Vertex Detector

The heart of any asymmetric B Factory detector aimed at the study of CP violation is the vertex detector. Primary emphasis is on resolution along the flight path of the B mesons, that is, the beam (z) direction, and not on resolution in the transverse plane, as in current silicon vertex systems. 

---

Vertex detection precision close to the beam direction is made difficult by the increased multiple Coulomb scattering caused by the passage of charged particles through increased amounts of material (in the beam pipe and first layer of vertex detection) at small polar angles. The best strategy is to make the first 2 measurement as close as possible to the initial scatter in the beam pipe. If this is done, studies show that the detailed arrangement of silicon vertex detector planes has little effect on z resolution (see Chapter 2). 

Multiple Coulomb scattering is, in fact, a serious issue for the main tracking drift chamber as Well as for the vertex detector. Figure 1.10 shows the momentum distribution of pions, kaons, electrons and photons produced by B mesons at the T(4S). The particle spectra peak at low momenta, although the high momentum tails are not unimportant, as we shall see. 

Tlie priniary task of a vertex detector at an asymmetric D Factory is the meiisure-ment of tlie time evolution of the asymmetry in decays to CP eigenstates,... 

We have calculated the expected increase in lealcage current in the silicon strips in the first barrel layer due to the lost particle fluence, as W(dl as tlie corresponding increase in noise. The increase in leakage current due to the charged particde flux crossing a given element of the silicon vertex detector is given by ... 

We have focused so far on the measurement of the imi)act parameters, especially in the coordinate. The vertex detector, how( ver, will also improve the measurement of the momentum for high-momentum tracks considerably l)y ( xtending the lever arm of the measurements in the central drift chamber (CDC), which begins at 22.5 cm. In order to achieve both the d( sired impact j)aramet( r resolution and the improvement in momentum resolution, a vertex tletc ctor with two or tlin c laytus ( xtemding from close to the beampipe out to approximately 8 cm in raelius we)ulel l)c satisfactory. Tins we)ulel lea e the space from 8 te> 20 cm uninstruine iite el. 

A Five-Layer Double-Sided Silicon Microstrip Vertex Detector 2.3.1 Design Considerations... 

These calculations suggest that the best resolution can be achi( ved by varying the strip pitcli in. i, with a transition from 50 to 100 //m at A 40 - 50°. With this a.ssuinj)tion, the re.solution can be approximated by (j° j cos A, where a° 10 — r2//in. (It should be noted that existing vertex detectors, such as the ALEPH doul)le-sid( d silicon vertex detector [8], have already achievTd an average intrinsic re.solution of 12-13 /an in both (f> and... 

Figure 2.5. Single hit resolutions in the r - 0 and projections measured in tlie ALEPH vertex detector,...

Figure 2.7. Tlu thickness in radiation lengths vs, polar angle for the current ALEPH vertex detector system and the preliminary design of the proposed upgrade (10).

From this discussion we can deduce some basic design guidelines to optimize the impact parameter resolution. The vertex detector sltould have ... 

A Fivc Layer Double-Sided Silicon Microstriy Vertex Detector 43... 

Table 2.2, DimcnsiotiH and Sogmoutation of tlie Silicon Vertex Detector...

A Five.-Layer Doubh Sickd Silicon Micrasirip Vertex Detector 45... 

The silicon vertex detector discu.ssed liere places aljout 5 X 10 readout channels in a very limited space imposing severe restrictions on the volume occupied by the readout electronics, cables and connectors, and on the j)ower consumption. The specifications for the readout system include the following ... 

PrelimiTKn y Dasifjn of a PEP-II Vertex Detector Based on Hybrid Pixel Airays... 

---