Pair production by Linearly Polarized Gamma Rays - Five-dimensional (5D) Bethe-Heitler Model

The G4BetheHeitler5DModel described in Five-dimensional (5D) Bethe-Heitler gamma Conversion to e+e- can generate the conversion of either linearly polarized photons or non-polarized photons. The performance is compared to that of other models in [GB17b]. If the optimal estimate of the event azimuthal angle is used, that is, the bisector \(\phi_{+-}\equiv (\phi_+ + \phi_-)/2\) of the electron azimuthal angle \(\phi_-\) and of the positron azimuthal angle \(\phi_+\) [GB17a], the G4BetheHeitler5DModel is the only physics model with which the polarization asymmetry of nuclear conversion is verified to be compatible with the low-energy [GB17a] and with the high-energy [BP71] asymptotic expressions, that have been obtained from partial integration of the Bethe-Heitler differential cross section.

As expected (eg. from [May51, OM59]), the electron and the positron are preferentially emitted in the polarization plane (if azimuthal angles are measured from the polarization plane, the single-lepton azimuthal angles \(\phi_-\) and \(\phi_+\) take their maximum at zero). Accordingly, the bisector \(\phi_{+-}\) and the recoil azimuthal angle \(\phi_r\) take their maximum at \(\pm\pi/2\).

The polarization asymmetry is optimally extracted from the average value of a weight computed from the azimuthal angle of the event [Ber13, GB17a].

See also the extended electromagnetic example TestEm15.

Bibliography

Ber13

D. Bernard. Polarimetry of cosmic gamma-ray sources above e+e- pair creation threshold. Nucl. Instrum. Meth., A729:765–780, 2013.

BP71

V. F. Boldyshev and \relax Yu. P. Peresunko. Electron-positron pair photoproduction on electrons and analysis of photon beam polarization. Yad. Fiz., 14:1027–1032, 1971.

GB17a(1,2,3)

P. Gros and D. Bernard. gamma-ray polarimetry with conversions to e+e- pairs: polarization asymmetry and the way to measure it. Astropart. Phys., 88:30–37, 2017.

GB17b

P. Gros and D. Bernard. gamma-ray telescopes using conversions to e+e- pairs: event generators, angular resolution and polarimetry. Astropart. Phys., 88:60–67, 2017.

May51

M. M. May. On the polarization of high energy bremsstrahlung and of high energy pairs. Phys. Rev., (84):265, 1951.

OM59

H. Olsen and L.C. Maximon. Photon and electron polarization in high- energy bremsstrahlung and pair production with screening. Physical Review, 114:887–904, 1959.