= 'perturbative' and 'real' particles; the perturbative weigth =

== 'perturbative' and 'real' particles ==

(following text is taken from: O.Buss, PhD thesis, [http://www.uni-giessen.de/cms/fbz/fb07/fachgebiete/physik/einrichtungen/theorie/theorie1/publications/dissertation/buss_diss pdf], Appendix B.1) 

For some calculations, e.g. low-energetic πA or γA collision, it is a good assumption, that the
target nucleus stays very close to its ground state. Henceforth, one keeps as an approximation
the target nucleus constant in time. This basically means that the phase space density of
the target is not allowed to change during the run. The test-particles which represent this
constant target nucleus are called ''real'' test-particles. However, one also wants to consider the
final state particles. Thus one defines another type of test-particles which are called ''perturbative''.
The ''perturbative'' test-particles are propagated and may collide with ''real'' ones, the products are
''perturbative'' particles again. However, ''perturbative'' particles may not scatter among each other.
Furthermore, they are neglected in the calculation of the actual densities. One can simulate in
this fashion the effects of the almost constant target on the outgoing nucleons without modifying
the target. E.g. in πA collisions we initialize all initial state pions as ''perturbative'' test-particles.
Thus the target stays automatically constant and all products of the collisions of pions and
target nucleons are assigned to the ''perturbative'' regime.

Furthermore, since the ''perturbative'' particles do not react among each other or modify the ''real''
particles in a reaction, one can also split a ''perturbative'' particle in \(n\) pieces (several ''perturbative''
particles) during a run. Each piece is given a corresponding weight \(1/n\) and one simulates like
this \(n\) possible final state scenarios of the same ''perturbative'' particle during one run.



== the perturbative weigth ==

the variable `perWeight` in the [//Documentation2016/code/typeDefinitions/particleDefinition_f90.html#robo688 definition of the particle type]