2. gamma ray polarisation measurement
Context
gamma ray polarisation measurement
correct systematic bias in angular distribution
Description of the simulation
pair conversion generator, Geant4 model, custom TPC simulation
Calibration and validation
data sample: cosmic rays
validation of all simulation parameters
First results on gamma-ray data
Conclusion

3. Polarisation measurementϕ
The azimuthal angle ϕ is related to the polarisation direction ϕ0
𝑑Γ 𝑑φ ∝1+𝐴𝑃cos 2 φ− φ 0
A: polarisation asymmetry
P: polarisation fraction

4. Polarisation Beam data (2014) Large systematic bias cubic geometry
fixed orientation
Cancel systematic bias by taking the ratio between P=100% and P=0%

5. Azimuthal angle

6. Need for Simulation Complex systematic bias
reconstruction efficiency (especially low opening angles)
effect of pre-amplifier saturation in beam data
Very accurate simulation is needed
exact geometry
realistic signal shape
electronics saturation
Also...
track resolution, efficiency, ...

7. Detector simulation Electron and positron 4-vectors from generator
Track propagation and gas ionisation with Geant4
Custom simulation of TPC
drift, diffusion, electron capture, gain fluctuations, electronics response
output same format as HARPO data, identical analysis

8. TPC simulation Ionisation electrons treated individually
Fixed drift velocity
e- life time (exponential decay)
Gaussian diffusion
Gain fluctuations: Polya
Gaussian space response (~diffusion between GEMs)
AFTER time response
Electronics noise and saturation

9. Calibration/Validation with cosmic rays
Using cosmic ray data
Selection of low angle straight tracks
peak of the angular distribution of cosmic rays
close to drift direction => good profile of the TPC
cut on very low angles (saturation problem)
Simulation
5GeV muons
flat angular and position distributions

10. Calibration: Vdrift
Tdrift=FWHM
Vdrift=LTPC/Tdrift

11. Calibration: Cdiff
σ 2 = σ 𝐶 𝑑𝑖𝑓𝑓 2 𝑧 𝑣 𝑑𝑟𝑖𝑓𝑡 2

12. Calibration: average gain

13. Validation: space and time response
Average shape of a cluster in channel and time directions
Qi/Qcl
Qi/Qcl
Xi-Xcl
ti-tcl
AFTER time response from
electronics measurement
and simulation
Gaussian diffusion and PRF

14. Calibration: gain fluctuation (Polya)

15. Validation: Preamp saturation
Saturation on accumulated charge on a single channel
Model from electronics measurement and simulation

16. Validation/Calibration: summary
average gain,
electron capture
gain fluctuations
longitudinal diffusion
transverse diffusion
time response
space response

17. Polarisation: pure MC

18. Comparison data/sim
DATA/DATA
SIM/SIM
DATA/SIM

19. DATA/MC 4 TPC orientations ωTPC = -45o ωTPC = 0o ωTPC = +45o
Very promising preliminary results:
systematic bias is well reproduced and corrected by the simulation

20. Conclusions
HARPO made good measurement of gamma-ray polarimetry in beam, but relies on comparison P=100%/P=0%
A detailed simulation was developed to treat systematic bias
The simulation was successfully calibrated and validated using cosmic rays
This will allow to correct systematic bias and allow polarisation measurements
This is a crucial step towards a large balloon borne TPC