Speaker
Description
Summary
The SuperKEKB collider, an upgrade of the former KEKB, is under construction at KEK. It is an asymmetric (4 GeV, 7 GeV) $e^+e^-$ collider working at the center of mass energy of the $\Upsilon(4S)$ resonance. The design peak luminosity is 8$\times$10$^{35}$ cm$^2$s$^{-1}$, about 40 times larger than KEKB, aiming at an integrated luminosity of 50 ab$^{-1}$.
To fully exploit the higher luminosity the former Belle detector is being upgraded. This new detector must cope with the higher backgrounds and event rates and the corresponding larger radiation damage, occupancy and fake hit production. As a consequence, improved vertexing and tracking capabilities are needed in the Belle II detector. This converts the vertex detector in one of the key elements that drive the physics performance. It consists of four layers of double-sided silicon strip detectors in the outer radii, SVD, and two layers of highly granular pixel sensors in the innermost part, known as PXD.
The PXD, built with DEPFET pixel detectors, is intended to improve vertex resolution. As a consequence of the low momentum of the particles in the final state (< 1 GeV/$c$), the hit position determination is intrinsically limited due to the multiple Coulomb scattering. This sets a lower limit of 10$\mu$m for the spatial resolution in the PXD that can be achieved with a moderate pixel size of 50$\times$50 $\mu$m$^2$. This pixel size is also enough to cope with the expected occupancy of 0.4 hits $\mu$m$^{-2}$s$^{-1}$. For the same reason the material budget must be kept low, up to a maximum of $\sim$0.2 % X$_\circ$ per layer, implying a thickness of 75 $\mu$m of the sensitive part of the sensor. The acceptance of the detector must cover the range 17$^\circ$-155$^\circ$ in azimuth angle. The detector will be read continuously with a frame time of 20 $\mu$s keeping the occupancy below 3%. This continuous readout means that ASICs are ON all the time, which together with the restrictions on the material budget sets the requirements for cooling. Finally, according to the simulations, the radiation dose expected in the inner region of the detector is around 2 Mrad/yr.
