The Silicon Vertex Detector of Belle II is a
state-of-the-art tracking and vertexing system based on double-sided
silicon strip sensors, designed and fabricated by a large
international collaboration in the period 2012–2018. Since 2019 it
has been in operation providing high quality data with a small
number of defective channels (<1%), a large hit-finding
efficiency (>99%), a good signal-to-noise ratio (well in excess
of 10 for all sensor configurations and tracks). Together with the
good control over the alignment, these are all essential factors to
achieve good tracking reconstruction and physics performance. In
this extended paper we try to document all the aspects of the SVD
challenges and achievements, in the spirit of providing information
to the broader community and help the development of high quality
detector systems, which are essential tools to carry out physics
research.
A thick gas electron multiplier (THGEM) chamber with an effective readout area of 10×10 cm 2 and a 11.3 mm ionization gap has been tested along with two regular gas electron multiplier (GEM) chambers in a cosmic ray test system. The thick ionization gap makes the THGEM chamber a minidrift chamber. This kind mini-drift THGEM chamber is proposed as part of a transition radiation detector (TRD) for identifying electrons at an Electron Ion Collider (EIC) experiment. Through this cosmic ray test, an efficiency larger than 94% and a spatial resolution ∼220 µm are achieved for the THGEM chamber at -3.65 kV. Thanks to its outstanding spatial resolution and thick ionization gap, the THGEM chamber shows excellent track reconstruction capability. The gain uniformity and stability of the THGEM chamber are also presented.
We perform a model independent analysis of new physics in B Ã s → μ þ μ − decay. We intend to identify new physics operator(s) which can provide large enhancement in the branching ratio of B Ã s → μ þ μ − above its standard model prediction. For this, we consider new physics in the form of vector, axial-vector, scalar and pseudoscalar operators. We find that scalar and pseudoscalar operators do not contribute to the branching ratio of B Ã s → μ þ μ −. We perform a global fit to all relevant b → sμ þ μ − data for different new physics scenarios. For each of these scenarios, we predict BrðB Ã s → μ þ μ − Þ. We find that a significant enhancement in BrðB Ã s → μ þ μ − Þ is not allowed by any of these new physics operators. In fact, for all new physics scenarios providing a good fit to the data, the branching ratio of B Ã s → μ þ μ − is suppressed as compared to the standard model (SM) value. Hence the present b → sμ þ μ − data indicates that the future measurement of BrðB Ã s → μ þ μ − Þ is expected to be suppressed in comparison to the standard model prediction.
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