# Presentation: 108240

## Charged Particle Tracking Issues(Vertexing, Central & Forward Tracking)

- Keith Riles
- University of Michigan
- Chicago Linear Collider Workshop
- January 7, 2002

## Conventional Wisdom:	“Easy” to build linear collider detector	(e.g., clone SLD or a LEP detector)

- Statement more or less true, but maximizing physics output argues for more aggressive approach
- Will discuss here how to be more aggressive in tracking charged particles
- See talks by Frey / Fisk for discussion of calorimetry / muon system. See Graf talk on simulation infrastructure.
- See talk by Heuer for overview of international detector R&D effort.

## Acknowledgements

- Thanks to
- J. Brau, M. Breidenbach, C. Damerell,
- 	K. Fujii, T.Markiewicz, M. Ronan,
- 	B. Schumm, R. Settles

## Physics Drivers (a sampling)

- Good primary / secondary vertex reconstruction (b vs c):
- B(H->cc) [distinguish SM from SUSY Higgs]
- Charm-tag W+W- final states [strong coupling]
- Good momentum resolution: [ (1/pt)  ~ 5 * 10-5 GeV-1 ]
- Clean Higgs signal from dilepton recoil mass
- End-point mass spectra in SUSY cascades
- Good pattern recognition / 2-track separation
- Jet energies in W+W- final states (Energy-flow algorithm)

## Physics Drivers (a sampling)

- Good forward tracking [ |cos()|  0.99 ]
- [delta theta ~ 10-5 rad; (1/pt)  ~ 2 * 10-4 GeV-1 ]
- New t-channel processes (e.g., chargino production)
- Differential luminosity measurement
- 	(scanning top-pair threshold lineshape)
- LEP/SLC detectors not useless for these measurements,
- but one would like to do them very well

## What tracker designs have been studied?

- Asia:
- CCD vertex detector
- Large-volume drift chamber (DC)
- Europe:
- CCD, CMOS or hybrid pixel vertex detector
- Large-volume time projection chamber (TPC)
- Forward active pixel and silicon microstrip disks,
- 	straw chamber behind TPC endcap
- North America:
- CCD vertex detector
- Large-volume TPC or large-radius silicon tracker (drift / microstrip)
- Forward silicon microstrip disks

## Vertex detector baseline (Europe & North America)

## Central tracker LD baseline (North America)

## Central tracker SD baseline (North America)

## Technical Issues

- Radius of innermost layer of vertex detector:
- Fierce background from Bethe-Heitler pairs
- 	(see figure)
- 	 Drives B-field magnitude
- 	 Pushes tolerance on background calculations
- Neutron backgrounds drive required rad-hardness

## Pair Background (plot from T. Markiewicz)

## Technical Issues

- Tracker material
- Make vertex detector layers as thin as possible to reduce degradation of impact parameter resolution – Probably important
- Minimize material in central tracker too to reduce degradation of momentum resolution
- 	– Desirable, but perhaps not critical
- Reduce secondary backgrounds from machine

## Technical Issues

- Pattern recognition – Vertex Detector
- Want pixellated vertex detector
- 	(CCD vs Active (monolithic/hybrid) Pixels]:
- Reconstruct primary / secondary vertices accurately
- Provide “seed” tracks for central / forward trackers
- CCD’s provide superior spatial resolution, but readout time a problem with Tesla bunch train and expected backgrounds.
- Active pixels fast and radiation-hard, but thick & coarse.

## Technical Issues

- Pattern recognition – Central Tracker
- 3-D vs 2-D technologies:
- Gas: TPC vs DC
- Silicon: Drift vs Microstrips
- 3-D eases reconstruction and improves robustness against backgrounds (SR photons,   jets). May come at higher cost.
- Few precise hits (silicon) vs many coarse hits (gas)
- Effect on 2-track separation?  Energy flow
- Reconstruct long-lived decays?
- Cope with large machine backgrounds?
- Pointing to shower max in calorimeter  Energy flow
- Does pixel vertex detector provide enough “stand-alone” tracking (seeding) to make above choices non-critical?

## Technical Issues

- Intermediate Tracker (needed for gas trackers?)
- Depending on Rmax of Vdet and Rmin of central tracker, a precise silicon layer at gas chamber Rmin improves p by up  to factor of two
- Might help pattern recognition (might hurt!)
- Offers possible bunch tagging via precise timing to disentangle two-photon crud, machine backgrounds (e.g., scintillating fiber)

## Technical Issues

- What about dE/dx?
- Capability “comes for free” in gas chambers, but electronics to exploit it is not free
- Some capability possible with silicon, but useful mainly for tagging very heavy (exotic) particles
- Do we need it?
- Identifying high-energy electrons will be easy, anyway.
- Do we care enough about K/separation to let dE/dx influence tracker design choice?

## Technical Issues

- Mechanical / electronic ramifications of thin silicon
- Ultra-thin CCD’s can be “stretched” to maintain rigidity without support structure
- Mechanical challenge
- Silicon microstrip ladders can be built long to get front-end electronics out of fiducial volume.
- Affects shaping time of electronics, could be a problem in high-background environment

## How do we make choices?

- We need:
- Simulations, Simulations, Simulations!
- 	(fast and full Monte Carlo)
- Detector R&D to ground simulations in reality.
- Will present:
- My (abbreviated) tracking simulations wish list
- 	Note: much work already underway & reported
- Overview of ongoing tracking detector R&D

## A Tracking Simulations Wish List

- _Fast_ Monte Carlo:
- Where do we reach diminishing returns on impact parameter resolution in measuring Higgs charm vs bottom branching ratios? How thin do pixel layers really need to be?
- Where do we reach diminishing returns on momentum resolution in measuring Higgs recoil mass and slepton mass end-point spectra, taking into account particle decay widths, initial state radiation, and beam energy spread?

## A Tracking Simulations Wish List

- _Fast_ Monte Carlo:
- Compelling 500 GeV physics example where material budget in central tracker matters:
- What p/p do we need at 1 GeV? (10-2, 10-3, 10-4)?
- What photon conversion rate is unacceptable? (10%)?
- Compelling 500 GeV physics example where dE/dx buys us much.

## A Tracking Simulations Wish List

- _Full_ Monte Carlo:
- Robust, reasonably optimized track reconstruction for North American LD and SD baseline designs, including:
- Non-cheat reconstruction from hits in Si barrel microstrip option
- Non-cheat reconstruction from hits in Si forward disk microstrips
- Self-contained vertex detector tracking with extrapolation outward
- Comparison of energy flow performance among the 3-D, 2-D, silicon, gaseous options
- 	(e.g., WW vs ZZ all-hadronic final states,
- 	 overlaps with calorimeter wish list!)

## A Tracking Simulations Wish List

- _Full_ Monte Carlo:
- Realistic study of benefits arising in LD design from:
- Intermediate silicon layer just inside the TPC (pat. rec., p/p)
- Intermediate sci-fiber layer in same place (timing)
- Outer “z” (straw/silicon) layer (pointing into calorimeter)
- Outer endcap (straw/silicon) layer (better p/p at low 
- 	(“realism” includes, e.g., systematic alignment errors, backgrounds from multiple bunches, and calorimeter backsplash)

## A Tracking Simulations Wish List

- _Full_ Monte Carlo:
- TPC E-field distortion by ionic space charge
- Proponents confident that new readout schemes (GEM, MicroMEGAS) and gating grid adequately suppress avalanche ion feedback
- Primary ionization said to be okay too for expected machine backgrounds
- What if backgrounds are much worse?
-     (need _really_ full Monte Carlo to study!)

## A Tracking Simulations Wish List

- _Full_ Monte Carlo:
- Wire saturation in drift chamber from larger-than-expected accelerator backgrounds:
- Synchrotron radiation background (1 MeV Compton curlers)
- Muons from beam halo hitting collimators

## Ongoing or Planned R&Dfor Vertex Detector (overview)

- CCD’s
- Europe, North America, Asia
- Hybrid, Monolithic, & DEPFET  Pixels
- Europe

## Ongoing or Planned CCD R&D

- Minimizing material: (JLC, LCFI*, Oregon, Yale)
- Thinner silicon
- Stretched silicon
- Room-temperature operation
- Coping with radiation (JLC, LCFI, Oregon, Yale)
- Manufacture of harder detectors
- Techniques for reducing / coping with damage (charge injection, lower temperature)
- Speed up readout (LCFI, Oregon, Yale)
- Higher clock speed
- Parallel column readout
- Integration
- *LCFI Collaboration: Bristol, Glasgow, Lancaster, Liverpool, Oxford, RAL

## Ongoing or Planned Hybrid Pixel R&D(CERN, Helsinki, INFN, Krakow, Warsaw)

- Reducing total thickness
- Improving spatial resolution
- Smaller pitch
- Interleaved sensors exploiting capacitive induction

## Ongoing or Planned CMOS Pixel R&D(also known as MAPS = Monolithic Active Pixel Sensor) (Strasbourg)

- Development (!)
- Larger wafers
- Thinner substrate
- More integrated readout

## Ongoing or Planned DEPFET* Pixel R&D (MPI)

- Development (!)
- Thinner layer and readout
- Thinner, integrated readout
- Improving spatial resolution (smaller pitch)
- *Similar to MAPS but with high-resistivity silicon, FET in readout chain,
-  readout from sides (for now)

## Ongoing or Planned R&Dfor Central Trackers (overview)

- Time Projection Chamber
- Mostly Europe, some Canada, U.S.
- Concrete design, R&D focused, funded
- Drift chamber
- Mostly Japan
- Concrete design, R&D well focused, funded
- Silicon (drift & microstrip)
- Mostly U.S.
- Competing designs, R&D strapped for funds

## Ongoing or Planned TPC R&D 

- Readout scheme (Aachen, Carleton, DESY, Karlsruhe, LBNL, MIT, MPI, NIKHEF, Novosibirsk, Orsay, Saclay)
- Optimizing spatial resolution for given electronics channel count
- GEM vs MicroMEGAS vs wires
- Suppressing ion feedback (e.g., multi-GEMS, gating grid)
- Readout pad shape (Aachen, Carleton, DESY, LBNL, MPI)
- Affects channel count, intrinsic spatial resolution, 2-track resolution, and dE/dx resolution
- Chevrons (clever splitting/ganging) vs induction
- Gas mixture (DESY, Krakow, MIT, Saclay, Novosibirsk, MPI)
- Drift velocity (resolution vs fast clearing)
- Quenching with hydrocarbons vs reducing neutron backgrounds
- Aging
- Affects field cage design

## Ongoing or Planned TPC R&D 

- Electronics (Carleton, LBNL, NIKHEF, MPI)
- Need O(106) pads to exploit intrinsic x-y TPC granularity
- Need high-speed sampling (~100 MHz) to exploit intrinsic z granularity and dE/dx
- Mechanics (LBNL, MPI)
- Minimize material in inner/outer field cages, endplates
- Eliminating wire readout helps!
- But high-speed sampling may require cooling, despite low duty cycle
- Calibration (LBNL, NIKHEF, MPI)
- Laser system?
- “Z” chamber at outer radius?
- Simulation (Aachen, Carleton, DESY, NIKHEF)
- Readout scheme modelling for design optimization
- Optimizing pad size & shape

## Ongoing or Planned Drift Chamber R&D(KEK)

- Controlling/monitoring wire sag over 4.6 meters
- Uniform spatial resolution (85 microns) over chamber volume
- Good 2-track resolution (<2 mm)
- Stable operation of stereo cells
- Gas gain saturation (affects dE/dx, 2-track resol)
- Lorentz angle effect on cell design
- Wire tension relaxation (Al)
- Optimizing gas mixture
- Neutron backgrounds (planned)

## Ongoing or Planned Silicon R&D

- Thinner silicon strips (LPNHE-Paris, Santa Cruz, SLAC)
- Reduce material of tracker
- Presents support / stabilization challenge
- Short vs long strips (LPNHE-Paris, Santa Cruz, SLAC)
- Short gives timing precision but more FEE in fiducial volume
- Long minimizes material, reduces noise,
- 	but sacrifices timing
- Choice dependent on expected backgrounds

## Ongoing or Planned Silicon R&D

- Barrel/disks support structure (LPNHE-Paris, Santa Cruz, SLAC, Wayne State)
- Want low-mass, stiff support
- ATLAS alignment scheme reduces stiffness demands
- Power-switching strip readout chip (LPNHE-Paris, Santa Cruz, SLAC)
- Exploiting low duty cycle of collider
- Reduce cooling infrastructure material
- Stability?

## Ongoing or Planned Silicon R&D

- Other strip readout issues (LPNHE-Paris, Santa Cruz, SLAC)
- Lorentz angle in high B-field
- p-side readout for “stereo”?
- Time-walk compensation, dE/dx measurement?
- More electronics integration
- Specific Silicon Drift Detector Issues (Wayne State)
- Improve spatial resolution to <10 microns (x-y, r-z)
- Increase drift length
- Low-mass readout for FEE in fiducial volume

## Vertexing 
Tracking 
Simulations 

 Summary:
Much work to be done in detector design optimization
Much work to be done in detector R&D, especially for silicon designs

Help is needed and welcome!

- Tracking
- Simulations
-  Summary:
- Much work to be done in detector design optimization
- Much work to be done in detector R&D, especially for silicon designs
- Help is needed and welcome!

- To learn more about many of these simulation and R&D issues, attend tomorrow afternoon’s parallel sessions on