First measurements of beam backgrounds at SuperKEKB

Abstract

The high design luminosity of the SuperKEKB electron-positron collider is expected to result in challenging levels of beam-induced backgrounds in the interaction region. Properly simulating and mitigating these backgrounds is critical to the success of the Belle II experiment. We report on measurements performed with a suite of dedicated beam background detectors, collectively known as BEAST II, during the so-called Phase 1 commissioning run of SuperKEKB in 2016, which involved operation of both the high energy ring (HER) of 7 GeV electrons as well as the low energy ring (LER) of 4 GeV positrons. We describe the BEAST II detector systems, the simulation of beam backgrounds, and the measurements performed. The measurements include standard ones of dose rates versus accelerator conditions, and more novel investigations, such as bunch-by-bunch measurements of injection backgrounds and measurements sensitive to the energy spectrum and angular distribution of fast neutrons. We observe beam-gas, Touschek, beam-dust, and injection backgrounds. As there is no final focus of the beams in Phase 1, we do not observe significant synchrotron radiation, as expected. Measured LER beam-gas backgrounds and Touschek backgrounds in both rings are slightly elevated, on average three times larger than the levels predicted by simulation. HER beam-gas backgrounds are on on average two orders of magnitude larger than predicted. Systematic uncertainties and channel-to-channel variations are large, so that these excesses constitute only 1-2 sigma level effects. Neutron background rates are higher than predicted and should be studied further. We will measure the remaining beam background processes, due to colliding beams, in the imminent commissioning Phase 2. These backgrounds are expected to be the most critical for Belle II, to the point of necessitating replacement of detector components during the Phase 3 (full-luminosity) operation of SuperKEB.

Compact, directional neutron detectors capable of high-resolution nuclear recoil imaging

“…Second, the self-trigger via firmware is beneficial in highbackground applications. Vetoing electron-recoil events in the readout firmware, and not issuing a trigger for such events, was the key ingredient that allowed the BEAST TPCs to operate successfully in the high-radiation environment at SuperKEKB, without suffering from significant deadtime [14].…”

“…We use both prototype and final TPCs in the first phase of SuperKEKB commissioning [14], and only final TPCs in later phases. Both types of TPCs are roughly shoebox-sized aluminum vessels with a single FE-I4B pixel chip, aluminum field cage, two GEMs, embedded alpha sources, and Parylene C coating as described in the previous section.…”

Silicon photomultipliers in particle and nuclear physics

“…From this system, a detector using to the latest generation of SiPMs and more capable digitizers was derived for the study of beam injection background during the commissioning of Su-perKEKB. Here, minimum-ionizing particles and synchrotron radiation photons are measured with a continuous readout window of up to 100 ms, corresponding to 10000 revolutions of the accelerator, by the CLAWS detector consisting of 16 scintillator tiles with SiPM readout [78,79]. The sub-ns time resolution allows the association of the observed particles to a specific bunch in the SuperKEKB machine.…”

Prospects for $\tau$ lepton physics at Belle II

“…Detector recorded 500 pb −1 of data at Υ(4S) energy with the BEAST II detector installed, instead of the vertex detector. BEAST II is used to study the beam background components [3]. Next year, in 2019, Belle II will start the Phase III with all the subsystems installed, expecting a full dataset of 50 ab −1 by the end of the data taking, in 2025.…”