  Many models beyond the Standard Model predict light and feebly interacting
particles that are often long-lived. These long-lived particles (LLPs) in many
cases can be produced from meson decays. In this work, we propose a simple and
quick reinterpretation method for models predicting LLPs produced from meson
decays. With the method, we are not required to run Monte-Carlo simulation,
implement detector geometries and efficiencies, or apply experimental cuts in
an event analysis, as typically done in recasting and reinterpretation works.
The main ingredients our method requires are only the theoretical input,
allowing for computation of the production and decay rates of the LLPs. There
are two conditions for the method to work: firstly, the LLPs in the models
considered should be produced from a set of mesons with similar mass and
lifetime (or the same meson) and second, the LLPs should, in general, have a
lab-frame decay length much larger than the distance between the interaction
point and the detector. As an example, we use this method to reinterpret
exclusion bounds on heavy neutral leptons (HNLs) in the minimal ``3+1''
scenario, into those for HNLs in the general effective-field-theory framework
as well as for axion-like particles. We are able to reproduce existing results,
and obtain new bounds via reinterpretation of past experimental results, in
particular, from CHARM and Belle.
