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

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 ${}^{\mathrm{nat}}$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.

accepted for publication in Phys. Lett. B

References in corpus (9)

• Upgrade of the Glasgow photon tagging spectrometer for Mainz MAMI-C
• New results on the Roper resonance and the P_{11} partial wave
• On the size of the sigma meson and its nature
• Photoproduction of pi0-mesons from nuclei
• Photoproduction of pi0pi0 and pi0pi+-pairs off the proton from threshold to the second resonance region
• Radiative pi0 photoproduction on protons in the Delta+(1232) region
• Low-energy pions in nuclear matter and 2pi photoproduction within a BUU transport model
• Coherent photoproduction of pi^0- and eta-mesons off 7Li
• Double pion photoproduction off 40Ca

Cited by in corpus (2)

• Quasi-free photoproduction of eta-mesons off 3He nuclei
• Experimental study of the $\gamma p\to K^0{\Sigma }^{+}$, $\gamma n\to K^0\Lambda$, and $\gamma n\to K^0{\Sigma }^0$ reactions at the Mainz Microtron