| 1. |
- Hederström, Veronica, et al.
(author)
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Pollinator-mediated effects of landscape-scale land use on grassland plant community composition and ecosystem functioning – seven hypotheses
- 2024
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In: Biological Reviews. - 1464-7931. ; 99:3, s. 675-698
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Journal article (peer-reviewed)abstract
- Environmental change is disrupting mutualisms between organisms worldwide. Reported declines in insect populations and changes in pollinator community compositions in response to land use and other environmental drivers have put the spotlight on the need to conserve pollinators. While this is often motivated by their role in supporting crop yields, the role of pollinators for reproduction and resulting taxonomic and functional assembly in wild plant communities has received less attention. Recent findings suggest that observed and experimental gradients in pollinator availability can affect plant community composition, but we know little about when such shifts are to be expected, or the impact they have on ecosystem functioning. Correlations between plant traits related to pollination and plant traits related to other important ecosystem functions, such as productivity, nitrogen uptake or palatability to herbivores, lead us to expect non-random shifts in ecosystem functioning in response to changes in pollinator communities. At the same time, ecological and evolutionary processes may counteract these effects of pollinator declines, limiting changes in plant community composition, and in ecosystem functioning. Despite calls to investigate community- and ecosystem-level impacts of reduced pollination, the study of pollinator effects on plants has largely been confined to impacts on plant individuals or single-species populations. With this review we aim to break new ground by bringing together aspects of landscape ecology, ecological and evolutionary plant–insect interactions, and biodiversity–ecosystem functioning research, to generate new ideas and hypotheses about the ecosystem-level consequences of pollinator declines in response to land-use change, using grasslands as a focal system. Based on an integrated set of seven hypotheses, we call for more research investigating the putative pollinator-mediated links between landscape-scale land use and ecosystem functioning. In particular, future research should use combinations of experimental and observational approaches to assess the effects of changes in pollinator communities over multiple years and across species on plant communities and on trait distributions both within and among species.
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| 2. |
- Kren, F., et al.
(author)
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Exclusive measurements of pp -> d pi(+)pi(0) : Double-pionic fusion without ABC effect
- 2010
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In: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 684:2-3, s. 110-113
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Journal article (peer-reviewed)abstract
- Exclusive measurements of the reaction pp -> d pi(+)pi(0) have been carried out at T-p = 1.1 GeV at the CELSIUS storage ring using the WASA detector. The isovector pi(+)pi(0) channel exhibits no enhancement at low invariant pi pi masses, i.e. no ABC effect. Therefore this most basic isovector double-pionic fusion reaction qualifies as an ideal test case for the conventional t-channel Delta Delta excitation process. Indeed, the obtained differential distributions reveal the conventional t-channel Delta Delta mechanism as the appropriate reaction process, which also accounts for the observed energy dependence of the total cross section.
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| 3. |
- Petrén, Henrik
(author)
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eta MESON PRODUCTION IN PROTON-PROTON COLLISIONS AT 40 AND 72 MeV EXCESS ENERGY
- 2011
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In: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 26:3-4, s. 681-682
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Journal article (peer-reviewed)abstract
- Differential cross sections for eta production in proton-proton collisions at 40 and 72 MeV above threshold show that higher partial waves are necessary to describe the reaction. A simple model assuming constant production amplitudes and including most partial waves up to two units of angular momentum reproduces the data rather well, and also describes the invariant mass distributions found at 15.5 MeV excess energy.
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| 4. |
- Petrén, Henrik, et al.
(author)
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eta-meson production in proton-proton collisions at excess energies of 40 and 72 MeV
- 2010
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In: Physical Review C. Nuclear Physics. - 0556-2813 .- 1089-490X. ; 82:5, s. 055206-
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Journal article (peer-reviewed)abstract
- The production of eta mesons in proton-proton collisions has been studied using the WASA detector at the CELSIUS storage ring at excess energies of Q = 40 MeV and Q = 72 MeV. The eta was detected through its 2 gamma decay in a near-4 pi electromagnetic calorimeter, whereas the protons were measured by a combination of straw chambers and plastic scintillator planes in the forward hemisphere. About 6.9 x 10(4) and 9.3 x 10(4) events were found at Q = 40 MeV and Q = 72 MeV, respectively, with background contributions of less than 5%. A simple parametrization of the production cross section in terms of low partial waves was used to evaluate the acceptance corrections. Strong evidence was found for the influence of higher partial waves. The Dalitz plots show the presence of p waves in both the pp and the eta {pp} systems and the angular distributions of the eta in the center-of-mass frame suggest the influence of d-wave eta mesons.
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| 5. |
- Petrén, Henrik, 1975-
(author)
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η Meson Production in Proton-Proton Collisions at Excess Energies of 40 and 72 MeV
- 2009
-
Doctoral thesis (other academic/artistic)abstract
- The production of η mesons in proton-proton collisions has been studied with the WASA detector using internal pellet targets in the CELSIUS storage ring at the The Svedberg Laboratory in Uppsala. Data were taken at two beam energies, 1360 MeV and 1445 MeV, corresponding to CM excess energies of 40 and 72 MeV, respectively. The η was detected via its 2γ decay in a near-4π electromagnetic calorimeter, whereas the protons were measured by a combination of straw chambers and plastic scintillator planes in the forward direction. The measurements were kinematically complete. The analysis yielded 69·103 events at 1360 MeV and 93·103 events at 1445 MeV, with a background contribution of less than 5%. Data were acceptance-corrected using a parametrization of a matrix element which includes all states up to two units of total angular momentum. The final state interaction between protons in the 1S0 state was included by a momentum-dependent enhancement factor in the relevant amplitudes. Angular distributions of the final state, invariant mass spectra and Dalitz plots are presented. The cos θη-distribution is found to be anisotropic with its maximum at 90° at both energies. From the parametrization it is inferred that this is due to interference between the Ss and Sd final states. A significant contribution from the Pp final state is also needed to describe data.
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| 6. |
- Schönning, Karin, et al.
(author)
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The pd →3Heηπ0 reaction at Tp = 1450 MeV
- 2010
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In: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 685:1, s. 33-37
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Journal article (peer-reviewed)abstract
- The cross section for the pd→3Heηπ0 reaction has been measured at a beam energy of 1450 MeV using the WASA detector at the CELSIUS storage ring and detecting one 3He and four photons from the decays of the two mesons. The data indicate that the production mechanism involves the formation of the Δ(1232) isobar. Although the beam energy does not allow the full peak of this resonance to be seen, the invariant masses of all three pairs of final particles are well reproduce dby a phase space Monte Carlo simulation weighted with the p-wave factor of the square of the π0 momentum in the 3He π0 system.
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| 7. |
- Skorodko, T., et al.
(author)
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Delta Delta excitation in proton-proton induced pi(0)pi(0) production
- 2011
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In: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 695:1-4, s. 115-123
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Journal article (peer-reviewed)abstract
- Exclusive measurements of the pp -> pp pi(0)pi(0) reaction have been performed at CELSIUS/WASA at energies from threshold up to T-p = 1.3 GeV. Total and differential cross sections have been obtained. Here we concentrate on energies T-p >= GeV. where the Delta Delta excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional t-channel Delta Delta excitation. This holds also for the double-pionic fusion to the quasibound He-2. The data are compared to model predictions, which are based on both pi- and rho-exchange. Total and differential cross sections are at variance with these predictions and call for a profound modification of the rho-exchange. A phenomenological modification allowing only a small rho-exchange contribution leads to a quantitative description of the data.
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| 8. |
- Skorodko, T, et al.
(author)
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Two-pion production in proton-proton collisions : experimental total cross sections and their isospin decomposition
- 2009
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In: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 679:1, s. 30-35
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Journal article (peer-reviewed)abstract
- The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to T-P = 1.36 GeV. Total and differential Cross sections have been obtained for the channels pn pi(+)pi(0), pp pi(+)pi(-), pp pi(0)pi(0) and also nn pi(+)pi(+). For intermediate incident energies T-P > 1GeV, i.e. in the region which is beyond the Roper excitation but at the onset of Delta Delta excitation, the total pp pi(0)pi(0) cross section falls behind theoretical predictions by as much as all order of magnitude near 1.2 GeV, whereas the nn pi(+)pi(+) cross section is a factor of five larger than predicted. An isospin decomposition of the total cross sections exhibits a s-channel-like energy dependence in the region of the Roper excitation as well as a significant contribution of an isospin 3/2 resonance other than the Delta(1232). As possible candidates the Delta(1600) and the Delta(1700) are discussed. (C) 2009 Elsevier B.V. All rights reserved.
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