The goal of experiments in light-quark spectroscopy is to find and test the predictions of QCD for the bound-state and decay characteristics of low-lying hadrons. All except for the few lightest states, these hadrons and very short-lived and are not directly detected in an experiment. Instead they appear in the experiment as clusters of particles into which they have decayed, with correlations between the daughter particles bearing the imprint of the parent, known as resonances. The delicate job of untangling the correlations to get at the hadron spectrum is known as partial wave analysis.
Besides the technical challenge of partial wave analysis, there sometimes also arise questions of interpretation. This is because not all correlations are necessarily due to resonance decays; there are also effects from inter-hadron scattering in the continuum. Provided that the resonances are reasonably narrow, these non-resonant re-scattering effects are as a rule gradual enough not to hamper the extraction of resonance parameters from the data. A notable exception occurs near a threshold, where two or more particles are created nearly at rest with respect to each other. In this case re-scattering effects can be strong and produce correlations that might otherwise be interpreted as resonance behaviour.
Thus the extraction of the hadronic spectrum from experimental data requires some knowledge of the scattering-states of strong QCD in addition to the bound states. In fact the two are connected because of mixing between the two sectors. With the spectrum still under investigation, it is not possible today to say much about the scattering states from first principles. However a considerable phenomenology exists in this area, from 30 years of experiments in hadron scattering over a wide range in energies. Requiring a consistent interpretation from a variety of experiments plays a critical role in forming reliable judgements about the results of any particular analysis. In the interpretation of the Jetset data, I am working together with theorist Eric Swanson (NCSU) to quantify the expected re-scattering in the final state.
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