BEAM BACKGROUND SIMULATION FOR SUPERKEKB / BELLE-II

Abstract The Belle experiment is now being upgraded to the Belle II experiment designed for a 40 times higher luminosity. Such a high luminosity is realized by the SuperKEKB collider where beam-induced background rates are expected to be much higher than those of KEKB. This poses a serious challenge for the design of the machine-detector interface. We have thus carried out a GEANT4-based beam background simulation for Touschek effect. We describe the method of generating background particles and present the result of simulation. SUPERKEKB AND BELLE-II SuperKEKB, an upgraded of the KEKB collider, will provide a prove to search for new physics beyond the Standard Model, thanks to much larger data sample. The target luminosity of SuperKEKB, 80 × 10 34 cm −1 s −1 , is 40 times higher than that of KEKB. The upgrade is based on so-called "Nano-beam scheme". The basic idea of this scheme is to squeeze the vertical beta function at the interaction point (IP). The vertical beta function at IP is 20 times smaller than KEKB. In addition, the total beam currents will be doubled to achieve 40 times higher luminosity. The basic parameter of SuperKEKB is summarized in TOUSCHEK EFFECT Touschek effect is the most dangerous background at SuperKEKB with "Nano-beam" scheme. The effect is an intra-bunch scattering. Elastic scattering between two particles in a same beam bunch changes their energy to deviate from the beam bunch, one with too much and the other with too little energy. The scattering rate of the Touschek effect is proportional to the inverse beam size, inverse third power of the beam energy, the number of bunches and second power of the bunch current. Since the beam size of SuperKEKB is much smaller than that of KEKB, background from the Touschek effect will become much higher. The contribution from the LER is higher than the HER due to asymmetric beam energy. As shown in At SuperKEKB, simple extrapolation using the machine parameters predicts that Touschek background will increase by factor of ∼20 compared to that of KEKB. However, Touschek background is reduced than this prediction because we introduce improved countermeasures to cope with the background. Touschek-scattered particles are lost by hitting the beam pipe inner wall while they propagate around the ring. If their loss position is close to the detector, generated shower might reach the detector. Fake hits generated by the background shower particles deteriorate the detector's physics resolution. Radiation dose by gammas or neutrons in the background shower damage the Silicon devices used in the detector. To cope with Touschek background, there are two countermeasures: movable collimators and heavy-metal shield. The movable collimators located along the ring can stop the deviated particles before they reach close to the detector. Touschek background can be reduced effectively by collimating the beam horizontally from both inner and outer sides, since Touschek-scattered particles have too much or too little energy. At KEKB, we had horizontal collimation only from inner side. The heavy-metal shield is located outside the detector acceptance, between the beam pipe and inner detectors. The shield is made of Tungstenalloy whose radiation length is short, and effectively stops the background showers before they reach the inner detectors

The K / ratio and strangeness suppression in p and anti p charged current interactions

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