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STAR TPC Luminosity Limitations

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)

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STAR TPC Luminosity Limitations

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  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

  3. 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

  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 < 70 pileup tracks

  5. 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

  6. 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%

  7. Space charge distortion – what to expect r distortion from radial E field component and EXB

  8. 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

  9. 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

  10. 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)

  11. 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)

  12. 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

  13. Space charge summary

  14. 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

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