Revive decays and polarization

[Juergen Reuter]

Starting working on this ...

(and anticipated delay due to the long weekend ahead)

[Juergen Reuter]

Should be the next one ...

[Juergen Reuter]

After discussion with the LC generator group, WK will revise this in an easier setup (bascially an array of one or two [possibly complex] entries) that allow for all the possibilities written down in the manual (incl. the possibility to loop over helicities!), but in a unified/simplified approach.

[kilian]

The low-level infrastructure for decays is now available, modulo some additions and tunings (r4879). Next step: make this available in the event record, and activate the Sindarin commands and parameters for decays.

Polarization is implicit in the decay infrastructure, also has to be made accessible to the user.

[Juergen Reuter]

Reinstated the polarization unit test in #4881. Clearly, this has to be adapted for WK's new structure of polarization specifiers, but it's a good reminder of what cases should be covered.

[kilian]

Status as of r4921

• decay chains (also cascades) should work again. Tests are in testproc_8 and smtest_4.
• work with explicit decay processes and unstable command.
• spin correlations fully respected.

To Do:

• reinstate uncorrelated decays (and classical spin correlations)
• automatic generation of decay channels (done at low level as method of model_t)
• polarized final states

[kilian]

With r4971, I consider this task as completed.

Isotropic and diagonal decays are available by the corresponding options. Polarized particles are also available. The states of unstable and polarized are mutually exclusive (setting one turns off the other one). If no decays are defined, the program will calculate decays if ?auto_decays is set. The parameter auto_decays_multiplicity (default 2) and ?auto_decays_radiative (default no) control this. The user may add further decays that the program does not generate automatically. (This happens if a vertex is missing in the model file; however, there may also be a phase-space problem then.)

There are lots of ideas for improvement, but for the initial 2.2 release this may be sufficient.