Generate transitions without spin projections

[redeboer]

Problem description

QRules is intended to be a generic system of input (topologies), constraints (initial and final state) and selection rules (conservation laws). Currently it produces specific state transition objects with Particle instances on that edges that each have a float for spin projections.

These spin projections are however rather a kinematic thing (it depends on rest frame) and there is no quantum conservation rule for these projections alone (in other words, it's not an inherent quantum number that each state caries independent of reference frame).

Some issues related to generating all pin projections:

• It can result in a large number of generated transitions due to combinatorics, making the program slow. See e.g. Illustrate 4- or even 5-body isobar decay in the visualization notebook #27 (comment)
• The spin projections are not really needed for AmpForm. In fact, it forms a problem for the implementation of Dalitz-plot decomposition, see Reconsider amplitude builder design to improve implementation of spin alignment ampform#318.

It should be possible to select a different number of input conditions (here: dropping spin projections of the initial and final states). This would allow generating transitions purely with spin magnitudes.

[redeboer]

@grayson-helmholz I updated the issue with a sketch of the steps we discussed. Can you create an issue for the first step? Either the fancy way (you should have the rights for that now) or just posting a new issue.

So, just a link to the python-constraints2 examples, sketch of the example you had in mind, and goal of the issue/task.