document: One major goal of current nuclear physics is the observation of at least partial restoration of chiral symmetry. Since the chiral order parameter 〈q̄q〉 is expected to decrease by about 30% already at normal nuclear matter density [1–4], any in-medium change due to the dropping quark condensate should in principle be observable in photonuclear reactions. The conjecture that such a partial restoration of chiral symmetry causes a softening and narrowing of the σ meson as the chiral partner of the pion in the nuclear medium [5,6] has led to the idea of measuring the π0π0 invariant mass distribution near the 2π threshold in photon induced reactions on nuclei [7]. In contrast to its questionable nature as a proper quasiparticle in vacuum, the σ meson might develop a much narrower peak at finite baryon density due to phase-space suppression for the σ→ππ decay, hence making it possible to explore its properties when embedded in a nuclear many-body system [8–11]. Measuring a threshold enhancement of the π0π0 invariant mass spectrum might serve as a signal for the partial restoration of chiral symmetry inside nuclei and, therefore, give information about one of the most fundamental features of QCD.
doc_bio_tags: O O O O O B I O O B I I I I I I I I O O B I I O O O O O O O O O O O B I O O O O O O O O O B I O O O O O O B I O O O O O B I I I I O O O O O O O B I O O B I O O B O O B I O O O O O O O B I I I I I O O O O O B O O O B O O O O O O O O O O B O O O B I O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O B I I I I I I I I I O O O O O O O B I I I I O B O O O O O O O O O O O O B
extractive_keyphrases: chiral order parameter chiral partner measuring a threshold enhancement of the π0π0 invariant mass spectrum measuring the π0π0 invariant mass distribution nuclear matter nuclear medium nuclear physics nuclei observation of at least partial restoration of chiral symmetry partial restoration of chiral symmetry photon photonuclear reactions pion qcd quark condensate quasiparticle σ meson σ→ππ decay
abstractive_keyphrases: nuclear many-body system photon induced reactions q̄q