Plans & Prospects for W Physics with STAR

1. Plans & Prospects for W Physics with STAR Frank Simon, MIT for the STAR Collaboration Parity Violating Spin Asymmetries at RHIC, BNL, April 27, 2007

2. Outline • STAR: Present Capabilities • W Production and Detection • Electron ID in the Calorimeter • Forward Tracking Upgrade: The Forward GEM Tracker • Simulations: • tracking and charge sign reconstruction efficiency • influence of vertex distribution • Requirements for Forward Tracking Technology • GEM Trackers • Technology • COMPASS Experience • STAR R&D • Summary

3. The STAR Experiment Central Tracking • Large-volume TPC • || < 1.3 Calorimetry • Barrel EMC (Pb/Scintilator) • || < 1.0 • Shower-Maximum Detector • Pre-Shower Detector • Endcap EMC (Pb/Scintilator) • 1.0 <  < 2.0 • Shower-Maximum Detector • Pre- and Post-Shower Detectors 2005 run … and many other detectors not discussed here

4. W Kinematics at RHIC • large x accessible at manageable rapidities!

5. ≈Δu/u ≈Δd/d W Production: What Asymmetries do we expect? • Largest sensitivity at forward rapidity, in particular for W- ≈Δd/d ≈Δu/u

6. Forward W production: Leptonic Signals • W production is detected through high pT electrons / positrons • Rapidity cut on electron reduces the pT: pT(lepton) = MW/2 x sin*

7. W Decay Kinematics • Partonic kinematics related to W rapidity: • W rapidity related to lepton rapidity: • lepton rapidity determined from pt:

8. W Production in STAR • 400 pb-1 will result in 47 (12)k W+(-) events • Every event counts, certainly for W-!

9. A W event in STAR • Charged tracks at mid-rapidity to reconstruct the primary event vertex • outgoing electron tends to be isolated e

10. Backgrounds • Simulations for PHENIX geometry at mid-rapidity, also applicable for STAR • Dominating QCD charged hadron background • clean electron / hadron separation mandatory

11. Electron/Hadron Separation in EEMC electron + • Difference in Shower Shape can be exploited to reject hadrons

12. Electron/Hadron Separation • EEMC provides a wealth of shower shape information • Hadrons have different longitudinal profile than electrons • high rejection power! Additional separation cuts: • E/p (especially at mid-rapidity) • isolation • large missing pt Preshower 1 Preshower 2 SMD 2 SMD 1 Tower Postshower

13. Effectiveness of cuts • Isolation cut R = 0.26 • Large missing pt • Together ~ x100 reduction of charged hadrons, only small reduction of signal

14. Forward Tracking: The Challenge • To provide charge identification at forward rapidity the sign of the curvature of tracks with a sagitta of less than 0.5 mm has to be correctly identified • Presently not possible in STAR! simulated electrons: 1 <  < 2, 5 GeV/c < pT < 40 GeV/c, flat distributions

15. Forward Tracking: Baseline Design I Forward Tracking Inner Tracking

16. Forward Tracking: Baseline Design II • 6 triple-GEM disks covering 1 <  < 2 • outer radius ~ 43 cm • inner radius varies with z position • size and locations driven by the desire to provide tracking over the full extend of the interaction diamond (±30 cm)

17. Forward Tracking Simulations • Simulations used to investigate: • Capabilities: • tracking efficiency • charge sign reconstruction efficiency • acceptance of vertex distribution • Detector configurations: • currently existing STAR Detector • baseline design: 6 triple-GEM disks • Resolution requirements • beam line constraint sufficient as transverse position of the primary vertexassumed resolution 200 µm (200 GeV: 250 µm, transverse size scales with √E) • constraints on the spatial resolution of the chosen detector technology • Simulation Procedure: • single electrons, pT = 30 GeV/c, 1 <  < 2, vertex positions at -30 cm, 0 cm, +30 cm • Full GEANT simulations with STAR detector • smearing of the hits in each detector by the respective resolution • reconstruction with helix fit (2 stage: circle fit in x,y; straight line fit in r,z)

18. Hit distribution vs  vtx z =-30 cm vtx z =0 cm vtx z =+30 cm EEMC SMD TPC ≥ 5 hits FGT SSD+IST vertex • Position of the primary vertex determines which detectors see tracks at a given

19. Simulations: Present Capabilities • Spatial resolution of the EEMC SMD: ~1.5 mm • Charge sign reconstruction impossible beyond  = ~1.3 TPC + EEMC SMD TPC Only

20. Simulations: Baseline Design • 6 triple-GEM disks, assumed spatial resolution 60 µm in x and y • charge sign reconstruction probability above 80% for 30 GeV pT over the full acceptance of the EEMC for the full vertex spread, >90% out to  = 1.8 • the addition of two high-resolution silicon disks does not provide significant improvement and is thus not considered further • 4 GEM disks might be sufficient, but the added redundancy of 6 disks comes at low cost

21. 100 µm 120 µm 80 µm Simulations: How Critical is Spatial Resolution? • Simulations with different spatial resolutions for the triple GEM disks: • 80 µm, 100 µm, 120 µm • Charge Sign resolution deteriorates with decreasing resolution • 80 µm spatial resolution is certainly sufficient, 100 µm might also do

22. Technology Requirements • Spatial resolution ~80 µm (or better) • High intrinsic speed: Discrimination of individual bunch crossings mandatory for the Spin program (107 ns) • Rate capability: Detector upgrade has to be able to handle RHIC II luminosities ( 4 x 1032 cm-2s-1 at 500 GeV p+p) • Low cost to cover larger areas (~ 3 m2) • GEM Technology a natural choice

23. GEM: Gas Electron Multiplier Metal-clad insulator foil with regular hole pattern • Hole Pitch 140 µm • Outer diameter ~70 µm, Inner diameter ~60 µm • Voltage difference between foil sides leads to strong electric field in the holes  Electron avalanche multiplication F.Sauli, 1997

24. GEM Detector Principles • Amplification stage separated from readout: Reduced risk of damage to readout strips or electronics • Readout on ground potential • Fast signal: Only electrons are collected • Intrinsic ion feedback suppression • Several foils can be cascaded to reach higher gains in stable operation • typical choice for MIP tracking: triple GEM • Many different readout designs possible (1D strips, 2D strips, pads, …)

25. GEM Trackers: First Large-Scale Use: COMPASS • Mechanical stability provided by honeycomb plates • average material budget 0.71 % radiation length • reduced material in the center (where the beam passes through) ~ 0.42 X0 • 2D orthogonal strip readout Small angle tracker uses GEMs • Triple GEM design, low mass construction, 30 cm x 30 cm active area

26. COMPASS: Readout: Cluster Size • 400 µm strip pitch chosen to get good spatial resolution while keeping number of channels reasonable

27. COMPASS Trackers: Efficiency • Efficiency for space points ~ 97.5% (stays above 95% for intensities of 4 x 107+/s, at rates of up to 25 kHz/mm2) 2D Efficiency • uniform efficiency over detector area (no effects from particle density) • local reductions in efficiency due to spacer grid

28. COMPASS Trackers: Resolutions • spatial resolution ~ 70 µm in high intensity environment with COMPASS track reconstruction • 50 µm demonstrated in test beams • time resolution ~ 12 ns (convolution of intrinsic detector resolution and 25 ns sampling of APV25)

29. Establishing a Commercial Source • Currently CERN is the most reliable supplier of GEM foils • Essentially a R&D Lab, not well suited for mass production: quite high price, limited production capability • Small Business Innovative Research: Funded by DOE • Phase I: Explore feasibility of innovative concepts with an award of up to $100k • Phase II: Principal R&D Effort with award of up to $750k • Phase III: Commercial application • Collaborative effort of Tech-Etch with BNL, MIT, Yale • Development of an optimized production process • Investigation of a variety of materials • Study post-production handling (cleaning, surface treatment, storage…) • Critical Performance Parameters • Achievable gain, gain uniformity & stability • Energy resolution • SBIR Phase II approved, $750k awarded

30. Testing of Foils at MIT: Optical Scanning • Electrical tests • Foils are required to have a high resistance (>> 1 G) • GEM foils are tested in nitrogen up to 600 V : no breakdowns • Optical tests • 2D moving table, CCD camera, fully automated, developed at MIT • Scan GEM foils to measure hole diameter (inner and outer) • Check for defects • missing holes • enlarged holes • dirt in holes • etching defects U. Becker, B. Tamm, S.Hertel (MIT)

31. Optical Scanning: Hole Parameters • Geometrical parameters are similar for foils made at Tech-Etch and foils made at CERN Tech-Etch CERN

32. Optical Scanning: Homogeneity • Homogeneity for CERN and TE foils similar Outer holes Inner holes Tech-Etch CERN

33. Triple-GEM Test Detector at MIT Components: 1. 2D readout board (laser etched micro-machined PCB) 3. Bottom Al support plate 4. Top spacer (G10): 2.38mm 5. Bottom spacer (G10) 6. plexiglass gas seal frame 7. Top Al support cover 8. GEM 1&2 frames (G10): 2.38mm 9. GEM 3 frame (G10): 3.18mm 10. Drift frame (G10) Detector constructed to allow rapid changes of foils, readout board and other components, not optimized for low mass Detector operated with Ar:CO2 (70:30) gas mixture

34. 55Fe Tests • Triple GEM test detectors are tested with a low intensity 55Fe source (main line at 5.9 keV) • Both Detectors (based on CERN and on Tech-Etch foils) show similar spectral quality and energy resolution (~20% FWHM of the Photo Peak divided by peak position) CERN TechEtch

35. Gain Uniformity CERN • Good uniformity of the gain (measured after charging up of the detectors) for both the CERN foil based and the TE foil based detector RMS = 0.064 TechEtch RMS = 0.077

36. Electronics & Data Acquisition • Detector electronics based on APV25S1 front-end chip • Front-end chips and control unit designed and available, undergoing tests • Proof of principle with the full STAR trigger and DAQ chain APV chip & front-end board Test Interface Control Unit (programmable FPGAs)  Beam test with full electronics & 3 test detectors starting at FNAL next week!

37. Electronics Test with RPC • First tests at ANL with a RPC on top of the test detector readout board • Induced signals (GEM: electron collection) => Very wide signals • Very high amplitudes (RPCs in avalanche mode, signals typically 0.2 to 2 pC (GEM: ~10 fC) Typical Signal in RPC

38. Towards a “real” detector • Development of a low mass prototype • use of low mass materials, e.g. carbon foam or honeycomb for mechanical structure, thin readout board,… • Disk design: similar to the one used by the TOTEM experiment at LHC (forward region of CMS) • FGT significantly larger than the TOTEM detectors • Tech-Etch can provide GEM foils at least 40 cm x 40 cm • build the detector from 90° quarter sections • 12 GEM foils per detector disk needed (get at least 24 to be safe) • total number of foils ~200 including some spare detector modules

39. Towards a “real” detector II • Readout Geometry: Currently under investigation, for example 2D strips (as in COMPASS) • strip pitch ~ 400 µm • shorter strips at inner radius to allow for high occupancy • challenge to produce, investigating with company • ~50 k to 70 k channels total • ~400 to 550 APV chips total

40. Mechanical Design: Support Structure

41. Construction Schedule • Design phase (Support structure / Triple-GEM chambers):12 weeks • Procurement of material:6 weeks • Construction of detector quarter sections:18 weeks • Delivery of 10 GEM foils from Tech-Etch per week • Test of GEM foils (Electrical tests, optical scan on flatbed scanner): 0.5 week • Test of readout board (Parallel to GEM foil tests): 0.5 week • Construction of GEM detectors: Mechanical assembly, foil mounting, testing between each gluing step: 2 weeks • Test of assembled chamber: Gas tightness, X-ray test, Gain map: 2 weeks • Estimated total construction of one quarter section: 5 weeks • Assume: 2 detectors in parallel starting every week • Construction of full system:10 weeks • Assemble 6 disks on support frame from 4 quarter sections each: 1 week • Assemble electrons and test: 2 weeks • Test disk electrons and detectors and full system test (Cosmic ray test): 7 weeks • Installation:3 weeks • Integration: 5 weeks • total construction time: ~54 weeks • Aim for Installation for FY2010 run, total project costs below $2M

42. Institutes on the FGT Project • Argonne National Laboratory • Indiana University Cyclotron Facility • Kentucky University • Lawrence Berkeley National Laboratory • Massachusetts Institute of Technology • Valparaiso University • Yale University

43. Summary • STAR is in a good position to make competitive W measurements • Forward Tracking Upgrade is needed to ensure charge sign identification for high pT electrons from W decays in the forward region • Baseline design: 6 triple-GEM tracker disks • cover the region 1 <  < 2 for vertex distributions of ±30 cm • Extensive simulations with GEANT modeling of the detector • spatial resolution of ~80 µm necessary • GEM technology satisfies the requirements of forward tracking in STAR • R&D Effort currently under way to establish commercial GEM foil production • Phase II of a funded SBIR proposal, collaboration of Tech-Etch, BNL, MIT, Yale • Promising results with detector prototypes • First successful tests with APV25 electronics and DAQ integration, Beam test at FNAL coming up • Design effort for final disk configuration • low mass materials • large area GEM foils • specialized readout geometry