941 research outputs found

    The ππ\pi\pi interaction in nuclear matter from a study of the π+Aπ+π±A\pi^+ A \to \pi^+ \pi^{\pm} A' reactions

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    The pion-production reactions π+Aπ+π±A\pi^+ A \to \pi^+\pi^{\pm} A' were studied on 2H^{2}H, 12C^{12}C, 40Ca^{40}Ca, and 208Pb^{208}Pb nuclei at an incident pion energy of Tπ+T_{\pi^{+}}=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\cal CππA_{\pi\pi}^A between the π+π±\pi^{+}\pi^{\pm} invariant masses on nuclei and on the nucleon is also presented. Near the 2mπ2m_{\pi} threshold pion pairs couple to (ππ)I=J=0(\pi\pi)_{I=J=0} when produced in the π+π+π\pi^+\to \pi^+\pi^- reaction channel. There is a marked near-threshold enhancement of C\cal Cπ+πA_{\pi^+\pi^-}^A which is consistent with theoretical predictions addressing the partial restoration of chiral symmetry in nuclear matter. Furthermore, the behaviour of C\cal Cπ+πA_{\pi^+\pi^-}^A is well described when the restoration of chiral symmetry is combined with standard P-wave renormalization of pions in nuclear matter. On the other hand, nuclear matter only weakly influences C\cal Cπ+π+A_{\pi^+\pi^+}^A, which displays a flat behaviour throughout the energy range regardless of AA.Comment: 30 pages, 16 figures, PS format, accepted for publication in Nucl. Phys

    General properties of the pion production reaction in nuclear matter

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    The pion production reaction π+π+π±\pi^+ \to \pi^+\pi^{\pm} on 45Sc^{45}Sc was studied at incident pion energies of Tπ+T_{\pi^{+}} = 240, 260, 280, 300, and 320 MeV. The experiment was performed using the M11M11 pion-channel at TRIUMF, and multiparticle events, (π+,π+π±\pi^+,\pi^+\pi^{\pm}) and (π+,π+π±p\pi^+,\pi^+\pi^{\pm}p), were detected with the CHAOS spectrometer. Results are reported in the form of both differential and total cross sections, and are compared to theoretical predictions and the reaction phase space. The present investigation of the T-dependence of the π+Aπ+π±A\pi^+ A \to \pi^+\pi^{\pm} A' reaction complements earlier examinations of the A-dependence of the reaction, which was measured using 2H^{2}H, 4He^{4}He, 12C^{12}C, 16O^{16}O, 40Ca^{40}Ca, and 208Pb^{208}Pb targets at \sim280 MeV. Some general properties of the pion-induced pion production reaction in nuclear matter will be presented, based on the combined results of the two studies.Comment: 23 pages, Latex, accepted for publication in Nucl. Phys.

    The pi -> pi pi process in nuclei and the restoration of chiral symmetry

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    The results of an extensive campaign of measurements of the pi -> pi pi process in the nucleon and nuclei at intermediate energies are presented. The measurements were motivated by the study of strong pi pi correlations in nuclei. The analysis relies on the composite ratio C_{pi pi}^A, which accounts for the clear effect of the nuclear medium on the (pi pi) system. The comparison of the C_{pi pi}^A distributions for the (pi pi)_{I=J=0} and (pi pi)_{I=0,J=2} systems to the model predictions indicates that the C_{pi pi}^A behavior in proximity of the 2m_pi threshold is explainable through the partial restoration of chiral symmetry in nuclei.Comment: accepted for publication in Nucl. Phys.

    First determination of the one-proton induced Non-Mesonic Weak Decay width of p-shell {\Lambda}-Hypernuclei

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    Previous studies of proton and neutron spectra from Non-Mesonic Weak Decay of eight Lambda-Hypernuclei (A = 5-16) have been revisited. New values of the ratio of the two-nucleon and the one-proton induced decay widths, Gamma_2N/Gamma_p, are obtained from single proton spectra, Gamma_2N/Gamma_p = 0.50 +/- 0.24, and from neutron and proton coincidence spectra, Gamma_2N/Gamma_p = 0.36 +/- 0.14stat +0.05sys -0.04sys , in full agreement with previously published ones. With these values, a method is developed to extract the one-proton induced decay width in units of the free Lambda decay width, Gamma_p/Gamma_Lambda, without resorting to Intra Nuclear Cascade models but by exploiting only experimental data, under the assumption of a linear dependence on A of the Final State Interaction contribution. This is the first systematic determination ever done and it agrees within the errors with recent theoretical calculations.Comment: 16 pages, 3 figures, 2 table

    Enhancement of πAππA\pi A \to \pi\pi A Threshold Cross Sections by In-Medium ππ\pi\pi Final State Interactions

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    We address the problem of pion production in low energy π\pi-nucleus collisions. For the production mechanism we assume a simple model consisting of a coherent sum of single pion exchange and the excitation---followed by the decay into two pions and a nucleon---of the N(1440)N^*(1440) resonance. The production amplitude is modified by the final state interaction between the pions calculated using the chirally improved J\"ulich meson exchange model including the polarization of the nuclear medium by the pions. The model reproduces well the experimentally observed πAππA\pi A \to \pi\pi A cross sections, especially the enhancement with increasing AA of the π+π\pi^+\pi^- mass distribution in the threshold region.Comment: 5 pages RevTeX, 3-eps figure

    Σp\Sigma^- p emission rates in KK^- absorptions at rest on 6^6Li, 7^7Li, 9^{9}Be, 13^{13}C and 16^{16}O

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    An experimental study of the KstopAΣpAK^-_{stop}A\rightarrow \Sigma^- p A' reaction on A=6A=^6Li, 7^7Li, 9^9Be, 13^{13}C and 16^{16}O pp-shell nuclei is presented. The data were collected by the FINUDA spectrometer operating at the DAΦ\PhiNE ϕ\phi-factory (LNF-INFN, Italy). Emission rates for the reaction in the mentioned nuclei are measured and compared with the few existing data. The spectra of several observables are discussed; indications of Quasi-Free absorptions by a (np)(np) pair embedded in the AA nucleus can be obtained from the study of the missing mass distributions.Comment: Version accepted by PR

    Mass discrimination using double-sided silicon microstrip detectors for pions and protons at intermediate energies

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    Prototype silicon detector modules for the vertex detector of the FINUDA experiment were tested at TRIUMF using pions and protons at 270 and 408 MeV/c in order to study their use for mass discrimination based on energy deposition. The detector modules were constructed using double-sided silicon detectors based on the ALEPH design, read out by VA1 integrated circuits. The test modules and apparatus are described, details of the data analysis are discussed, and the results are presented together with GEANT simulations. Particular attention is given to the detector response for the various particles, with signals ranging from the minimum-ionising pions at 408 MeV/c up to 20 times minimum-ionising for the protons at 270 MeV/c. ( 1999 Elsevier Science B.V. All rights reserved

    Hypernuclear spectroscopy with K^- at rest on 7^7Li, 9^9Be, 13^{13}C and 16^{16}O

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    The FINUDA experiment collected data to study the production of hypernuclei on different nuclear targets. The hypernucleus formation occurred through the strangeness-exchange reaction K^-_{stop} + \; ^AZ \rightarrow \; ^A_{\Lambda}Z + \pi^-. From the analysis of the momentum of the emerging π\pi^-, binding energies and formation probabilities of Λ7^7_{\Lambda}Li, Λ9^9_{\Lambda}Be, Λ13^{13}_{\Lambda}C and Λ16^{16}_{\Lambda}O have been measured and are here presented. The behavior of the formation probability as a function of the atomic mass number A is also discussed.Comment: Accepted for publication in PL
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