II experiment at the SuperKEKB electron-positron collider aims to collect an unprecedented data set of 50 ab −1 to study CP -violation in the Bmeson 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 6.3 × 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 β * y = 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 2.8 × 10 35 cm −2 s −1 is reached at β * y = 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.

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

“…In Ref. [16], we have proposed a few possible solutions to partially cure beam instabilities and resolve the specific luminosity and dynamic aperture degradation, where the latter affects beam lifetime, as mentioned above. Nevertheless, the upshot is that the target machine lattice and beam parameters are still too uncertain to make an accurate background prediction for the target luminosity.…”

Luminosity performance of SuperKEKB

“…Installing a nonlinear collimator (it will replace a smallgap collimator and consequently reduce the vertical impedance significantly.) to the LER during LS1 [26] is an expected solution for curing the vertical blowup due to collective effects. The effectiveness of crab waist relies on a clean IR, which means less nonlinearity in optics and less background to the Belle II.…”

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

“…The most crucial upgrades under consideration are discussed in Ref. [14]. The Belle II detector, shown in Fig.…”

“…In Ref. [14], we have proposed a few possible solutions to partially cure beam instabilities and resolve the specific luminosity and dynamic aperture degradation, where the latter affects beam lifetime, as mentioned above. Nevertheless, the upshot is that the target machine lattice and beam parameters are still too uncertain to make an accurate background prediction for the target luminosity.…”

“…In Ref. [14], we review ongoing activities and options for further background mitigation, and background predictions for even higher luminosities, up to 6.3 × 10 35 cm −2 s −1 .…”