Abstract
The photoproduction of π 0 π 0 and π 0 π +/− pairs has been investigated in view of the of in-medium modifications the σ -meson. The measurements have been done at the Mainz MAMI accelerator, using the Glasgow photon tagging spectrometer and a 4π electromagnetic calorimeter, consisting of the Crystal Ball and TAPS detectors. The measurements covered different nuclear targets (2 H, 7 Li, 12 C, 40 Ca, and nat Pb) and the statistical quality of the data was improved compared to the previous experiments. The energy and mass dependence of the invariant mass distributions suggests that Final State Interactions play a very important role, but some of the observed effects can also be related to a possible in-medium modification of the σ -meson.
Highlights
The properties of hadrons in nuclear matter is a much discussed topic in nuclear physics as it is related to a regime where the Quantum-Chromo Dynamics (QCD) is not accessible to perturbative methods.At low energies the spontaneous breaking of the chiral-symmetry occurs and without it, hadrons would appear as mass degenerate parity doublets
The restoration of the symmetry is related to the temperature and density dependence of the quark condensate which is indirectly connected to the in-medium properties of hadrons via QCD
Invariant mass distributions of the π0 π0 and the π0 π± for the different targets have been extracted for the energy range from 400 to 460 MeV and show an accumulation of strength at small invariant masses with increasing nuclear mass in both channels (Fig. 2)
Summary
At low energies the spontaneous breaking of the chiral-symmetry occurs and without it, hadrons would appear as mass degenerate parity doublets. This symmetry breaking can be seen in the hadron spectrum and in particular for the σ-meson. A partial symmetry restoration at high density or temperature can be possible according to many models. The restoration of the symmetry is related to the temperature and density dependence of the quark condensate which is indirectly connected to the in-medium properties of hadrons via QCD sum rules. An effect on the invariant mass distribution of pion pairs is expected to be seen since the σ-meson couples strongly to such iscoscalar pairs
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