BELLE/SVD2 status and performance
Section snippets
Introduction: Description and design
The BELLE detector [1] operating at the asymmetric energy collider KEKB [2] is a large solid angle magnetic spectrometer, with the aim of studying the CP violation in the decays of neutral B mesons. The key point of this experiment is to measure the distance between B meson decay verteces, 200 , with good precision. For this purpose, a silicon vertex detector (SVD1) [3] was employed and it has brought us many remarkable results [4], [5], [6]. However, SVD1 covers the polar angle range
Performance
During the BELLE shutdown in 2003, the SVD2 was installed in the place of the SVD1 and its operation has started in October 2003 successfully. Now about a couple of months have passed without any trouble.
Reducing the radius of the innermost layer implies that the detector will suffer from a higher radiation and particle background. The expected dose level based on the current radiation is [9] that is far less than the radiation tolerance of the SVD2. As shown in Fig. 2, the
Summary
For the further precise measurement of the CP violation in the neutral B meson system, the silicon vertex detector was upgraded and its operation has started successfully in October 2003. This upgrade is designed to provide a larger polar angle acceptance, a more precise vertex resolution and a better radiation tolerance than the previous vertex detector.
The observed performances are: gain stability, the ratio of each half-ladder ranges from 16 to 36, the hit efficiency is more than 96%
Acknowledgements
The authors would like to express appreciation to other members in the BELLE Collaboration for many supports and advices. The authors are also deeply indebted to all institutes and universities related to the SVD2.
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- 1
Present address: KEK.
- 2
Present address: IHEP Beijing.
- 3
Visitor from Toronto University.
- 4
On leave from IFJ Krakow.