The ππ interaction in nuclear matter from a study of the πA→ππA′ reactions

Abstract

The pion-production reactions π + A → π + π ± A ′ were studied on 2 H, 12 C, 40 Ca, and 208 P b nuclei at an incident pion energy of T π + =283 MeV. Pions were detected in coincidence using the CHAOS spectrometer. The experimental results are reduced to differential cross sections and compared to both theoretical predictions and the reaction phase space. The composite ratio C A ππ between the π + π ± invariant masses on nuclei and on the nucleon is also presented. Near the 2m π threshold pion pairs couple to (ππ) I=J=0 when produced in the π + → π + π − reaction channel. There is a marked near-threshold enhancement of C A π + π − which is consistent with theoretical predictions addressing the partial restoration of chiral symmetry in nuclear matter. Furthermore, the behaviour of C A π + π − is well described when the restoration of chiral symmetry is combined with standard Pwave renormalization of pions in nuclear matter. On the other hand, nuclear matter only weakly influences C A π + π + , which displays a flat behaviour throughout the energy range regardless of A.

In-medium properties of hadrons—observables II

“…This leads to a threshold enhancement in the ππ invariant mass spectrum due to suppression of the phase-space for the σ → ππ decay. A first measurement of the two pion invariant mass spectrum has been obtained by the CHAOS collaboration in pion induced reactions on nuclei [44]. The authors of [44] claimed to indeed have seen an accumulation of spectral strength near the 2π threshold for heavy target nuclei in the π + π − mass distribution.…”

“…A first measurement of the two pion invariant mass spectrum has been obtained by the CHAOS collaboration in pion induced reactions on nuclei [44]. The authors of [44] claimed to indeed have seen an accumulation of spectral strength near the 2π threshold for heavy target nuclei in the π + π − mass distribution. According to the arguments presented in the introduction, photon induced reactions in nuclei are much better suited to investigate double pion pion production at finite baryon densities.…”

Pion properties at finite nuclear density based on in-medium chiral perturbation theory

“…[18,19], the in-medium ππ interaction in the scalar-isoscalar part will also has attractive enhancement thanks to the partial restoration of chiral symmetry in nuclear medium and the experimental observation of the invariant mass spectrum of the ππ production off nuclear targets performed in Refs. [20][21][22] could have a hint of such a enhancement.…”

Medium Modifications of Hadrons in Photon Induced Reactions

“…In-medium modifications of the ππ interaction have been studied in pion-induced reactions on nuclei like A(π · ,π · π ) [20] and A(π ,π 0 π 0 ) [21]. However, pion-induced reactions occur at fractions of the normal nuclear density.…”