# Presentation: 161412

## Optimization of  the SiD Detector ?

- Adam Para, Sept. 18, 2006

## Bottom Line

- Silicon-based tracking offers new possibilities for the optimization of the colliding-beam detector in area of:
- Physics capabilities
- Detector performance
- Cost
- Engineering and construction
- Note: in the following the angle theta will be used in an inconsistent fashion:  90o-Substitutesin  cos, as necessary

## SiD – a Fine Example of a Cylindrical Detector

- Cylindrical Detectors – Advantages:
-  Battle-proven, well understood design. Several existing examples
-  uniform (axial) magnetic field, No radial field component
- Established construction techniques, support, services...
-  Phi-symmetry (construction, analysis)
-  significant effort in simulation, analysis, physics performance, detector optimization

## A Cylindrical Detector: Weak Points

-  Good momentum resolution in the barrel region only. It degrades rapidly at lower angles, whereas the particle density increases. For e+e- f fbar
- For other processes (like *t*-channel exchange) more forward-peaking
- Ugly corners/transitions (often used: this analysis for |>45°|, ‘forward tracking’, ...)
- Calorimetric sampling/resolution degrades with 1/sintoo thin at =90°, unnecessarily thick at =45°
-  cost of covering the solid angle increases as 1/sin

## Why Build a Cylindrical Detector? 

- This is the way the colliding-beam detectors are usually built
- 4 gaseous tracking:
-  axial wires + (usually awkward) forward disks
- TPC (axial drift of electrons)
- ExB, Lorentz angle effects: need homogenous axial magnetic field => solenoid
- The main (the only good?) reasons for the cylindrical geometry seem to be related to gas-based tracking detectors.
- Silicon-based tracker offers new flexibility: one may construct a traditional cylindrical detector, but other geometries are possible too.

## Why not a Spherical Detector?

- Maximal symmetry of the detector
- Equal treatment of high and low angle regions, no corners and transition regions
- Best detector performance: detector surfaces ~orthogonal to the measured particles trajectories
- (Probably) the best use of the materials strength, the minimal need for the support structures
- Cost! Example:
- A detector with radius R and length L=2R: area = (4+2)R2
- A spherical detector with radius R: area  = 4R2
- For the same detector radius a spherical detector is 1.5 times ‘cheaper’
- For the ‘same cost’ the spherical detector can be 1.2 times bigger
- For detector with L>2R the cost savings are even bigger

## A Spherical Detector? 

- Nested shells, inner shells supported form outer ones
- Vertex detector and tracker : spherical space frames
- Hadron calorimeter supported from an outer strong back shell
- EM Calorimeter supported from the HAD calorimeter
- Uniform calorimetry (identical ‘towers’)

## Size Comparison: SiD vs a SSD (Spherical Silicon Detector)

- Postpone muons and coil discussion (coming)
- All sub-detectors of the spherical detector have 1.5-2 times smaller volumes/surfaces than those of the SiD

## Construction the Spherical Detecor

- (Some of) the problems: Inner detectors are trapped inside.
-  How can you build/install the detector
- Access for repairs/maintenance
- Cables/services
- Solution(s):
- Split the detector in two halves at =90°.
- Run most of (all?) the cables out in the =90° ‘crack’. Crack may not be projective. The particle density is at minimum and the available area is maximal there, hence the impact on physics is minimized
- Detectors can be constructed (if this is a desirable scenario) with hemispheres openings facing up (ease of access, minimize the support problems) and subsequently rotated to the vertical position
- Easy access to the innermost detectors after the opening

## ‘Open’ Detector (for Access or before Installation)

## Tracking resolution/magnetic field

- Spherical detector may be good for calorimetric measurements, perhaps for the structural reasons. But.. If immersed in the solenoidal magnetic field, the momentum resolution is rapidly degraded with decreasing polar angle
- Two avenues for the improvement of the momentum resolution:
- Make the detector bigger (p/p~L2)
- ‘bend’ magnetic field to provide Br component to magnify BxR
- Shaping of the magnetic field:
- Wind the coil on the spherical shell
- Optimize the current density as a function of the polar angle

## ‘Spherical Solenoid’ ?

- Strong (hemi)spherical shell with notches. Outside the detector volume, no material constraints. Two separate cryostats.
- Wind the coil on the notches
- Coil radius decreasing with the polar angle:
- Reduce the distance to the tracking volume  increase the field
- Reduce the volume of the superconductor (if the same current density) or increase the current density
- Reduce the stresses on the superconductor: most of the compressive load transferred to to support shell, as opposed to the solenoid where the superconductor has to take the entire load

## Initial Field Calculations (B. Wands)

- This is NOT a design of an optimized coil, but rather some toy-examples to understand the issues:
- Field strength and the field lines
- Role of the iron flux return
- BL2 as a function of the polar angle
- Model:
- CMS-like current density at =0°(90°)
- Current density ~ 1/sin2, down to =80°(10°,r=0.5 m)
- Current sheet at the radius of 3 m
- ∫BxRdR evaluated up to R=1.5 m

## Field Strength: No Flux Return, I(0) = 0.5ICMS

## Field Strength: Flux Return, I(0) = 0.5ICMS

- Geometry of the flux return: naïve attempt to force the flux lines to bend more

## ∫BxLdL for I(0) = ICMS

- Significant amount of bending down to ~10-15°. Proportional to I, of course
- Relatively small effect of the iron flux return. In retrospect: obvious. Iron saturates a values way below the actual field strength.            Q: What is the iron for??? Muon ID? Shielding of the hall? How much is needed?

## Field Lines: No Iron

## Field Lines: Iron Flux Return

## Spherical Solenoid: The Issues

- Impact on the electron beam? The field has (nearly) axial symmetry  very small radial component
- Non-homogenous magnetic field
- Very strong magnetic field in the detector volume
- Very, very strong magnetic field at the tip: need different superconducor (high Tc?)
- Optimize the current density as a function of a polar angle
- Optimize the angular range
- Few (3-4?) traditional solenoids of decreasing radius to restore a complete axial symmetry?
-  others?