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)

• Conclusions

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

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

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

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

Space charge distortion – what to expect

VE

VEXB

VE

VEXB

+

DCA

B out ofplaner distortion from

radial E field component and EXB

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

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

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

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

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

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

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