April 2002 K. Riles List of Linear Collider Tracking Simulation and R&D Issues ---------------------------------------------------------- (with some prioritizing) Simulation ---------- Some of the issues below can be addressed with fast Monte Carlo studies; others require full simulation, including non-cheat track reconstruction. Each issue is assigned a priority (high, medium, or low) and method (fast or full MC). 1) What momentum resolution is really needed at cos(theta) ~ 0? High priority, fast MC There are two benchmark signatures commonly quoted as drivers for good momentum resolution in the barrel. In each case, full account should be taken of a) initial state radiation & beamsstrahlung; b) beam energy spread; c) kinematic fitting improvement; d) realistic final mass determination; and e) any relevant particle decay widths. * Dilepton recoil mass in Higgsstrahlung (HZ -> X l+l-) (H. Yang - Michigan has done much work on this and should have complete study soon) * Lepton spectrum end-points from slepton production followed by decay to lepton and neutralino (Preliminary work in Europe and U.S.) 2) What thickness (radiation lengths) for the central tracker is acceptable? High priority, fast MC There has been strong emphasis on developing thin-material tracking (e.g., reducing inner field cage thickness in TPC, thinning layer of silicon microstrips or silicon drift detectors) What important physical processes at 500 GeV justify this aggressive R&D? Specifically, what deltap/p is good enough for a 1 GeV pion? 1.0%? 0.1%? 3) What precision (if any) is needed in dE/dx measurements? Medium priority, fast MC What important physical processes at 500 GeV would argue for good dE/dx precision? 4) Track reconstruction infrastructure development / improvement High priority, full MC * We need non-cheat reconstruction for projective barrel tracking technologies (silicon microstrips barrel highest priority, axial/stereo drift chamber at low priority). * We need non-cheat reconstruction for forward disks (Si strips) * We need self-contained vertex detector tracking with extrapolation outward to central tracker * We need better optimized track reconstruction for 3D barrel tracking technologies (TPC, silicon drift detector), with much better tolerance of high machine backgrounds. 5) Evaluate impact of tracker design on energy flow measurement Medium priority, fast & full MC If aggressive jet energy flow jet measurement is adopted (using expensive, fine-grained calorimetry), then ideally every charged particle should be tracked well, i.e., precise momentum measurement and trajectory extrapolation into the calorimeter to allow charged / neutral shower separation. * Examine the effect of 2-track separation in both r-phi and r-z. For example, is this where a drift chamber has an advantage over a TPC? Does a 3-D device give better track separation on the whole? * Examine the benefit of a high-precision outer barrel tracking layer in calorimeter extrapolation. 6) Evaluate advantages/disadvantages of intermediate tracking layer between pixelated vertex detector and large gas chamber Medium priority, fast & full MC An intermediate tracking layer has been proposed to give three potential advantages: 1) Improved momentum resolution 2) Improved pattern recognition (bridging the gap) 3) More precise timing information for distinguishing tracks from different bunches in a train Full study should include consideration of systematic alignment errors, non-cheat track reconstruction/fitting, backgrounds from multiple bunches. 7) Evaluate advantages/disadvantages of tracking layer between gas chamber endcap and calorimeter endcap Medium priority, fast & full MC An endcap tracking layer has been proposed to give three potential advantages: 1) Improved momentum resolution at low angles 2) Improved polar angle resolution (useful in differential luminosity measurement of Bhabha kinking) 3) Improved extrapolation of tracks into calorimeter (see related issue #4 above) Full study should include consideration of systematic alignment errors, backgrounds from multiple bunches, and calorimeter backsplash. 8) Ionization distortion of electric field in a TPC Low priority, (very) full MC Conventional wisdom is that new TPC readout schemes, combined if necessary with a gating grids, can control ion presence from avalanche feedback and that primary ionization from annihilation and two-photon processes is okay. But what if machine backgrounds are much worse than expected? What level of synchrotron radiation, for example, can be tolerated without significant degradation of momentum resolution? 9) Wire saturation in a drift chamber design Medium priority, full MC How well does an axial / stereo drift chamber cope with high synchrotron and beam halo muon backgrounds? 10) "Stereo" layers for silicon microstrip option High priority, full and fast MC Should "stereo" be small-angle to reduce ambiguity in jet environment or large-angle (90 degrees) to optimize z resolution, or something in between (e.g., 45 degrees)? Detector R&D ------------ No attempt is made below to prioritize between technologies. Until many of the simulation issues above have been well addressed, it's premature to assign priority to these technologies. Instead, prioritizing below is assigned below only to issues for a given technology. The drift chamber option is not discussed below, given the comprehensive R&D program well underway in Japan, but please note the simulation issues abov. TPC issues ---------- 1) Readout scheme - High priority There are many related issues here, and much work has already been done. The essential problem is to come up with an affordable readout at the TPC ends that exploits the intrinsic r-phi and r-z resolution of the chamber and doesn't add excessively to the material of the endcap. Traditional readout has been via inductive pads near high tension wires. Supporting wire tension tends to add material to the endcap, and feasible wire spacing leads to degradation of resolution. New proposed avalanche schemes include GEMs and MicroMEGAS with inductive pad readout. The main problem lies in a pad layout that takes advantage of the intrinsic resolution of the avalanches without requiring millions of electronic channels. High clocking speed with multiplexing is one approach, but dE/dx precision and necessity of cooling become issues. 2) Gas mixture - Medium priority Issues to explore include 1) the tradeoff between degraded spatial resolution and fast clearing of the chamber with faster gases; 2) quenching with hydrocarbons vs reducing susceptibility to neutron backgrounds vs aging; and 3) affect on field cage design. 3) Mechanical support - Low priority It is generally desirable to minimize material in the field cages and endplates. Readout scheme and cooling requirements play a role. 4) Calibration - Low priority Can one rely on running at the Z pole to calibrate the tracking system? Does a laser system work well enough for the gas chambers? Can one rely on auxiliary tracking layers, e.g., intermediate tracker or outer barrel "Z" layer? Silicon detector issues (common to drift and strips options) ----------------------- 1) Thinner layers - Medium priority (see simulation issue #2) Everything else being equal, it's desirable to minimize material in each tracking layer. But thinness presents challenges of mechanical support and stability. There are two fronts to work on: * Developing thin detectors and readout electronics * Supporting the detectors stably.Tension stretching is one suggestion; another is living with instability, but tracking it in real-time with an Atlas-style chirped IFO system). 2) Longer ladders - High priority (again, see simulation issue #2) Short ladders can give better timing precision and background rejection, but introduce more front-end electronics into the fiducial volume, increasing material burden. 3) Readout electronics - Medium priority Increase level of integration, exploit low duty cycle of accelerator via power switching while maintaining stability. Reduce mass for FEE in fiducial volume. Silicon drift detector issues ----------------------------- 1) Improve r-phi resolution to below 10 microns - High priority 2) Improve r-z resolution to below 10 microns - High priority Silicon microstrip detector issues ---------------------------------- 1) Stereo readout - High priority 2) Lorentz angle in high B field - Medium priority 3) p-side readout - Medium priority 4) dE/dx measurement - Low priority