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Fix recently introduced bug in EvtDalitzReso for parent with significant natural width

Thomas Latham requested to merge dalitzreso_broadparent into master

In !22 (c51a5f79), which was cherry-picked into both D119 and D124, several changes were made to EvtDalitzReso to make the evaluate method const. The previous code was not const because a data member was modified in a particular case where the parent mass had changed. Following the changes, the data member is left unmodified and a warning is printed instead. The warning was never seen in any existing tests. However, the existing tests do not cover the case where the parent particle has a natural width, which implies that the per-event mass of the parent is not constant.

This MR:

  • adds a test for just such a case: \eta_c \to K^+ K^- \pi^0
  • modifies the code to restore the correct behaviour

TODO:

  • Check that the new behaviour is identical to the behaviour before the bug was introduced
  • Add a test that actually uses the LASS and Flatte functionality to ensure the changes here don't break anything

This fix will also need to be propagated to the R02-02-00-patches branch for inclusion in LHCb Sim10.

Edited by Thomas Latham

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