2. HARPO at the NewSUBARU photon beam
Event Reconstruction
Simulation
Angular Resolution
Polarimetry
Conclusions and Perspectives

3. Polarised photon beam 1.74 – 74.3MeV NewSUBARU, Japan

4. Measurement in polarised photon beam
Demonstrator built and tested in polarised photon beam in NewSUBARU, Japan
13 Energy points, 1.6 to 74MeV, ~60Mevents
Laser
gamma beam
electron beam

5. TPC: photon conversion
The incoming photon interacts with the gas and decays into an electron-positron pair
γ 𝑍 + → 𝑒 + 𝑒 − 𝑍 +

6. TPC: Gas ionisation Ar++e-
The electron and positron travel through the gas (mostly Argon) and ionises it, freeing many electrons and positive ions
This takes a few nanoseconds

7. TPC: Drift and Readout
The electrons drift along the electric field in a few microseconds.
The are amplified and read out on the readout plane

8. Pair Event Reconstruction

9. Reconstruction 3.0 No tracking: local vertices Find ROI
φ>π
φ<π
No tracking: local vertices
Find ROI
Find peaks in polar distribution around point
match 2D vertices to get 3D picture

10. Vertex Reconstruction
Simple and Robust:
ignores obvious scattering and background
potential for small opening angle
resistant to electronics saturation
No event categorisation
many fakes (including track ends)
space cut in beam configuration
Can be improved with tracking (from vertex seeds)
better event selection
better resolution (?)
momentum from multiple scattering (?)

11. Simulation

12. 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

13. Validation/Calibration with cosmic rays
average gain,
electron capture
gain fluctuations
longitudinal diffusion
transverse diffusion
time response
space response

14. Angular Resolution

15. Vertex Selection Space cut on beam inside gas volume Opening angle
reject single tracks (especially Compton)

16. Angular Resolution 68% containment 3 main effects nucleus recoil
unknown momentum
detector/reconstruction resolution

17. Angular Resolution
E<10MeV
recoil
E>10MeV
momentum & detector

18. Polarimetry

19. Azimuthal angle Azimuthal angle Ω definitions recoil angle φr
pair plane angle ω
pair bisector ϕ=(ϕ++ϕ-)/2
Angle ω used in previous publications underestimates A at low energy
ϕ appears in Bethe-Heitler formula, agrees with asymptotic values ω ϕ+ ϕ- φr pr
𝑑Γ 𝑑Ω ∝1+𝐴𝑃cos 2 Ω− Ω 0

20. Azimuthal angle Cubic geometry Fixed photon direction
Large systematic bias
Same bias on polarised and unpolarised
Cancel by ratio

21. Azimuthal angle DATA/DATA

22. Azimuthal angle Sim/Sim

23. Azimuthal angle DATA/Sim

24. Polarimetry Measured up to 20MeV Consistent with expectation
QED limit + angular resolution
Precise beam polarisation to be confirmed
apparently <100%

25. Azimuthal angle Isotropic photons
Full simulation
as in previous results
Isotropic photons
polarised or unpolarised
uniform vertex distribution in as volume
selection using MC position
Resolution/Asymmetry depends on fiducial cuts

26. Conclusion/Perspective

27. Conclusions
The HARPO demonstrator operated successfully in a gamma-ray beam
Good resolution with simple reconstruction
reconstruction can be improved
strong effect of unknown momentum
Beam polarisation measurement up to 20MeV
Excellent agreement with simulation
Reliable Simulation for future developments

28. Backup

29. Vertex Finder
φ>π
φ<π

30. Vertex Fitting
Polar charge distribution around vertex
delta electron

31. Vertex Fitting
Clean up: keep only straight lines

32. Vertex Matching
Assign signal to tracks

33. Vertex Matching
Compare profiles: X(1,2) ↔ Y(1,2) “same”

34. Vertex Matching
Compare profiles: X(1,2) ↔ Y(2,1) “switch”

35. Azimuthal angle Azimuthal angle Ω definitions recoil angle φr
pair plane angle ω
pair bisector ϕ=(ϕ++ϕ-)/2
Angle ω used in previous publications underestimates A at low energy
ϕ appears in Bethe-Heitler formula, agrees with asymptotic values ω ϕ+ ϕ- φr pr
𝑑Γ 𝑑Ω ∝1+𝐴𝑃cos 2 Ω− Ω 0

36. Azimuthal angle Azimuthal angle Ω definitions recoil angle φr
pair plane angle ω
pair bisector ϕ=(ϕ++ϕ-)/2
Angle ω used in previous publications underestimates A at low energy
ϕ appears in Bethe-Heitler formula, agrees with asymptotic values
π/4
1/7
D.Bernard and PG arXiv: [astro-ph.IM]
submitted to astroparticle physics

37. Gas stability Recovery of full performance after 6 month sealed
pressure temperature
gain
attenuation
drift velocity
Recovery of full performance after 6 month sealed
START PURIFICATION
Frotin et al., MPGD2015, EPJ Web of Conferences, arXiv:

38. The MeV Gap Sensitivity gap no Compton above ~100keV
no Pairs below ~100MeV
Polarimetry cut
no polarimetry above 1MeV no
pola
polarimetry
Spectra from blazars

39. Polarimetry: example Separating models
Leptonic synchrotron self Compton (SSC)
Hadronic proton synchrotron
Polarisation can give the answer
no difference in X
visible in gamma
1MeV
1GeV
H. Zhang and M. Böttcher,
A.P. J. 774, 18 (2013)

40. HARPO Demonstrator scintillators micromegas +GEM readout strips
field cage

41. Expected performance
D. Bernard,
NIM A 701 (2013) 225

42. Detector monitoring using cosmic rays Gas stability

43. Diagnostics: Q vs Tdrift
electron capture
Tmax = LTPC/Vdrift
gain
The charge is normalised with regard to the track angle
The profile is obtained from a Landau fit (of slices) (mean value affected by threshold/saturation effects)
Vdrift is also easily extracted from this plot

44. Cosmic runs LLR Relative measurements
First run as reference (“clean gas”)
Weekly data taking of ~1.5h, for 6 months
Clear degradation of gain and e- capture

45. Gas stability Very encouraging results
excellent performance after 6 month sealed
detector not optimised for outgassing
Test done without HV
monitoring necessary: risk of damage on µM
=> HV turned on only for data taking ~1h/day
New test ongoing
HV on all the time, data taking at regular interval
µM HV control using

46. Examples of events 13 Energy points, 1.74 to 74 MeV
Experimental setup presented at TeVPA 2015 in Kashiwa

47. Examples of events 13 Energy points, 1.74 to 74 MeV
Experimental setup presented at TeVPA 2015 in Kashiwa

48. Examples of events 13 Energy points, 1.74 to 74 MeV
Experimental setup presented at TeVPA 2015 in Kashiwa

49. Examples of events 13 Energy points, 1.74 to 74 MeV
Experimental setup presented at TeVPA 2015 in Kashiwa

50. Polarisation Modulation of the azimuthal angle ω
𝑑Γ 𝑑ω ∝1+𝐴cos 2 ω− ω 0

51. Angular resolution
VERY PRELIMINARY

52. Angular resolution Agreement with theoretical prediction
relatively small contribution of tracking
Excellent agreement with simulation
effect of saturation dominates at high energy
Potential for improvement
estimation of track momentum
even 100% resolution should significantly improve

53. Gas Flow

54. Triplet event

55. Efficiency

56. Polarisation asymmetry