star tpc luminosity limitations
DESCRIPTION
STAR TPC Luminosity Limitations. Bar Harbor June 2002 Howard Wieman. Outline. Efficiency dependence on luminosity (hit density) Momentum dependence on luminosity (hit density) Space charge distortions Normal collisions (luminosity dependent) Beam gas showers (beam current dependent) - PowerPoint PPT PresentationTRANSCRIPT
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STAR TPC Luminosity Limitations
Bar Harbor
June 2002
Howard Wieman
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Outline
• Efficiency dependence on luminosity (hit density)
• Momentum dependence on luminosity (hit density)
• Space charge distortions– Normal collisions (luminosity dependent)– Beam gas showers (beam current dependent)
• Conclusions
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Method for efficiency estimate as a function of luminosity, i.e. pileup
0 200 400 6000
50
100
effi
cien
cy (
%)
dN
d
Use Bum Choi’s embedding analysis of efficiency for the high Pt paper.
This gives efficiency as a function of track multiplicity.
Estimate pileup track multiplicity as a function of luminosity.
Multiplicities are expressed as dN/d
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Tracking efficiency in central events as a function of luminosity
• Mean dN/ = 164 from high Pt paper
• Time for pileup: 2 x drift, 70 s
• Linear extrapolation0 5 1027 1 1028
0
50
100
effi
cien
cy (
%) 2 1026 80 1026
Luminosity 1
cm2
s
Result: 41% at upgrade luminosity
13% events have < 70 pileup tracks
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Method for Pt resolution estimate as a function of luminosity – effects do to pileup
Use Bum Choi’s embedding analysis of Pt resolution for the high Pt paper.
This gives Pt resolution as a function of track multiplicity.
Use track pile up multiplicity expected for different luminosities
Multiplicities are expressed as dN/d
0 200 400 6000
0.01
0.02
dN
d
Pt
Pt0.013 Pt b
b
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Pt resolution in central events as a function of luminosity
• Mean dN/ = 164 from high Pt paper
• Time for pileup: 2 x drift, 70 s
• Linear extrapolation
Result: Pt/Pt = 7.4% at upgrade luminosity, up from 6.1%
Pt/Pt at Pt = 3 GeV
0 2 1027 4 1027 6 1027 8 10270
10
20
Pt r
esol
utio
n (%
)
2 1026
Luminosity 1
cm2
s
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Space charge distortion – what to expect
VE
VEXB
VE
VEXB
+
DCA
B out ofplaner distortion from
radial E field component and EXB
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Space charge from normal collisions
Positive Ion Density
Q
r (cm) 50 200
z (cm)210
0
+ ion charge density
peak: 3000 +e/cm3
ionization density rate
50 100 150 200
ionization density rate as a function of r
r (cm)
5000 ions/cm3 s
HIJET
1/r2
•Design luminosity: 2 x 1026 1/cm2 s
•Mean dN/d = 400
•dN/d = constant gives uniform ionization in z
•dN/d = constant gives ionization 1/r2
•Ionization density for dN/d = 400 event at inner radius: ~ 4 ion-e pairs/cm3
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space charge error potential
Up
volt
Space charge error potential in the TPC gas volume
Central Membrane
z (cm)
r (c
m)
2 volts
Space charge from normal collisions at design luminosity
Solution for designated charge distribution in a conductive 0 volt box with the STAR field cage geometry
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Calculated distortion from normal collisions (beam axis view)
• Mean dN/d = 400• Design Luminosity –
2 x 1026 (1/cm2 s)• Full drift length• DCA = 700 m• Dunlop DCA = 3 mm
Calculated distortion at design L
0.1 0.05 0 0.050
50
100
150
200
0
Space charge distorted track
Undistorted track Pt =
Circle fit
r (cm)
x (cm)
Apparent DCA700 m
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Calculated distortion at 40 x design L
3 2 1 0 10
50
100
150
200
0
Calculated distortion from normal collisions (beam axis view)
• Average dN/d = 400• 40 x Design
Luminosity – 80 x 1026 (1/cm2 s)
• Full drift length• DCA = 2.7 cm
Space charge distorted track
Undistorted track
Circle fit
r (cm)
x (cm)
Apparent DCA
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r distortion as a function r and z
• 3 methods of calculation– 1/r2 charge distribution, no
end cap coax geometry
– HIJET r dependence, coax
– Full 2D solution
• Note z dependence shows advantage of TPC with shorter drift distance
50 100 150 200800
600
400
200
0
200
400
z (cm)
r-phi
dist
ortio
n (m
icro
ns)
r = 50, 60, 75, 100, 195 cm
r = 50 cm
r = 195 cm
210 z (cm) 0
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Space charge summary
L DCA measured (beam gas)
DCA expected (beam gas)
DCA calculated (normal collisions)
Year 1 ~0.5x1026 3 mm 0.2 mm
Design 2x1026 3 mm 0.7 mm
Upgrade 80x1026 3 mm 27 mm
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Conclusion
• Pt resolution loss is not significant• Tracking efficiency drop to 40% is a problem, but
this is a trade off with efficiency. Efficiency can be increased at the expense of Pt resolution
• Space charge distortion with a DCA = 2.7 cm is a real problem that requires a 100 to 1 correction to reach TPC design specification – but, not as much to be equal to what we have today
• Additional issues to be resolved: wire chamber aging