Created attachment 833 [details] Z position of charged secondary pions from pbar annihilation at rest We observe anisotropic distributions of secondary pions from antiproton annihilation at rest while using FTFP_INCLXX physics in Geant4.11.2.beta. The distribution is strongly skewed in the direction of negative Z (see the figure attached). The problem is analogue to the Problem ID 1910 (Geant4.10.2 FTFP_BERT_TRV).
Dear Przemysław, Could you give me more information? I'm not sure to understand "Z position of charged secondary pions". Do you mean that charged pions are preferentially emitted in the negative Z direction? If yes, why is this a problem? At rest the pbar is annihilated at the border of your nucleus. Then pions are produced in all directions, but toward the positive Z they can be absorbed by the nucleus, which is not the case for the negative Z. Regards
(In reply to jean-christophe.david from comment #1) > Dear Przemysław, > > Could you give me more information? > I'm not sure to understand "Z position of charged secondary pions". > Do you mean that charged pions are preferentially emitted in the negative Z > direction? > If yes, why is this a problem? > At rest the pbar is annihilated at the border of your nucleus. > Then pions are produced in all directions, but toward the positive Z they > can be absorbed by the nucleus, which is not the case for the negative Z. > > Regards Daer Jean-Christophe, Thanks for a quick response and sorry for my imprecise language. I wll try to better describe the problem. By the „Z position of charged secondary pions” I mean the z-coordinate of the detection point of the secondary pion on a sphere surrounding the target. The center of the sphere is located in the middle of the annihilation target, and the radius of the sphere is much larger than dimensions of the annihilation target. Everything but the target is made of vacuum. The target is a tiny cube made of lead, and with sides 0.1 mm long. The antiproton is initially placed at rest, i.e., with zero kinetic energy, in the middle of the target. In the simulation I am just recording position of pions resulting from the pbar annihilation as they are crossing the sphere. It is true that pions emitted in the direction of the nucleus are partially absorbed. Let this direction be the positive Z direction in the local, antiproton – nucleus coordinate system (in this case we let the center of this local coordinate system be at the annihilation point, and it’s Z-axis pointing towards the center of the target nucleus). Nonetheless, the rotation of this local coordinate system w.r.t. the global (i.e. lab’s or my spherical detector’s) coordinate system should be random, and therefore, I would expect isotropic distribution of secondary pions observed in my spherical detector. Am I right? Regards, Przemysław Adrich
Dear Przemysław, It seems to me that's you are right. But who should care of the isotropy in the case of zero energy? User? Geant4? INCLXX? I don't know. INCLXX was not designed for zero energy (but it works, except for this isotropy). I'll see (sorry, I'm too busy for the next few weeks) if this can be taken into account in INCLXX. Maybe, but I can't promise. Thank you for pointing out this problem. Regards
