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Development of the Silicon Vertex Detector for Belle II experiment

Belle II experiment at KEK (Tsukuba, Japan) is planning to start to collect e+e− data from 2016 with SuperKEKB accelerator and Belle II detector, where the luminosity of the accelerator will be 80 × 1034cm−2s−1, 40 times as large as that of the former accelerator KEKB. In order to accommodate its high luminosity, the upgrade of the detector is now ongoing. We are responsible for the development of an outermost layer of the SiStrip Vertex Detector (SVD). SVD is one of the Belle II subdetectors, which precisely measures position of charged particles by using Double-sided SiStrip Detector (DSSD). The outermost layer of SVD is composed of sixteen ladders and we plan to start mass production from April 2014. It is necessary to develop a ladder assembly procedure that can accomplish alignment precision within a limited production time. For example, a B meson, which is one of the main target of Belle II, flies approximately 100 μm from interaction point. In order to achieve the vertex resolution required for the measurement of time dependent decay rate of B mesons, we set the goal of alignment precision to be 10 μm. In order to proceed to the mass production, we developed ladder assembly procedures. We have developed support fixtures (jigs) so that everyone can fix the components of ladder. However, because the positioning precision of jigs is approximately 100 μm, we need to develop an additional alignment procedure of DSSDs. We have successfully developed special jig and an alignment software to align DSSDs. After the assembly of the ladder, the displacement of DSSDs are measured and the information will be utilized for physics analysis. In order to evaluate the alignment precision of DSSDs in the finished ladder, we have assembled a dummy mockup by substituting aluminum plates for the DSSDs, where we found a horizontal displacement of the DSSDs by ∼100 μm. We have identified several possible sources of the displacement. Further study is necessary to accomplish the required alignment precision of assembled ladders.