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

Polarised photon beam 1.74 – 74.3MeV NewSUBARU, Japan

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

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

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

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

Pair Event Reconstruction

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

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 (?)

Simulation

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

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

Angular Resolution

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

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

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

Polarimetry

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

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

Azimuthal angle DATA/DATA

Azimuthal angle Sim/Sim

Azimuthal angle DATA/Sim

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

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

Conclusion/Perspective

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

Backup

Vertex Finder φ>π φ<π

Vertex Fitting Polar charge distribution around vertex delta electron

Vertex Fitting Clean up: keep only straight lines

Vertex Matching Assign signal to tracks

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

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

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

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:1611.05179 [astro-ph.IM] submitted to astroparticle physics

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:1512.03248

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

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)

HARPO Demonstrator scintillators micromegas +GEM readout strips field cage

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

Detector monitoring using cosmic rays Gas stability

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

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

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 http://llr.in2p3.fr/sites/pyrame

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

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

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

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

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

Angular resolution VERY PRELIMINARY

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

Gas Flow

Triplet event

Efficiency

Polarisation asymmetry