JT: 1 The Berkeley Lab STAR TPC Distortions in the Transverse Plane: An Update Jim Thomas
JT: 2The Berkeley Lab
STARA Mnemonic for Listing the Potential Distortions
ZCal
Barrel EM Calorimeter
Endcap Calorimeter
Magnet
Coils
TPC Endcap & MWPC
ZCal
FTPCs
Vertex Position Detectors
Central Trigger Barrel or TOF
Time Projection Chamber
Silicon Vertex Tracker
RICH
Central Membrane Endwheel and PadplaneOuter Field Cage
Inner Field Cage
JT: 3The Berkeley Lab
STARThe List of Distortions in the Transverse Plane
The list can be enumerated by surfaces:
• Outer field cage corrections • Inner field cage corrections• Central membrane corrections• End-wheel and pad-plane corrections • Pad Row 13 corrections and other local electrostatic defects • Rotation and miss-alignment of sectors with respect to their ideal locations• Rotation of either TPC end-wheel with respect to its ideal location
and by volume:
• Space Charge corrections due to charge in the volume of the TPC • Magnetic field corrections due to B fields in the volume of the TPC• Twist of the TPC with respect to the magnetic field axis and/or the measured map• General coordinate transformations
A few additional items are listed for completeness. (These items affect the drift of the electrons in the Z direction but do not strongly affect the distortions in the transverse plane.)
• Gas composition and variations in the drift velocity• Barometric pressure changes and variations in the drift velocity• Pressure variations as a function of height in the TPC• Temperature gradients in the TPC
JT: 6The Berkeley Lab
STARSpaceCharge from (all) events cause distortion
Z / 5 cm
Rad
ius
/ 5 c
m
Z
Radius
Dis
tort
ion
JT: 7The Berkeley Lab
STARModel for the distortions
• Old Model– Beam gas events leave a uniform deposition of
charge in the TPC
– The charge from the collisions is not significant
• New Model– Beam gas events leave a 1/R2 distribution of
charge in the TPC
– The charge from the collisions is not significant in the 2001 data, but will be in the future (?)
JT: 8The Berkeley Lab
STARUniform .vs. 1/R2 Space Charge Distribution
RadialDistortions
Z Z
Radius
RadiusD
isto
rtio
n
Dis
tort
ion
JT: 9The Berkeley Lab
STARTwo sources of SpaceCharge
• Beam gas and other up stream events– not synchronous with our trigger
– Scales with beam intensity (not Luminosity)
– 1/R2 distribution of charge (?)
• The collisions at STAR– synchronous with our trigger
– Scales with Luminosity
– A/R + B/R2 distribution of charge (?)
• In the future, the average Luminosity will go up a factor of 40 but the beam intensity will only go up a factor of 2 to 4
– We have to prepare for a significant increase in space charge due to the collisions in the detector.
We have to be able to distinguish the two sources of distortion
JT: 10The Berkeley Lab
STARA Wide Range of Charge Distributions
1/R3 1/R2 Wieman’s HiJet
1/R Linear 2:1 Linear
These (and other) Distributions are Available in StMagUtilities
Radius
Vo
ltag
e
JT: 11The Berkeley Lab
STAR2 Equations, 2 Unknowns
• We can simultaneously fit the DCAs and match the steering at the RICH due to the beam gas induced space charge
– Choose the right charge distribution 1/R, 1/R2, HiJet, etc.
– Choose the RICH scaler normalization constant
Variations in Charge Shape
-1500.0
-1000.0
-500.0
0.0
500.0
1000.0
1500.0
1 3 4 51/
R 6 10 16
Wiem
an
1/R**2
1/R**3
Shape Parameter
Dis
pla
cem
ent
at R
ICH
in
mic
ron
s
delta Rich
with the DCA held constant
JT: 12The Berkeley Lab
STARThese Items on the List Are Ready to Go
The list can be enumerated by surfaces:
Outer field cage corrections Inner field cage corrections• Central membrane corrections• End-wheel and pad-plane corrections Pad Row 13 corrections and other local electrostatic defects Rotation and miss-alignment of sectors with respect to their ideal locations Rotation of either TPC end-wheel with respect to its ideal location
and by volume:
Space Charge corrections due to charge in the volume of the TPC Magnetic field corrections due to B fields in the volume of the TPC Twist of the TPC with respect to the magnetic field axis and/or the measured map General coordinate transformations
A few additional items are listed for completeness. (And these I don’t know about.)
• Gas composition and variations in the drift velocity• Barometric pressure changes and variations in the drift velocity• Pressure variations as a function of height in the TPC• Temperature gradients in the TPC
JT: 13The Berkeley Lab
STARConclusions
• Tools are available to calculate all known distortions – in the transverse plane
– gain, t0, and drift velocity corrections have not been discussed
• SpaceCharge corrections are significant – especially at 40x <L>
• We need to track the beam current and/or the distribution of charge in the TPC for untriggered events. We will need scalers and diagnostics for each source of charge
– Monitor L– Monitor beam current
• The RICH Mult Scaler is gone and we need a replacement
• Recent progress with the laser cluster finder means we might be able to use this data …
JT: 14The Berkeley Lab
STARDCA, Steering at the RICH, and p
• The RICH Scaler has an arbitrary normilization
• The DCA’s have been tuned to be the same in both cases
• p is different by a factor of 2
• Steering at the Rich changes sign under these conditions
Uniform Charge Distribution (0.004)
-2500.0
-2000.0
-1500.0
-1000.0
-500.0
0.0
500.0
0 50 100 150 200 250
Z axis
Dis
pla
cem
ent
(mic
ron
s o
r M
eV)
DCA
delta Rich
delta p (x10)
Event Charge Distribution (0.0021)
-2500.0
-2000.0
-1500.0
-1000.0
-500.0
0.0
500.0
1000.0
0 50 100 150 200 250
Z axis
Dis
pla
cem
ent
(mic
ron
s o
r M
eV)
DCA
delta Rich
delta p (x10)