Andrey Chernogorov , Sergey Belogurov, ITEP, Moscow

1. Optimization of the Silicon Tracking System (STS) layout and beam pipe configuration for the CBM experiment. • Outline • The main idea. “Pipe” = “Window”+”Tube”. Balance between measurable particles (window) and ions (tube) • Beam pipe first section and vacuum chamber’s window effect on the STS performance • Effects of the junction between STS and RICH sections • The implementation into cbmroot package of spatial and angular distributions of ions incident to the target for realistic beam emittance simulation • Stereo angle effects on STS performance • Sensors’ overlapping effects on STS performance • Conclusions • Plans Andrey Chernogorov, Sergey Belogurov, ITEP, Moscow FAIR seminar, 21-22 June 2011, Moscow

2. Beam pipe first section and vacuum chamber’s window effect on the STS performance Studied configurations Some of “Ideal ” configurations. Effects: cylinder-cone; Be-Al. Al: window – scaling from VELO, cone – “simple” manufacturing. 1st “Realistic” configurations. Effects of bellows, width of “Tube”. 1.6º configuration fits to carbon fiber (CF)ladders without cutting the central rib. Any configuration has a weld 1x10 mm A.Chernogorov, S.Belogurov, ITEP, Moscow

3. Range distribution window welding IR,cm • Results for UrQMD central events at 25 AGeV: IR Figure of merit: integrated range in the pipe: IR=Σli·qi2·ne li – range of a particle inside the pipe qi - its charge ne – relative electron density Total ionization losses and number of d-electrons are roughly proportional to IR Distribution of integrated ranges of charged particles in the beam pipe’s walls for several configurations. Cylinder is optimal with MF, cone – without MF (The same for 8 AGeV). No contradiction with the conclusions of HERAb and LHCb. A.Chernogorov, S.Belogurov, ITEP, Moscow

4. Results for UrQMD central events at 25 AGeV: Track finding quality Clear trends for “ideal” configurations (the narrower the better), for “real” ones no big difference. A.Chernogorov, S.Belogurov, ITEP, Moscow

5. Results for UrQMD central events at 25 AGeV Comparison of Al and Be cones 30%/7% Momentum distribution of ghost tracks Efficiency of tracks’ secondary particles restoration as a function of momentum 30-50%, mainly soft ghosts A.Chernogorov, S.Belogurov, ITEP, Moscow

6. Results for UrQMD central events at 25 AGeV : reconstruction of short lived particles The narrower the better for “ideal” configurations, for “real” ones more narrow seems slightly better. Vertex resolution much worse than expected from multiple scattering and not indicative. Thanks to I. Vassiliev for the basic analysis routine A.Chernogorov, S.Belogurov, ITEP, Moscow

7. Results for UrQMD central events at 8 AGeV : • Track finding quality STS with given layout works perfectly with any beam pipe. As any optimistic conclusion should be double checked. Probably for SIS-100, STS layout may be simplified and made cheaper. However, taking into account realistic STS simulation may change the answer! A.Chernogorov, S.Belogurov, ITEP, Moscow

8. Results for UrQMD central events at 8 AGeV : • reconstruction of short lived particles Only S/B and mass resolution are somewhat sensitive to Al beam pipe. A.Chernogorov, S.Belogurov, ITEP, Moscow

9. Flange/window effects on RICH Default cbmroot geometry: 1-st section of pipe with Be window h0.1mm on the end with flange 2.0x2.5cm on the end with a weld 2.5x0.2cm on the end A.Chernogorov, S.Belogurov, ITEP, Moscow

10. Flange/window effects on RICH The influence of the flange is almost imperceptible. There is a slight decrease of parameters when using Be window, while passing 20 or more ions. In general, when the number of ions no more than 20, the configuration of the window can be used. A.Chernogorov, S.Belogurov, ITEP, Moscow

11. The implementation into cbmroot package of spatial and angular distributions of ions incident to the target for realistic beam simulation with collimators. Section of the beam on the CBM target without collimation (left) and with collimation (right) for SIS-300 energy.Simulations by M. Kats. - The description of the ion beam by a Gaussian distribution (class FairPrimaryGenerator now) is not adequate; - More correct to describe the collimated beam by the product of a Gaussian distribution and a trapezium (symmetricnow and more complex function if it is necessary in next amendment process); - The distributions for description of realistic beam emittance are done and will be implemented in next CBMROOT release after completion of testing. A similar smearing of the angular distribution is implemented too. Parameters of the projection of beam cross section on the CBM target without collimation (left) and with collimation (right) for SIS-300 energy after the fitting. A.Chernogorov, S.Belogurov, ITEP, Moscow

12. Stereo angle effects on STS performance Big number of fakes is a big problem for track finding, especially for extending the algorithm to the segments with missed points. We looked at sensors with smaller stereo angles… Stereo angle – between strips on the opposite sides of double side sensor. A.Chernogorov, S.Belogurov, ITEP, Moscow

13. Stereo angle effects on STS performance Big number of fakes is a big problem for track finding, especially for extending the algorithm to the segments with missed points. We looked at sensors with smaller stereo angles…. SIS-300 A.Chernogorov, S.Belogurov, ITEP, Moscow

14. Stereo angle effects on STS performance SIS-100 SIS-100 For both SIS 100 and SIS 300, stereo angle of 6 ° is the best for track finding. Momentum resolution becomes worse bellow 6°. In simulations of short-lived particle reconstruction 8° is considered because technologically preferable (dm). A.Chernogorov, S.Belogurov, ITEP, Moscow

15. Sensors’ overlapping effects on STS performance (SIS-300) 1.2-1.4% 5% 10% 5% Overlap is due to the guard rings but could be bigger due to geometrical reasons (DZ between neighboring sensors up to 1 cm) A.Chernogorov, S.Belogurov, ITEP, Moscow

16. Sensors’ overlapping effects on STS performance (SIS-100) 1.2-1.4% 4% 5% 10% • Study the influence of sensors’ overlapping on tracking system properties showed that the properties dependence of the overlap is sufficiently weak. Overlap does not represent a problem and allowed. A.Chernogorov, S.Belogurov, ITEP, Moscow

17. Conclusions • Influence of shape and material of the beam pipe on the STS performance is studied. Pipe is considered as a composition of “window” and “tube”; • A comprehensive study of the effect of the beam pipe’s first section and the vacuum chamber window on tracking system properties for central collisions of gold-gold at 25 AGeV (SIS-300) and 8 AGeV (SIS-100) was completed. For 8 AGeV only S/B and mass resolution are somewhat sensitive to Al beam pipe. It should be checked with more realistic STS; • Decrease of the stereo angle may help to reduce number of fakes and improve track finding performance; • Study the influence of sensors’ overlapping on tracking system properties showed that the properties dependence of the overlap is sufficiently weak. Overlap does not represent a problem and allowed. • The influence of the flange/window between the first and the second sections of the beam pipe was studied; • The distributions for description of realistic beam emittance are done and will be implemented in next CBMROOT release after completion of testing. A similar smearing of the angular distribution is implemented too. A.Chernogorov, S.Belogurov, ITEP, Moscow

18. Plans • Tuning and adjustment of the realistic (i.e. including noises and clustering) STS Monte Carlo model for correct description of low energy secondaries generation and accounting for small energy releases in the sensors. This model is necessary for further design justification; • Feasibility study of 8 degree stereo angle in silicon sensors. Realistic STS and track propagation to downstream detectors will be taken into account; • Careful Monte Carlo justification will be done for reduction of the number of channels in the STS for SIS 100 by increasing the length of the sectors. This work will allow to save funds for STS construction without loss of the physical performance. However in such a scenario an upgrade of STS will be needed before operation at SIS300; • The effects of the auxiliary equipment in the gap of the magnet on the performance of the STS will be studied. It is important for engineering of the service equipment for STS and the vertex detector; • Feasibility study for the central part of the ECAL made of the tungsten based modules with small Moliere radius will be performed. This work is important for having maximum physical information from the compact start version of the ECAL. The work will include both simulation of the ECAL and building of the Prototype module; • The design optimization and justification work for the RICH and downstream sections of the beam pipe will be continued. A.Chernogorov, S.Belogurov, ITEP, Moscow