Double pion photoproduction off nuclei - are there effects beyond final-state interaction?

TL;DR

This study measures pion pair photoproduction off various nuclei to analyze final-state interactions and explore potential in-medium modifications of the sigma meson, finding significant effects that complicate the search for such modifications.

Contribution

It provides the first model-independent analysis of pion final-state interactions in nuclear photoproduction, highlighting their large influence on invariant mass spectra.

Findings

Pion invariant mass spectra shift to smaller masses in heavy nuclei.

Model calculations reproduce mixed-charge pair spectra better than neutral pairs.

Results suggest limited room for sigma meson in-medium modifications due to FSI effects.

Abstract

Photoproduction of $\pi^{0}\pi^{0}$ and $\pi^{0}\pi^{\pm}$ pairs from nuclei has been measured over a wide mass range ($^2$H, $^{7}$Li, $^{12}$C, $^{40}$Ca, and $^{\rm nat}$Pb) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4$\pi$ electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant mass spectra for heavy nuclei to small invariant masses has been observed for $\pi^0$ pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the $\sigma$-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for $\sigma$-in-medium modification.