In order to test the dynamics of a system it is common to use probes that only interact feebly with the system, which dynamics is driven by stronger interactions. The former weaker process can then drive to different final states that interact strongly among them.

1. 1.

   Depending on the nature of the initial state, i.e., the probe, it might be necessary to decompose it as well in PWAs. For example, for\(\gamma \gamma \) to meson–meson the reader could consult Refs. [73, 74] where the\(\gamma \gamma \) state is decomposed in helicity amplitudes.

Authors and Affiliations

1. Departamento de Física, Universidad de Murcia, Murcia, Spain

   José Antonio Oller

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Correspondence toJosé Antonio Oller.

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