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 Measure moments of Q ID: 796369
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Slide1
The QWeak Experiment Event tracking, luminosity monitors, and backgrounds
John LeacockVirginia Techon behalf of the QWeak collaborationHall C Users Meeting23 January 2010
Slide2QWeak 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
Why is event tracking needed?
Luminosity monitors
Slide3Two 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
Slide42.5% shift in acceptance-averaged Q
2
Detector Response vs. Position
Slide5Trigger 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
Slide6Region I GEMs
Gas electron multiplier Registers spatial coordinates of event 100 μ
m resolution Radiation hard (near target)Louisiana Tech
Slide7Region I GEMs
Slide8Region I GEM Rotator
Slide9Region II HDCs
Residuals from track reconstructionHorizontal Drift Chambers
When combined with GEMs gives accurate scattering angleVirginia Tech
Six layers:X,U,VX’,U’,V’ offset to resolve left right ambiguities
Slide10Region II HDCs
Slide11Region II HDC Rotator
Slide12Region 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
=223
μ
m
Slide13Region III VDC Rotator
Slide14Focal 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
Slide15Downstream:
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
Slide16Downstream Luminosity Monitors
Excess statistical broadening:
LUMI 1
<
pe
> = 8.8
σ
pe
= 6.1
LUMI 2
<
pe
> = 8.9
σ
pe
= 5.6
LUMI 3
<
pe
> = 8.4
σ
pe
= 5.5
LUMI 4
<
pe
> = 9.2
σ
pe
= 5.7
LUMI 5
<
pe
> = 8.4
σ
pe
= 5.3
LUMI 6
<
pe
> = 7.9
σ
pe
= 5
LUMI 7
<
pe
> = 10.6
σ
pe
= 7.6
LUMI 8
<
pe
> = 8
σ
pe
= 4.9
Slide17BackgroundsTwo background contributions considered here:
Inelastic electronsProblem: 1% of asymmetry weighted signal is inelastic, 10 times the asymmetry of elastic eventsSolution: 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 windowsProblem: Aluminum windows have asymmetry weighted background contribution of 30% (cross section ~Z
2 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
Q
p
Weak
is 0.5%
Slide18Extra Slides
Slide19GEM Hit GUI