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 2016-12-07

Polarisation measurement ϕ 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 2016-12-07

Polarisation Beam data (2014) Large systematic bias cubic geometry fixed orientation Cancel systematic bias by taking the ratio between P=100% and P=0% 2016-12-07

Azimuthal angle 2016-12-07

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, ... 2016-12-07

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 2016-12-07

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 2016-12-07

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 2016-12-07

Calibration: Vdrift Tdrift=FWHM Vdrift=LTPC/Tdrift 2016-12-07

Calibration: Cdiff σ 2 = σ 0 2 + 𝐶 𝑑𝑖𝑓𝑓 2 𝑧 𝑣 𝑑𝑟𝑖𝑓𝑡 2 2016-12-07

Calibration: average gain 2016-12-07

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 2016-12-07

Calibration: gain fluctuation (Polya) 2016-12-07

Validation: Preamp saturation Saturation on accumulated charge on a single channel Model from electronics measurement and simulation 2016-12-07

Validation/Calibration: summary average gain, electron capture gain fluctuations longitudinal diffusion transverse diffusion time response space response 2016-12-07

Polarisation: pure MC 2016-12-07

Comparison data/sim DATA/DATA SIM/SIM DATA/SIM 2016-12-07

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 2016-12-07

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 2016-12-07