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The Q Weak Experiment Event tracking, luminosity monitors, and backgrounds. John Leacock Virginia Tech on behalf of the Q Weak collaboration Hall C Users Meeting 23 January 2010. Q W eak Event Tracking. Why is event tracking needed?. Luminosity monitors. Measure moments of Q 2

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The q weak experiment event tracking luminosity monitors and backgrounds

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


The q weak experiment event tracking luminosity monitors and backgrounds

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


The q weak experiment event tracking luminosity monitors and backgrounds

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.


The q weak experiment event tracking luminosity monitors and backgrounds

Detector Response vs. Position

2.5% shift in acceptance-averaged Q2


The q weak experiment event tracking luminosity monitors and backgrounds

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


The q weak experiment event tracking luminosity monitors and backgrounds

Region I GEMs

  • Gas electron multiplier

  • Registers spatial coordinates of event

  • 100 μm resolution

  • Radiation hard (near target)

  • Louisiana Tech




The q weak experiment event tracking luminosity monitors and backgrounds

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




The q weak experiment event tracking luminosity monitors and backgrounds

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



The q weak experiment event tracking luminosity monitors and backgrounds

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


The q weak experiment event tracking luminosity monitors and backgrounds

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


The q weak experiment event tracking luminosity monitors and backgrounds

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:


The q weak experiment event tracking luminosity monitors and backgrounds

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%