1. 1 J.M. Heuser et al. CBM Silicon Tracker Requirements for the Silicon Tracking System of CBM Johann M. Heuser, M. Deveaux (GSI) C. Müntz, J. Stroth (University of Frankfurt) for the CBM Collaboration 10 th Semiconductor Detector Symposium, Wildbad Kreuth, June 2005 Overview: The future accelerator facility FAIR in Darmstadt The CBM experiment: Physics Motivation Detector Concept Silicon Tracker – Physics Requirements and Impact on the Design

2. 2 J.M. Heuser et al. CBM Silicon Tracker Facility for Antiproton and Ion Research Future accelerator complex FAIR at GSI, Darmstadt: Research program includes: Radioactive Ion beams: Structure of nuclei far from stability Anti-proton beams: hadron spectroscopy, anti hydrogen Ion and laser induced plasmas: High energy density in matter High-energy nuclear collisions: Strongly interacting matter at high baryon densities SIS 100 Tm SIS 300 Tm U: 35 AGeV p: 90 GeV Compressed Baryonic Matter Experiment

3. 3 J.M. Heuser et al. CBM Silicon Tracker Facility for Antiproton and Ion Research Photomontage of the existing and the planned research facility at GSI/FAIR.

4. 4 J.M. Heuser et al. CBM Silicon Tracker Strong-interaction physics: confinement, broken chiral symmetry, hadron masses. CERN-SPS and RHIC: indications for a new state of matter: „Quark Gluon Plasma“. Produced at high T and low  B. LHC: even higher T, lower  B. QCD phase diagram:  poorly known at low T, high  B :  new measurements at FAIR: with highest baryon densities! CBM Physics Motivation SIS100/300  CBM Experiment

5. 5 J.M. Heuser et al. CBM Silicon Tracker Physics and Observables Transition of normal nuclear matter to a deconfined quark gluon plasma. PhysicsObservables In-medium modifications of hadrons: onset of chiral symmetry restoration , ,   e + e - (μ + μ - ) open charm: D 0, D ± Strangeness in matter: enhanced strangeness production K, , , ,  Indications for deconfinement: anomalous charmonium suppression ? D 0, D ±, J/   e + e - (μ + μ - ) Critical point: event-by-event fluctuations π, K Open charm measurement: one of the prime interests of CBM, and one of the most difficult tasks.

6. 6 J.M. Heuser et al. CBM Silicon Tracker The CBM Experiment - Conceptional Design - Tracking, momentum measurement, vertex reconstruction: Exclusively with a Silicon Tracking System (STS) Electron ID: RICH & TRD (& ECAL) Hadron ID: TOF (& RICH) Photons,  0,  : ECAL High interaction rates beam target STS (5, 10, 20, 40, 60, 80, 100 cm) TRDs (4,6, 8 m) TOF (10 m) ECAL (12 m) RICH magnet

7. 7 J.M. Heuser et al. CBM Silicon Tracker Tracking Nuclear Collisions Central Au+Au collision, 25 AGeV: URQMD + GEANT 160 protons 170 neutrons 360  - 330  + 360  0 41 K + 13 K - 42 K 0 ~500 charged primary particles in acceptance 50-500 mrad Tracking challenge: ~ 1000 charged particles/event momentum measurement with good resolution secondary vertex detection

8. 8 J.M. Heuser et al. CBM Silicon Tracker Open Charm Reconstruction D0→KD0→K Some hadronic decay modes: D  (c  = 317  m): D +  K -  +  + (9  0.6%) D 0 (c  = 124.4  m): D 0  K -  + (3.9  0.09%)  High-granularity sensors.  Thin tracking stations.  High level charm trigger. target primary tracks

9. 9 J.M. Heuser et al. CBM Silicon Tracker The Silicon Tracking System  Assume 7 planes - 3 or 2 thin pixel planes - 4 or 5 thin strip planes  Acceptance: 50 to 500 mrad  First plane: z=5cm ; size 25 cm 2 covers 100 to 500 mrad  Last plane: z=100cm; size ~ 1 m 2  Magnetic dipole field: ~ 1Tm,  p/p <1% @ p=1 GeV vacuum 1 7 3 4 vertexing tra c king 2 5 6 pixel detectors strip detectors z = 5,10,(20) cm z = (20),40,60,80,100 cm Conceptional geometry: – mostly guided by open charm needs. – focus on the pixel planes for secondary vertex detection – tracking

10. 10 J.M. Heuser et al. CBM Silicon Tracker D 0 vertex resolution primary vertex resolution MC Studies on Vertex Finding I. Vassiliev et al., GSI What kind of pixel detectors can do the job? Material budgets, pixel sizes: 10% D impact parameter detection 100 µm thick 750 µm thick  small pixels < 25 x 25 µm 2  ( thin sensors ~ 100 µm )

11. 11 J.M. Heuser et al. CBM Silicon Tracker Studies on D 0 → K - π + reconstruction study by I. Vassiliev, GSI D 0  K -,  + signal Background Reconstructed events Z-vertex (cm) ~10 % efficiency signal

12. 12 J.M. Heuser et al. CBM Silicon Tracker Silicon Detectors Requirements  Small pixels – less than 25 x 25 µm 2  Thin – less than ~100 µm silicon  Radiation hard – > 10 13 n _equiv /cm 2  Fast readout – Interaction rate up to 10 7 /s Such a detector does not exist ! R&D on existing detector types. Two possibilities: Monolithic Active Pixel Sensors: - small pixels: 25 x 25 µm 2 - thin: standard 120 µm, study: 50 µm - achieve spatial resolution: ~3 µm - TOO SLOW for CBM: ~ ms/Mpixel - rad. hard. limited by bulk damage  Improve r/o time, radiation hardness. Hybrid Pixel Detectors: - fast readout, - radiation hard - larger pixels: 50 x ~400 µm 2 - spatial resol.: ~15 (115) µm - thick: standard > 350 µm  Reduce pixel size and thickness. We started persuing the MAPS option.

13. 13 J.M. Heuser et al. CBM Silicon Tracker Pixel Detector Stations Detector module: BTeV inspired design ladders mounted on either side of a substrate providing (active?) cooling. Active cooling support: a carbon fibre structure with micro pipes? ~ 0.3% X 0 glass or silicon wafers with buried micro channels? ~ 0.1-0.3% X 0 CMOS MAPS chips for CBM: - size: ~0.5 x 1 cm 2 - 50% sensor, 50% r/o. - column readout in 5-10 µs CBM MAPS ladders with 4 or 5 chips.

14. 14 J.M. Heuser et al. CBM Silicon Tracker Silicon Strip Detector Stations Basic sensor elements: 200  m thick silicon wafers. double-sided, rad-tolerant. 25  m strip pitch. Inner : 6x4 cm Middle : 6x12 cm Outer :6X20 cm Open questions: strip length, stereo angle (to reduce fake hits) location of read-out (on sensor, all at edge ?) Four detector stations: built from few wafer types

15. 15 J.M. Heuser et al. CBM Silicon Tracker Tracking in the Silicon Strip Stations Cellular automaton/Kalman filter method. First attempts: Problem - High occupancy with many combinatorial hit points. Recent approach: - start with long “stiff” tracks - 4 strip stations + 1 pixel station (“no pile-up”) - attached strips are removed - this cleans up the hit pool, reduces the fake strip combinations. Work in progress. Ideal conditions: full sensor efficiency, no pile-up: ~ 1% “ghost tracks” remain.

16. 16 J.M. Heuser et al. CBM Silicon Tracker Consider more Tracking Redundancy vacuum 1 11 3 vertexing track finding track seeding 4 8 9 larger area ?larger acceptance, faster r/o? Under real conditions: - event pile-up in pixels - fake hits in strips At present: - forward tracking hampered by pile-up - backward tracking hampered by fakes - need for clean track seeds Consider: - more tracking stations - shorter strips – lower occupancy - fast pixels with clean event association

17. 17 J.M. Heuser et al. CBM Silicon Tracker Low Mass Dilepton Spectroscopy If missed...... fake open pair is formed. Signal: vector meson decays , ,   e + e - Background:  0 decay (365/event)  0  e + e -  (1.2%)  0   98.8%) conversion   e + e - Detector requirements: first stations with large acceptance tracking efficiency down to p = 0.1 GeV/c to suppress background detect conversion pairs: → small pixels

18. 18 J.M. Heuser et al. CBM Silicon Tracker Delta Electrons study by I. Vassiliev, GSI Beam ions on target:  produce delta-rays  dominate occupancy when integrated over many events.  high local radiation damage  hits spoil track finding  limits rate capability Only way out: Fast detector readout to avoid electron hit pile-up.  hits in 1st MAPS station: 1000 min. bias URQMD events, Au+Au 25 AGeV.

19. 19 J.M. Heuser et al. CBM Silicon Tracker Particle Fluence – Detector Lifetime Impact on D Meson Measurements Fluence at 1 st MAPS station, z = 5cm, r = 5mm: ~ 10 1 MeV n equiv per event. MAPS rad. tolerance: ~10 12 1 MeV n equiv. ; development goal: 10 13 1 MeV n equiv. MAPS readout in potentially 10  s. No pileup at event rate <10 5 /s. The MAPS lifetime is then about 1  10 12 /10 5 s = 1  10 7 s = 16 weeks. D 0 →Kπ: 4  10 -5  0.038  0.05 = 7.6  10 -8 per event. In MAPS lifetime: 7.6  10 4 D 0. Fluence of 1 MeV n equiv. /cm 2 in 1 st MAPS station at z = 5cm

20. 20 J.M. Heuser et al. CBM Silicon Tracker CBM Silicon Tracker Requirements - Summary - Tracking exclusively performed with the Silicon Tracker: Very important detector, key to the physics of CBM. The Silicon Tracker must deliver high performance: Open charm detection is the bench mark. Pixel planes near the target are of special importance: Thin, small pixels, fast readout, radiation hard. Such detector does not exist. What comes closest ? What R&D? We chose to start investigating MAPS. Alternative ? Discussions on latest detector technologies are very welcome!