119 research outputs found

    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 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

    In-medium pi-pi Correlation Induced by Partial Restoration of Chiral Symmetry

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    We show that both the linear and the non-linear chiral models give an enhancement of the pi-pi cross section near the 2pi threshold in the scalar-iso-scalar (I=J=0) channel in nuclear matter. The reduction of the chiral condensate, i.e., the partial chiral restoration in nuclear matter, is responsible for the enhancement in both cases. We extract an effective 4pi-nucleon vertex which is responsible for the enhancement but has not been considered in the non-liear models for in-medium pi-pi interaction. Relation of this vertex and a next-to-leading order terms in the heavy-baryon chiral lagrangian, L_piN^(2), is also discussed.Comment: 5 pages, 5 eps figure, REVTe

    πππ\pi \to \pi\pi results in nuclei

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    The Crystal Ball (CBCB) collaboration at BNLBNL has recently presented results regarding a study of the πAπ0π0A\pi^- A \to \pi^0\pi^0 A^\prime reaction on H,D,C,AlH, D, C, Al and CuCu, using a nearly 4π\pi detector. Similar results, but for the π+Aπ+π±A\pi^+ A \to \pi^+\pi^{\pm} A^\prime reaction on 2H^{2}H, 12C^{12}C, 40Ca^{40}Ca, and 208Pb^{208}Pb, have been published earlier by the CHAOSCHAOS collaboration at TRIUMFTRIUMF. In this Brief Report a comparison of the results of the two measurements is made, which shows that the CHAOSCHAOS and CBCB data share relevant common features. In particular, the increase in strength as a function of A seen in the near-threshold π+π\pi^+ \pi^- invariant mass spectra reported by the CHAOSCHAOS group, is also seen in the π0π0\pi^0 \pi^0 CB data, when the results from the two groups are compared in a way which accounts for the different acceptances of the two experiments.Comment: 7 pages, 1 figure, accepted for publication in Phy. Rev. C - Brief Repor

    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.

    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

    Low-energy pions in nuclear matter and 2pi photoproduction within a BUU transport model

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    A description of low-energy scattering of pions and nuclei within a BUU transport model is presented. Implementing different scenarios of medium modifications, the mean free path of pions in nuclear matter at low momenta and pion absorption reactions on nuclei have been studied and compared to data and to results obtained via quantum mechanical scattering theory. We show that even in a regime of a long pionic wave length the semi-classical transport model is still a reliable framework for pion kinetic energies greater than ~20-30 MeV. Results are presented on pion-absorption cross sections in the regime of 10 MeV < E(kin) < 130 MeV and on photon-induced double-pion production at incident beam energies of 400-500 MeV.Comment: 22 pages, 20 figures Replaced with a revised version. Accepted for publication in EPJ A. Added a short section on pion reaction and charge exchange cross-section

    Pionic Degrees of Freedom in Atomic Nuclei and Quasielastic Knockout of Pions by High-Energy Electrons

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    The nonlinear model of pionic condensate in nuclei by G. Preparata can be efficiently verified by investigation of the quasielastic knockout process of pions out of nuclei by high energy electrons. First, a momentum distribution (MD) of the collective pions has a bright maximum at q=0.3 Gev.Second the excitation spectrum of a recoil nucleus is concentrated at low energies E lesser than 1MeV. The results for the pion knockout from mesonic clouds of individual nucleons are absolutely different. The latter results are presented both for pion and rho-meson clouds localized on nucleons.Comment: 13 pages, 3 figure

    Mid-term results and factors affecting outcome of a metal-backed unicompartmental knee design: a case series

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    <p>Abstract</p> <p>Background</p> <p>Controversies exist regarding the indications for unicompartmental knee arthroplasty. The objective of this study is to report the mid-term results and examine predictors of failure in a metal-backed unicompartmental knee arthroplasty design.</p> <p>Methods</p> <p>At a mean follow-up of 60 months, 80 medial unicompartmental knee arthroplasties (68 patients) were evaluated. Implant survivorship was analyzed using Kaplan-Meier method. The Knee Society objective and functional scores and radiographic characteristics were compared before surgery and at final follow-up. A Cox proportional hazard model was used to examine the association of patient's age, gender, obesity (body mass index > 30 kg/m<sup>2</sup>), diagnosis, Knee Society scores and patella arthrosis with failure.</p> <p>Results</p> <p>There were 9 failures during the follow up. The mean Knee Society objective and functional scores were respectively 49 and 48 points preoperatively and 95 and 92 points postoperatively. The survival rate was 92% at 5 years and 84% at 10 years. The mean age was younger in the failure group than the non-failure group (p < 0.01). However, none of the factors assessed was independently associated with failure based on the results from the Cox proportional hazard model.</p> <p>Conclusion</p> <p>Gender, pre-operative diagnosis, preoperative objective and functional scores and patellar osteophytes were not independent predictors of failure of unicompartmental knee implants, although high body mass index trended toward significance. The findings suggest that the standard criteria for UKA may be expanded without compromising the outcomes, although caution may be warranted in patients with very high body mass index pending additional data to confirm our results.</p> <p><b>Level of Evidence</b>: IV</p

    Spectral functions in the sigma-channel near the critical end point

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    Spectral functions in the σ\sigma-channel are investigated near the chiral critical end point (CEP), that is, the point where the chiral phase transition ceases to be first-ordered in the (μ,T)(\mu,T)-plane of the QCD phase diagram. At that point the σ\sigma meson becomes massless in spite of explicit breaking of the chiral symmetry. It is expected that experimental signatures peculiar to CEP can be observed through spectral changes in the presence of abnormally light σ\sigma mesons. As a candidate, the invariant-mass spectrum for diphoton emission is estimated with the chiral quark model incorporated. The results show the characteristic shape with a peak in the low energy region, which may serve as a signal for CEP. However, we find that the diphoton multiplicity is highly suppressed by infrared behaviors of the σ\sigma meson. Experimentally, in such a low energy region below the threshold of two pions, photons from π02γ\pi^0\to2\gamma are major sources of the background for the signal.Comment: 12 pages, 8 figures, 1 figure replaced, minor modification
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