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

Plans & Prospects for W Physics with STAR

Frank Simon, MITfor the STAR Collaboration

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

Frank Simon: Plans & Prospects for W Physics at STAR 204/27/2007

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-

≈Δd/d

≈Δu/u

≈Δu/u

≈Δd/d


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+(-) eventsEvery 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 backgroundclean 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

80 µm100 µm120 µm

Charge Sign resolution deteriorates with decreasing resolution80 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

P

PP

D d

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, …)

GEM Detector Principles

Udrift

ΔUGEM

Uup

Ulow

E driftD

E collectionC

readout


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 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 E

fficiency


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 MITComponents:

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 massDetector 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

Documents

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