Re: Correlated polarized photons in GEANT4

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Hi Peter,

thanks a lot for the prompt relpy. Your method reproduces exactly what I did. When I say that the polarization that I get is not correct, what I mean is that the level of polarization (the amplitude of the modulation curve, i.e., the azimuthal distribution of the photons scattered) of the photons in the right is only half of the expected value.

Let's say, Photon1 goes to the left, and Photon2 goes to the right with opposite momentum and perpendicular polarization.

The problem in the method that you suggested and I used, is that the cross-section of Photon2 to interact in the right hand side of the setup, is completely independent from the interaction that Photon1 experienced in the left side. If photons are really correlated, this is not the case. There is a general cross section that describes both scattering processes at the same time.

Let's call P1 the probability of photon1 to be detected at a certain scattering angle Phi1 in the left side of the setup, and P2 the probability of photon2 to be detected at a certain scattering angle Phi2 in the right side of the setup. Then, when I simulate both photons separately I am considering that the probability of both things to happen at the same time is P1*P2. But this is not true. Calculating it analytically I get 1/2 of the expected modulation, what is what the simulations give as a result. If I understand correctly, one can not factorize the problem, because the quantum wave function is not factorizable.

I have seen a simulation of the problem that was not done in Geant4. What they did is to simulate the whole event (I mean, both photons, left and right). Then, they calculate according to quantum mechanics the probability that the observed scattering angles Phi1 and Phi2, actually happen, and they assign it as a weight to the event. In this way they introduce the "correlation" of both photons "by hand".

I will try to do something similar with Geant4, and see what I get. Please, let me know if you have some idea for another approach.

Cheers, Estela.