Date post: | 02-Jan-2016 |
Category: |
Documents |
Upload: | dean-jordan |
View: | 43 times |
Download: | 4 times |
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