The π→ππ process in nuclei and the restoration of chiral symmetry

Abstract

Abstract:The results of an extensive campaign of measurements of the π → ππ process in the nucleon and nuclei at intermediate energies are presented. The measurements were motivated by the study of strong ππ correlations in nuclei. The analysis relies on the composite ratio C A ππ , which accounts for the clear effect of the nuclear medium on the ππ system. The comparison of the C A ππ distributions for the (ππ) I=J=0 and (ππ) I=0,J=2 systems to the model predictions indicates that the C A ππ behavior in proximity of the 2m π threshold is explainable through the partial restoration of chiral symmetry in nuclei.

Photoproduction of Mesons off Nuclei - The Photonuclear Programs at ELSA and MAMI

“…Such effects have been sought in pion induced reactions at the CHAOS spectrometer (see e.g. [40,41]) and with the Crystal Ball detector at BNL [42], as well as with photon induced reactions at MAMI [43,44]. All experiments found some in-medium effects.…”

Search for the QCD critical point in nuclear collisions at158A GeV at the CERN Super Proton Synchrotron (SPS)

“…With this prescription one expects, by studying the behavior of π + π − pairs (near threshold) in the freeze-out states of nuclear collisions, to be able to capture the properties of the sigma field (order parameter) at the critical point. It is of interest to note that the spectral function of sigma in a thermal environment and near the two-pion threshold is based on general principles (partial restoration of chiral symmetry) and is not affected by the controversial issue of the σ meson (a broad resonance) in a vacuum (T = 0, ρ B = 0), which remains an open question in hadronic physics [4,5].…”

Photoproduction of mesons off nuclei

“…Since the σ meson couples strongly to scalar-isoscalar pion pairs the predicted effect should influence the invariant mass spectra of such pairs in the nuclear medium, producing a downward shift of the invariant-mass distributions of such pairs. This prediction has been experimentally investigated with pion-and photon-induced double pion production reactions [51,52,53,54,55,56,57,58]. The CHAOS collaboration [51,52,53,54,55] reported a downward shift for π + π − pairs with respect to π + π + pairs from pion induced reactions.…”

“…This prediction has been experimentally investigated with pion-and photon-induced double pion production reactions [51,52,53,54,55,56,57,58]. The CHAOS collaboration [51,52,53,54,55] reported a downward shift for π + π − pairs with respect to π + π + pairs from pion induced reactions. The Crystal Ball collaboration at BNL [56] observed a low-mass enhancement of strength for heavy nuclei in π − -induced π 0 π 0 production.…”