why polarization vector is not perpendicular to momenta ?

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Hi,

I am a new g4 member and now starting to simulate the physical progress of Compton scattering of polarized gamma rays.

// set polarization of incident gamma rays
  GPSource->SetParticlePolarization(polar);
// polar is perpendicular to momenta (example/novice/ExN06)

Partial Results of My program:

>> tracking/verbose 3
>> run/beamOn 1

...

 G4Track Information:   Particle = gamma,   Track ID = 1,   Parent ID = 0

      -----------------------------------------------------------------------
        StepPoint Information               PreStep            PostStep
      -----------------------------------------------------------------------
         ...
         Momentum Direct - x : -0.05637176954884118  0.4130663918928705
         Momentum Direct - y :  0.03700451044691715 -0.1472695079621853
         Momentum Direct - z :    0.997723854483051 -0.8987145530774392
         ...
         Polarization - x    :   0.8547874941346449  0.9193025924556333
         Polarization - y    :  -0.5189040340729129 0.06948359839555679
         Polarization - Z    : -0.008771732660759615 0.009372527261891497
         Weight              :                    1                   1
         Step Status         :           Geom Limit       PostStep Proc
         Process defined Step:       Transportation           pol-compt
      -----------------------------------------------------------------------

My questions:

(1) In PreStep (before Compton scattering), the polarization vector is set to be perpendicular to momenta. why, in PostStep (after Compton Scattering), those two vectors become not perpendicular with each other?

(2) why the polarization vector is not a unit vector?

(3) I learn preliminarily in Physics Reference Manual that there are three different coordinate systems: World frame, Particle frame, Interaction frame. Are the momenta and polarization vector calculated in the same frame? which frame?

Thanks for your help.

zhangyf