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3-body phase space fixes for pion production in hA/hN 2018 and 2025#498

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3-body phase space fixes for pion production in hA/hN 2018 and 2025#498
sjgardiner wants to merge 3 commits into
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sjgardiner:feature/gputnam-fix-ha-pionprod-with-2025

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This PR updates and replaces #448 originally prepared by Gray Putnam. Changes they originally developed for the 2018 model versions have also been applied consistently to the 2025 code. The main changes involve

  1. Sampling particle polar angle cosines rather than just the polar angles (a mistake present in the original code and commonly-encountered elsewhere, see https://mathworld.wolfram.com/SpherePointPicking.html).

  2. Introducing a bias parameter that allows reshaping of pion production kinematic sampling away from pure 3-body phase space. The parameter is XML-configurable and defaults to a value of zero (no bias at all).

  3. Updating interfaces to handle non-zero values of the bias parameter correctly.

gputnam and others added 3 commits June 2, 2026 09:52
…ematics. Add in optional bias towards forward nucleon momentum.
XML-based configuration of the bias parameter.
…tion

FSI to the hA/hN 2025 models. The adjustments are intended to match what
was previously done for hA/hN 2018.
@sjgardiner sjgardiner requested a review from dytman June 2, 2026 16:24
@sjgardiner sjgardiner self-assigned this Jun 2, 2026
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