| 1. |
- Bargholtz, Christoph, et al.
(author)
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Production of the ω meson in the pd -> 3He ω reaction at 1450 MeV and 1360 MeV
- 2009
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In: Physical Review C. Nuclear Physics. - 0556-2813 .- 1089-490X. ; 79:4, s. 044002-
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Journal article (peer-reviewed)abstract
- The production of ω mesons in the pd→3Heω reaction has been studied at two energies near the kinematic threshold, Tp=1450 MeV and Tp=1360 MeV. The differential cross section was measured as a function of the ω c.m. angle at both energies over the whole angular range. Whereas the results at 1360 MeV are consistent with isotropy, strong rises are observed near both the forward and backward directions at 1450 MeV. Calculations made using a two-step model with an intermediate pion fail to reproduce the shapes of the measured angular distributions and also underestimate the total cross sections
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| 2. |
- Bashkanov, M., et al.
(author)
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Double-Pionic Fusion of Nuclear Systems and the "ABC" Effect : Approaching a Puzzle by Exclusive and Kinematically Complete Measurements
- 2009
-
In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:5, s. 052301-
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Journal article (peer-reviewed)abstract
- The ABC effect-a puzzling low-mass enhancement in the pi pi invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn -> d pi(0)pi(0) at beam energies of 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a (pi pi)(I=L=0) channel phenomenon associated with both a resonancelike energy dependence in the integral cross section and the formation of a Delta Delta system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.
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| 3. |
- Bashkanov, M., et al.
(author)
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Low-mass pi pi enhancement in baryonic pi pi production : ABC effect revised by exclusive measurements
- 2007
-
In: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 22:2-3, s. 625-628
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Journal article (peer-reviewed)abstract
- The ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of double-pionic fusion to nuclear bound states - has been investigated since 40 years by inclusive measurements without providing a conclusive interpretation. First exclusive measurements to this topic have now been carried out at CELSIUS-WASA. They confirm a huge ππ threshold enhancement, prove it to be of scalar-isoscalar nature, i.e., a σ-channel phenomenon and reveal the theoretically predicted high-mass enhancement in inclusive spectra to be of πππ rather than of ππ nature - falsifying thus corresponding model predictions for the ABC effect. From the kinematically complete data samples we infer that the interaction between the two Δ particles created in this process is of crucial importance.
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| 4. |
- Bashkanov, M., et al.
(author)
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Measurement of the slope parameter for the η → 3π0 decay in the pp → ppη reaction
- 2007
-
In: Physical Review C. Nuclear Physics. - 0556-2813 .- 1089-490X. ; 76:4, s. 048201-
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Journal article (peer-reviewed)abstract
- The CELSIUS-WASA setup is used to measure the 3π0 decay of η mesons produced in pp interactions with beam kinetic energies of 1.36 and 1.45 GeV. The efficiency-corrected Dalitz plot and density distributions for this decay are shown, together with a fit of the quadratic slope parameter α yielding α = −0.026 ± 0.010(stat) ± 0.010(syst). This value is compared to recent experimental results and theoretical predictions.
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| 5. |
- Berlowski, M., et al.
(author)
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Measurement of eta meson decays into lepton-antilepton pairs
- 2008
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In: Physical Review D. Particles and fields. - : American Physical Society. - 0556-2821 .- 1089-4918. ; 77:3, s. 032004-
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Journal article (peer-reviewed)abstract
- A search for rare lepton decays of the eta meson was performed using the WASA detector at CELSIUS. Two candidates for double Dalitz decay eta -> e(+)e(-)e(+)e(-) events are reported with a background of 1.3 +/- 0.2 events. This allows to set an upper limit to the branching ratio of 9.7x10(-5) (90% CL). The branching ratio for the decay eta -> e(+)e(-)gamma is determined to (7.8 +/- 0.5(stat)+/- 0.8(syst))x10(-3) in agreement with world average value. An upper limit (90% CL) for the branching ratio for the eta -> e(+)e(-) decay is 2.7x10(-5) and a limit for the sum of the eta ->mu(+)mu(-)mu(+)mu(-) and eta ->pi(+)pi(-)mu(+)mu(-) decays is 3.6x10(-4).
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| 6. |
- Calén, Hans, et al.
(author)
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Measurement of η meson decays into lepton-antilepton pairs
- 2007
-
Reports (other academic/artistic)abstract
- A search for rare lepton decays of the η meson was performed using the WASA detector at CELSIUS. Two candidates for double Dalitz decay η→e+e−e+e− events are reported with a background of 1.3±0.2 events. This allows to set an upper limit to the branching ratio of 9.7×10−5 η(90% CL). The branching ratio for the decay η→e+e−γ is determined to (7.8±0.5stat ±0.8syst)×10−3 in agreement with world average value. An upper limit (90% CL) for the branching ratio for the η→e+e− decay is 2.7 × 10−5 and a limit for the sum of the η→μ+μ−μ+μ− and η→π+π−μ+μ− decays is 3.6 × 10−4.
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| 7. |
- Keleta, Samson, 1971-
(author)
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Double Pion Production in the dd to αππ reaction
- 2008
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Doctoral thesis (other academic/artistic)abstract
- The reaction dd → 4Heππ has been measured exclusively for the first time. The measurements were performed at Td = 712 MeV and Td = 1029 MeV, with the WASA detector assembly at CELSIUS (Uppsala-Sweden). The aim was to investigate a long standing puzzle, the so called ABC effect. This is a peculiar feature of double pion production in light nuclei collisions. In addition to confirming previous experimental observations in this regard, our results also reveal a strong angular dependence of the pions in the overall center of mass system as well as non-isotropic angular distribution of the low mass enhancement in the ππ invariant mass. The results are qualitatively reproduced by a theoretical model, according to which, the ABC effect is described as a result of kinematical enhancement in the independent production of the pion pairs from two parallel and independent NN → dπ reactions.
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| 8. |
- Keleta, S, et al.
(author)
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Exclusive measurement of two-pion production in the dd -> He-4 pi pi reaction
- 2009
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In: Nuclear Physics A. - : Elsevier BV. - 0375-9474 .- 1873-1554. ; 825:1-2, s. 71-90
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Journal article (peer-reviewed)abstract
- The results from the first kinematically complete measurement of the dd -> He-4 pi pi reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar pi pi-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV. with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low pi pi-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, Our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent NN -> d pi sub-processes. (C) 2009 Elsevier B.V. All rights reserved.
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| 9. |
- Khakimova, O., et al.
(author)
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Observation of the abc effect in the first exclusive measurements of pn -> d pi(0)pi(0)
- 2007
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In: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 22:2-3, s. 617-620
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Journal article (peer-reviewed)abstract
- The reaction pn → dπ0π0 constitutes the most fundamental system for the study of the ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of the ππ system in double-pionic fusion to nuclear bound states. The first exclusive measurements of this reaction have now been carried out at CELSIUS-WASA. They exhibit a striking low-mass enhancement in the ππ invariant mass spectrum, prove it to be a σ-channel phenomenon and falsify conventional theoretical models, which predict an associated high-mass enhancement not present in the new exclusive data. In the kinematically complete data sample we observe a ΔΔ excitation, where the interaction between the two Δ obviously plays a crucial role for the generation of the ππ low-mass enhancement.
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| 10. |
- Kren, F., et al.
(author)
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THE pp -> d pi(+)pi(0) REACTION - A CASE OF Delta Delta EXCITATION WITHOUT ABC-EFFECT
- 2009
-
In: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 24:2-3, s. 561-563
-
Journal article (peer-reviewed)abstract
- The ABC effect, a low-mass enhancement in the invariant pp mass, is observed in double-pionic fusion reactions leading to a bound nuclear system in the final state. From previous measurements there have been indications that this phenomenon is resctricted to the sigma channel of the pp system. With exclusive measurements of the pp -> d pi(+)pi(0) reaction at T-p = 1.1 GeV we demonstrate that, indeed, the ABC effect does not occur in the vector-isovector Delta Delta channel (rho channel) despite the fact that the Delta Delta excitation is oberved to be the dominant reaction process. We also show that this reaction is well described by a t-channel Delta Delta excitation with the subsequent decay into the vector-isovector Delta Delta channel given by the rho channel operator.
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