The Q Weak Experiment Event tracking, luminosity monitors, and backgrounds

1. The QWeak Experiment Event tracking, luminosity monitors, and backgrounds John Leacock Virginia Tech on behalf of the QWeak collaboration Hall C Users Meeting 23 January 2010

2. QWeak Event Tracking Why is event tracking needed? Luminosity monitors • Measure moments of Q2 • Determine main detector light response vs. angle and position • Sanity check on collimators and magnetic field • (Limited) Diagnostics on background origins • Radiative tail shape (benchmark simulation, E loss) • 0.5% measurement of Q2

3. QWeak Event Tracking Two opposing octants instrumented, rotator system for each region to cover all octants and to move to “parked” position for asymmetry measurement. Periodic tracking measurements at sub-nA beam current.

4. Detector Response vs. Position 2.5% shift in acceptance-averaged Q2

5. Trigger Scintillators • Located just in front of the main detector • Must have a fast response • Veto neutrals and have enough resolution to identify multiparticle events GWU

6. Region I GEMs • Gas electron multiplier • Registers spatial coordinates of event • 100 μm resolution • Radiation hard (near target) • Louisiana Tech

7. Region I GEMs

8. Region I GEM Rotator

9. Region II HDCs • Horizontal Drift Chambers • When combined with GEMs gives accurate scattering angle • Virginia Tech Residuals from track reconstruction Six layers: X,U,V X’,U’,V’ offset to resolve left right ambiguities

10. Region II HDCs

11. Region II HDC Rotator

12. Region III VDCs • Vertical Drift Chambers • Located after magnet • When combined with Region I+II and knowledge of magnetic field gives momentum of particle • William and Mary σ =223μm

13. Region III VDC Rotator

14. Focal Plane Scanner • Measures rates just behind the detector • Tracking will be inoperable at high current • Used to compare rates between low and high current • Has a small active area so it can be used in low and high current runs Scanner system on bottom octant

15. Luminosity Monitors • Luminosity monitors: • current mode operation • higher rates than main detectors • quartz Cerenkov radiators • air light guides • PMTs in “unity gain” mode • Downstream: • 8 detectors@  ~ 0.55° • 100 GHz / det • null asymmetry monitor • Upstream: 4 detectors @  ~ 5° • 130 GHz / detector • mainly detects Moller e- • target density monitor • insensitive to beam angle, energy changes

16. Downstream Luminosity Monitors LUMI 2 <pe> = 8.9 σpe = 5.6 LUMI 1 <pe> = 8.8 σpe = 6.1 LUMI 4 <pe> = 9.2 σpe = 5.7 LUMI 3 <pe> = 8.4 σpe = 5.5 LUMI 5 <pe> = 8.4 σpe = 5.3 LUMI 6 <pe> = 7.9 σpe = 5 LUMI 7 <pe> = 10.6 σpe = 7.6 LUMI 8 <pe> = 8 σpe = 4.9 Excess statistical broadening:

17. Backgrounds Two background contributions considered here: Inelastic electrons Problem: 1% of asymmetry weighted signal is inelastic, 10 times the asymmetry of elastic events Solution: Decrease magnetic field by 25% to focus inelastic peak on to the main detector. 30% of signal will be inelastic for a much quicker measurement Electrons that scatter off the target windows Problem: Aluminum windows have asymmetry weighted background contribution of 30% (cross section ~Z2asymmetry ~8 times) Solution: Use a thick aluminum dummy target at the upstream and downstream positions of the target windows to measure the asymmetry from the aluminum Goal for the contribution of the background error to the final error on QpWeak is 0.5%

18. Extra Slides

19. GEM Hit GUI