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In-medium pion-pion correlations were studied via their effect on experimental observables measured for the ϩ → ϩ Ϯ reaction in nuclei (A: 12 C, 40 Ca, and 208 Pb) and in deuterium, from which it is inferred the observables for the nucleon (N: n and p). The measurements were performed at TRIUMF using a positivepion flux with a bombarding energy of 283 MeV. The invariant mass distributions M were measured from the 2m threshold up to the kinematic maximum, and total cross sections T deduced. The medium modifications were investigated by forming the composite ratio Cproved to be only weakly affected by the (,2) reaction mechanism. In the IϭJϭ0 channel the C A distributions are peaked at the 2m threshold and increases with A. For pion pairs in the Iϭ2, Jϭ0 channel the C A distributions are nearly independent of A. The distinctive behavior of C A may be related to the appearance of the meson. ͓S0556-2813͑99͒00707-4͔ PACS number͑s͒: 25.80.Hp, 13.75.LbThe interaction in nuclei was studied at TRIUMF by means of the ϩ A→ ϩ Ϯ AЈ (,2) reactions, which were measured simultaneously. Although the measured ϩ ϩ invariant mass distributions M ϩ ϩ exhibited little A dependence, the same distributions in the JϭIϭ0 channel M ϩ Ϫ increased in strength near the 2m threshold with increasing nuclear mass number A ͓1͔. It was further realized that this increase was not due to the reaction mechanism since the quasifree nature of the interaction mechanism is independent of A ͓2͔, and the elementary ϩ n→ ϩ Ϫ p reaction has a ϩ Ϫ invariant mass distribution of negligible intensity near threshold ͓3͔. A similar behavior was presented in an earlier article describing the M ϩ Ϫ distribution as a function of A ͓4͔, although in that experiment the apparatus was only able to detect pairs with M ϩ Ϫ Ͼ300 MeV.These experimental results inspired a number of theoretical works ͓5-7͔ which addressed the issue of the influence of the nuclear medium on the () IϭJϭ0 properties, a topic extensively studied for vector mesons. All the models satisfy chiral constraints, and predict some common features, which for convenience of discussion are labeled Ti. T1: The mass distribution of the system at ϭ0 ͑i.e., in the vacuum͒ appears as a broad peak with a width ⌫ Ϸ500 MeV ͓5-8͔ while for 0Ͻр n , where n ϭ0.17 fm Ϫ3 ͑the saturation density͒, the mass distribution splits into two branches ͓5-7͔. The low energy branch peaks at, or even below, the 2m threshold. T2:The intensity of the mass distribution around the 2m threshold depends markedly on . The intensity increases as the average nuclear density increases.T3: The enhancement of the M ϩ Ϫ distribution around 2m is obtained for р n , that is, for nuclear densities of stable nuclei.T4: In spite of the fact that all the models use quite different approaches, they all find that the accumulation of strength around the 2m threshold is due to the () IϭJϭ0 interacting system.The broad mass distribution predicted by various models for the meson, ⌫ Ϸ500 MeV, indicates that it is a shortlived resonance with a lifetime Ϸ10 Ϫ...
͑The CHAOS Collaboration͒ ͑Received 19 December 1996͒The process of pion-induced pion production in nuclei has been studied with the A( ϩ , ϩ Ϫ ) and A( ϩ , ϩ Ϫ p) reactions for four nuclei 2 H, 12 C, 40 Ca, and 208 Pb at an incident pion energy of T ϩϭ 283 MeV. The ϩ Ϫ and ϩ Ϫ p particles in the final state were detected in coincidence with the CHAOS spectrometer at TRIUMF. By looking at kinematic quantities such as missing momentum and missing energy, we have been able to draw some model-independent conclusions about the dynamics of the → process in nuclei, which do not vary significantly with target mass. We find that the incident pion interacts mainly with one surface nucleon via the leading N→N channel. The pion pair releases no appreciable energy to the residual nucleus, which remains substantially bound throughout the reaction. These findings underline the quasifree nature of the pion production reaction in nuclei.
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