# Presentation: 889947

## Plans & Prospects for W Physics with STAR

- Frank Simon, MIT
- for the STAR Collaboration

- Parity Violating Spin Asymmetries at RHIC, BNL, April 27, 2007

## 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

## 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

## W Kinematics at RHIC

- large x accessible at manageable rapidities!

## W Production: What Asymmetries do we expect?

- Largest sensitivity at forward rapidity, in particular for W-

- ≈Δ**d/d**

- ≈Δ**u/u**

## 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*

## W Decay Kinematics

- Partonic kinematics related to W rapidity:
- W rapidity related to lepton rapidity:
- lepton rapidity determined from pt:

## W Production in STAR

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

## A W event in STAR

- Charged tracks at mid-rapidity to reconstruct the primary event vertex
- outgoing electron tends to be isolated

**e**

## Backgrounds

- Simulations for PHENIX geometry at mid-rapidity, also applicable for STAR

- Dominating QCD charged hadron background
- clean electron / hadron separation mandatory

## Electron/Hadron Separation in EEMC

- electron

- +

- Difference in Shower Shape can be exploited to reject hadrons

## 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 1

- SMD 2

- Tower

- Postshower

## Effectiveness of cuts

- Isolation cut R = 0.26
- Large missing pt
- Together ~ x100 reduction of charged hadrons, only small reduction of signal

## 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

## Forward Tracking: Baseline Design I

- Inner Tracking

- Forward Tracking

## 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)

## 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)

## Hit distribution vs 

- Position of the primary vertex determines which detectors see tracks at a given

- TPC ≥ 5 hits

- SSD+IST

- EEMC SMD

- vertex

- FGT

- vtx z =-30 cm

- vtx z =0 cm

- vtx z =+30 cm

## Simulations: Present Capabilities 

- Spatial resolution of the EEMC SMD: ~1.5 mm
- Charge sign reconstruction impossible beyond  = ~1.3

- TPC Only

- TPC + EEMC SMD

## 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

## 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

## 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

## 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

## 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, ...)

- 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, ...)

- GEM Detector Principles

## 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

## COMPASS: Readout: Cluster Size

- 400 μm strip pitch chosen to get good spatial resolution while keeping number of channels reasonable

## COMPASS Trackers: Efficiency

- Efficiency for space points ~ 97.5% (stays_x0010_  above 95% for intensities of 4 x 107 +/s, at rates of up to 25 kHz/mm2)

- uniform efficiency over detector area (no effects from particle density)
- local reductions in efficiency due to spacer grid

**2D Efficiency**

## COMPASS Trackers: Resolutions

- time resolution ~ 12 ns (convolution of intrinsic detector resolution and 25 ns sampling of APV25)

- spatial resolution ~ 70 μm in high intensity environment with COMPASS track reconstruction
- 50 μm demonstrated in test beams

## 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

## Testing of Foils at MIT: Optical Scanning

- 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

- 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

- U. Becker, B. Tamm, S.Hertel (MIT)

## Optical Scanning: Hole Parameters

- Geometrical parameters are similar for foils made at Tech-Etch and foils made at CERN

- CERN

- Tech-Etch

## Optical Scanning: Homogeneity

- Outer holes

- Inner holes

- Tech-Etch

- CERN

- Homogeneity for CERN and TE foils similar

## 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

## 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

## Gain Uniformity

- 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

- RMS = 0.077

- CERN

- TechEtch

## 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

- Control Unit (programmable FPGAs)

- Test Interface

-  Beam test with full electronics & 3 test detectors starting at FNAL next week!

## 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

## 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

## 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

## Mechanical Design: Support Structure

## 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

## 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

## 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