8. CENTRAL TRACKING

8.1 Performance requirements

A central tracking system represents one of the key elements of the CMS. The primary goal is to reconstruct all high Pt muons and isolated electrons in central rapidity region with a momentum precision of (Tev). The high momentum precision is a direct consequence of high magnetic field (4T). Taking into account the multiplicity (about 500 background low energy tracks accompanying interesting events) the problem of pattern recognition becomes essentially hard. High granularity of the system is necessary with a cell size of about 50 µm near the beam line (20-40 cm) and 200 µm at a distance of (40-120 cm). From occupancy considerations (probability of coincidences in one cell should be less than 1%), the length of strips should be less than 10 cm in a close region and less than 20-25 cm in a far region. For a good track reconstruction the system has to provide about 12 track points in the barrel and 15-20 in the forward region. The radiation environment in the central region is very hard. Equivalent flux of charged particles at 40 cm from the beam is close to 105 mm-2 s-1 which gives 1 MRad/year of integral dose. These two values demand the tracker to be rate capable and radiation hard.

8.2 Detector description and organization

The proposed design of the Central Tracker is shown in Fig. 8.1. It consists of the silicon pixel detector (r = 7.5-15 cm, z = -70-70 cm), silicon microstrip tracker (r = 20-40 cm, z = -1.2-1.2 m) and gas microstrip (MSGC) (r = 40-120 cm, z = -3-3 m). Each of the trackers consists in turn of barrel and forward parts.

Novosibirsk group is participating in the "hottest" part of the Gas Microstrip Tracker (see Fig.8.1) in the forward region (r = 40-80 cm, z = 120-300 cm). This region represents the highest flux and radiation dose in the MSGC tracker as all low energy background will concentrate along the beam line due to high magnetic field. Forward parts of MSGC tracker will consist of wheels of glass microstrip plates with strips in radial direction. Microstrip structure in each plate has thin ~7 µm anode strips and thicker ~70 µm cathode strips. Average strip pitch is 200 µm. In the inner region of the wheel (r<=60 cm) the plates will be 5 x 10 cm in size while at larger radius from the beam the size of the plates will be ~10 x 10 cm.

8.3 Research and development

To solve the problem of high rate capability and radiation hardness we are developing the technology which uses electronically conductive glass as a substrate and gold as a material for the strips. It was proven earlier in multiple studies that the substrate which has some conductivity behaves more stable than the ordinary glass and that gold as a material for the strips is the best from radiation hardness point. Budker INP together with NIIES (Moscow) developed glass with the required resistivity. NPO VOSTOK in Novosibirsk developed appropriate technological process for the MSGC plates production and good quality of the chambers produced was demonstrated. At present the main problem of the proposed technology is the thickness of the substrate which has to be made of 100 mm thick (MSGCs which were fabricated by BINP use 0.5 mm glass). We are developing pulling technology which will permit us to get thin semiconductive glass substrates cheaply and in large quantities. We plan to construct MSGCs on thin conductive glass substrates and test them at CERN in 1995. At the same time we are planning to begin production of the set of masks for wedge shaped detector with radial strips in order to begin (in the second half of the year), production of such MSGC plates and perform its tests by the end of the year. One of the main subjects of the R&D for the 1995 will be radiation hardness of the MSGC plates in a more real environment. Together with a group from Brussels University we are planning to perform tests in intensive X-ray beam.

On the basis of 1995 R&D results the final technology of MSGC will be chosen for the final design. RDMS group interested in Tracker detector will then work on the chosen baseline. The MSGCs can be produced in Novosibirsk and Brussels and will be tested in the prototype of the sector of the wheel. In 1996 assembling and testing of some limited size prototype will be continued and in 1997 we are planning together with other participants of forward tracker to assemble the prototype of the forward wheel.