1. Space-point Distortions
Particle Tracking and Identification at High Rates
Friday, 16th December 2016
Seminar - Talk
Yannik Vetter

2. Outline Motivation Motion of charged particles Static Distortions
Misalignment: Field Cage
Misalignment: E x B
Calibration
Dynamic Distortions
Space-Charge
ALICE TPC RUN2
Outlook: RUN3 GEM Update
Summary
Yannik Vetter Space-Point Distortions

3. Motivation reconstruct actual track identify particles (p, dE/dx,…)
highest possible resolution
several sources that distort path of ionization electrons
definition of distorting effects
correction necessary
‘cross-check’ with e.g. ITS and TRD tracks
Yannik Vetter Space-Point Distortions

4. Outline Motivation Motion of charged particles Static Distortions
Misalignment: Field Cage
Misalignment: E x B
Calibration
Dynamic Distortions
Space-Charge
ALICE TPC RUN2
Outlook: RUN3 GEM Update
Summary
Yannik Vetter Space-Point Distortions

5. Charged particle in Fields
Langevin-Equation:
with static solution:
three directional dependencies
ions move much slower than electrons:
 Electrons cleared from volume much faster than ions
 Ions create so called space charges
Yannik Vetter Space-Point Distortions

6. Outline Motivation Motion of charged particles Static Distortions
Misalignment: Field Cage
Misalignment: E x B
Calibration
Dynamic Distortions
Space-Charge
ALICE TPC RUN2
Outlook: RUN3 GEM Update
Summary
Yannik Vetter Space-Point Distortions

7. Static Distortions Misalignment: Field Cage
E-Field pointing towards central electrode
simplest analogy: plate capacitor
field cage to prevent inhomogeities at boundary
resistor rods and strips to correct these
last strip left out
<<<<<<<< WHY??? >>>>>>>>>>> [
Sketch: D.Vranic
Yannik Vetter Space-Point Distortions

8. Static Distortions Misalignment: Field Cage
need very precise resistor values for best result
deviations result in E-field distortion, especially at very low/high radii
distortions up to 6 mm due misalignment
E-field main component misalignment up to 2 mm distortions
also: misalignment of central electrode, ROCs and resistor rods
misalignment of 0.1 mm results in distortion of up to 6 mm
What else?
Yannik Vetter Space-Point Distortions
[Rossegger et al.]

9. Static Distortions Misalignment: E x B
(always) and components
E x B – term in static solution of EoM ≠ 0 
Rossegger et al.: 2nd order solution of the Langevin-Equation
expanding to only 2nd order terms
x- and y- velocities strongly suppressed, but not 0!
Rossegger et al.
Yannik Vetter Space-Point Distortions

10. Static Distortions Misalignment: E x B
z – component; with v0 nominal drift velocity at nominal field E0:
resulting distortions after integration of ux,y,z:
plug in equations above to get dependence on E and B
distortion of 1 cm in 2nd order known to 4 µm < resolution TPC
Delta_z??? How comes the form?
Yannik Vetter Space-Point Distortions

11. Static Distortions Misalignment: E x B
E x B biggest distortion effects, O(cm)
B-field inhom. up to 8 mm distortions
B-field main component misalignment up to 2 mm distortions
Yannik Vetter Space-Point Distortions

12. Static Distortions Misalignment: E x B
E x B corrections for B inhom. B = 0.5 T FC misalignment correction
Yannik Vetter Space-Point Distortions

13. Static Distortions Calibration
assume distortion transformations to commute and approximate as linear combination of partial distortions:
BUT: cannot observe distortions!
Define observables:
RUN1: ITS+TPC or TPC+TRD or TPC+primary vertex
RUN2+3: already have space charge and distortion maps, can use known calibration
Partial distortions, like E x B, misalignment etc.
Yannik Vetter Space-Point Distortions

14. Static Distortions Calibration
Partial distortions, like E x B, misalignment etc.
Yannik Vetter Space-Point Distortions

15. Static Distortions Calibration
Partial distortions, like E x B, misalignment etc.
Yannik Vetter Space-Point Distortions

16. Outline Motivation Motion of charged particles Static Distortions
Misalignment: Field Cage
Misalignment: E x B
Calibration
Dynamic Distortions
Space-Charge
ALICE TPC RUN2
Outlook: RUN3 GEM Update
Summary
Yannik Vetter Space-Point Distortions

17. Dynamic Distortions Space-Charge
accumulation of charges in space
TPC: clusters of ions from tracks (low effect), backflowing ions from readout planes (IBF, MWPC ~0, GEM ~1%)
varying over time  dynamic distortion
depending on interaction rate (track number)
parametrized by charge density
Laplace equation yields resulting E-field
obtain distortions with Langevin-equation
Yannik Vetter Space-Point Distortions

18. Dynamic Distortions Space-Charge
Low z and low r  high track number
Yannik Vetter Space-Point Distortions

19. Dynamic Distortions Space-Charge
hot spots of space charges
mostly in between the sector boundaries
resulting E-field distortion has focussing effect
Better picture!!!
Yannik Vetter Space-Point Distortions

20. Dynamic Distortions Space-Charge
Central Electrode
ROCs
Yannik Vetter Space-Point Distortions

21. Dynamic Distortions Space-Charge
Residuals,
Yannik Vetter Space-Point Distortions

22. Dynamic Distortions Alice tpc Run2
Measurement of space charges:
use data from ITS, TRD, (TOF)
compare tracks with TPC tracks
differences into distortion map
Space charge fluctuations observed
dominant term for F << 2 and F >> 2
Ntracks number of tracks; NQ charge for one track; F fraction where Q is deposited
See other slides! Step by step correction; add fluctuation picture
Yannik Vetter Space-Point Distortions

23. Dynamic Distortions Alice tpc Run2
track with default dist. map
match to ITS, TRD, TOF
refit latter part and interpolate to TPC (reference)
determine differences between distorted and reference
3D vector of distortion in each voxel
smooth parametrization; time dependence, redo every ~40 min
See other slides! Step by step correction; add fluctuation picture
Yannik Vetter Space-Point Distortions

24. Dynamic Distortions Alice tpc Run2
big change in distortions due to change of gas mixture after RUN1
Local distortions factor 10 – 20 higher in RUN2, factor 2 expected
dependence on interaction rate: non-linear, seems to saturate
distortions depend on the orientation of the B-field (++, --)
See other slides! Step by step correction; add fluctuation picture
Yannik Vetter Space-Point Distortions

25. Dynamic Distortions Alice tpc Run2
1 rel. IR = 5 kHz
Yannik Vetter Space-Point Distortions

26. Dynamic Distortions Alice tpc Run2
positive B-field
negative B-field
Yannik Vetter Space-Point Distortions

27. Outline Motivation Motion of charged particles Static Distortions
Misalignment: Field Cage
Misalignment: E x B
Calibration
Dynamic Distortions
Space-Charge
ALICE TPC RUN2
Outlook: RUN3 GEM Update
Summary
Yannik Vetter Space-Point Distortions

28. Outlook: RUN3 Gem update
ion backflow ~1% (MWPC ~0): GEMs are not opaque for ions like GG
much higher IR
new calibration necessary:
we measure I(x,y,t)
density maps become time dependent in RUN3
otherwise calibration is same as in previous run
Include calibration with epsilon(x,y,t), see TDR
Yannik Vetter Space-Point Distortions

29. Outlook: RUN3 Gem update
Partial distortions, like E x B, misalignment etc.
space charge density maps for different occupancies, normalized to 10 events, z = 10 cm
Yannik Vetter Space-Point Distortions

30. Outlook: RUN3 Gem update
Partial distortions, like E x B, misalignment etc.
distortion map for B = 0 (left) and B = 0.5 T (right); symmetry broken due E x B; RUN3 simulation
Yannik Vetter Space-Point Distortions

31. Outline Motivation Motion of charged particles Static Distortions
Misalignment: Field Cage
Misalignment: E x B
Calibration
Dynamic Distortions
Space-Charge
ALICE TPC RUN2
Outlook: RUN3 GEM Update
Summary
Yannik Vetter Space-Point Distortions

32. Summary
Yannik Vetter Space-Point Distortions