The QWeak Experiment Event tracking, luminosity monitors, and backgrounds

John LeacockVirginia Tech

on behalf of the QWeak collaboration

Hall C Users Meeting23 January 2010

QWeak Event Tracking

• 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

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A pWeak

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Why is event tracking needed?

35 cm Liquid Hydrogen Target

Polarized Electron Beam

Collimator With Eight Openings = 9 ± 2°

Toroidal Magnet

Eight Fused Silica (quartz)Cerenkov Detectors

5 inch PMT in Low GainIntegrating Mode on Each

End of Quartz Bar

Elastically Scattered Electrons

325 cm

580 cm

LuninosityMonitor

Region 3Drift Chambers

Region 2Drift Chambers

Region 1GEM Detectors

Luminosity monitors

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.

QWeak Event Tracking

2.5% shift in acceptance-averaged Q2

Detector Response vs. Position

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

Region I GEMs

Gas electron multiplier• Registers spatial coordinates of event• 100 μm resolution• Radiation hard (near target)

Louisiana Tech

Region I GEMs

Region I GEM Rotator

Region II HDCs

Residuals from track reconstruction

Horizontal Drift Chambers• When combined with GEMs gives accurate scattering angle

Virginia Tech

Six layers:X,U,VX’,U’,V’ offset to resolve left right ambiguities

Region II HDCs

Region II HDC Rotator

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

Region III VDC Rotator

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

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

Luminosity monitors: • current mode operation• higher rates than main detectors• quartz Cerenkov radiators• air light guides • PMTs in “unity gain” mode

Luminosity Monitors

Downstream Luminosity Monitors

22.1~70.1~1 2

2

pepe

Excess statistical broadening:

LUMI 1<pe> = 8.8σpe = 6.1

LUMI 2<pe> = 8.9σpe = 5.6

LUMI 3<pe> = 8.4σpe = 5.5

LUMI 4<pe> = 9.2σpe = 5.7

LUMI 5<pe> = 8.4σpe = 5.3

LUMI 6<pe> = 7.9σpe = 5

LUMI 7<pe> = 10.6σpe = 7.6

LUMI 8<pe> = 8σpe = 4.9

BackgroundsTwo 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 ~Z2 asymmetry ~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%

Extra Slides

GEM Hit GUI