STAR TPC Luminosity Limitations

1
STAR TPC Luminosity Limitations

• Bar Harbor
• June 2002
• Howard Wieman

2
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
Use Bum Chois 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?
4
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 extrapolation

Result 41 at upgrade luminosity
13 events have lt 70 pileup tracks
5
Method for Pt resolution estimate as a function
of luminosity effects do to pileup
Use Bum Chois 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 Multiplicitie
s are expressed as dN/d?
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
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Space charge distortion what to expect
r? distortion from radial E field component and
EXB
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Space charge from normal collisions
ionization density rate as a function of r

• 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

5000 ions/cm3 s
1/r2
HIJET
r (cm)
ion charge density
peak 3000 e/cm3
210
z (cm)
0
r (cm)
50
200
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Space charge error potential in the TPC gas volume
Solution for designated charge distribution in a
conductive 0 volt box with the STAR field cage
geometry
r (cm)
2 volts
z (cm)
Space charge from normal collisions at design
luminosity
Central Membrane
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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

r (cm)
Circle fit
Space charge distorted track
Apparent DCA 700 ?m
Undistorted track Pt ?
x (cm)
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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

r (cm)
Circle fit
Space charge distorted track
Apparent DCA
Undistorted track
x (cm)
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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

r 195 cm
r 50 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