# Presentation: 357541

## 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

- 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 extrapolation

- 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

- 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%

## Space charge distortion – what to expect

- r distortion from radial E field component and EXB

## Space charge from normal collisions

- r (cm)

- 50

- 200

- z (cm)

- 210

- 0

- + ion charge density

- peak: 3000 +e/cm3

- 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 in the TPC gas volume

- Central Membrane

- z (cm)

- r (cm)

- 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

- Space charge distorted track

- Undistorted track Pt = 

- Circle fit

- r (cm)

- x (cm)

- Apparent
-  DCA
- 700 m

## 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

- 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