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In the Belle II detector, the central drift chamber (CDC) plays three important roles. First, it reconstructs charged tracks and measures their momenta precisely. Second, it provides particle identification information using measurements of energy loss within its gas volume. Lowmomentum tracks, which do not reach the particle identification device, can be identified using the CDC alone. Finally, it provides efficient and reliable trigger signals for charged particles. As described in the LOI [1], the Belle CDC has worked well for more than ten years without any serious problems. Therefore, the Belle II CDC follows the global structure of its predecessor for the material of the major parts, the superlayer wire configuration, the cell structure, the wire material, and the gas mixture. The main parameters are listed in Table 6.1, together with those of the Belle CDC for comparison. The main differences between the two designs are described in the following paragraphs.
First, the new readout electronics system must handle higher trigger rates with less deadtime. The front-end electronics are located near the backward endplate and send digital signals to the electronics hut through optical fibers. The front end uses a new ASIC chip that incorporates an amplifier, shaper, and discriminator. The drift time is measured with a TDC that is implemented in an FPGA. A slow FADC (around 30 MHz) measures the signal charge, and is controlled by the same FPGA. The ASIC chips, the FADC, and the FPGA are mounted on a single board, which is described in Sec. 6.4.
Second, to avoid the high-radiation and high-background region near the interaction point and to provide more space for the SuperSVD, the CDC inner radius is 160 mm. Since the Belle II barrel particle identification device is more compact than in Belle, the CDC outer radius is 1130 mm (tentative value). A new wire configuration fills the modified volume.
Third, the CDC generates three-dimensional trigger information. A z trigger for charged particles is essential to reduce background without sacrificing physics events. In the original Belle CDC, there was a cathode chamber with three strip layers in the innermost region [2]. This chamber reduced the charged trigger rate by a factor of three. When the SVD2 was installed in Belle, the cathode chamber was removed to make space. In principle, the SVD2 would have been able to provide the z-trigger information. Fortunately, the charged-track trigger rate was low enough that this was not needed for background suppression. However, we expect that the
availability of a z trigger will be important for Belle II, particularly during the early stages when the beam backgrounds are expected to be high. The z trigger will be based on a 3D tracking method implemented in an FPGA using axial and stereo wires. (Charge division between the two ends of a sense wire is a possible alternative to provide the z information.) This approach is robust against high beam background and does not require additional material. The 2D and 3D charged-track triggers are described in Ch. 12.
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