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Register a new userDelta Delta Excitation in Proton-Proton Induced pi0pi0 Production
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Exclusive measurements of the $pp to pppi^0pi^0$ 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 $T_p ge$ 1 GeV, where the $DeltaDelta$ excitation becomes the leading process. No evidence is found for a significant ABC effect beyond that given by the conventional $t$-channel $DeltaDelta$ excitation. This holds also for the double-pionic fusion to the quasibound $^2$He. The data are compared to model predictions, which are based on both pion 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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Hard bremsstrahlung production in proton-proton collisions has been studied with the ANKE spectrometer at COSY-Juelich in the energy range of 353-800 MeV by detecting the final proton pair {pp}_s from the pp -> {pp}_s reaction with very low excitation energy. Differential cross sections were measured at small diproton c.m. angles from 0 to 20 degrees and the average over this angular interval reveals a broad peak at a beam energy around 650 MeV with a FWHM of about 220 MeV, suggesting the influence of Delta(1232)N intermediate states. Comparison with deuteron photodisintegration shows that the cross section for diproton production is up to two orders of magnitude smaller, due largely to differences in the selection rules.
The parity-violating asymmetry arising from inelastic electron-nucleon scattering at backward angle (~95 degrees) near the Delta(1232) resonance has been measured using a hydrogen target. From this asymmetry, we extracted the axial transition form factor G^A_{NDelta}, a function of the axial Adler form factors C^A_i. Though G^A_{NDelta} has been previously studied using charged current reactions, this is the first measurement of the weak neutral current excitation of the Delta using a proton target. For Q^2 = 0.34 (GeV/c)^2 and W = 1.18 GeV, the asymmetry was measured to be -33.4 pm (5.3)_{stat} pm (5.1)_{sys} ppm. The value of G^A_{NDelta} determined from the hydrogen asymmetry was -0.05 pm (0.35)_{stat} pm (0.34)_{sys} pm (0.06)_{theory}. These findings agree within errors with theoretical predictions for both the total asymmetry and the form factor. In addition to the hydrogen measurement, the asymmetry was measured at the same kinematics using a deuterium target. The asymmetry for deuterium was determined to be -43.6 pm (14.6)_{stat} pm (6.2)_{sys} ppm.
We report on the investigation of $Delta$(1232) production and decay in proton-proton collisions at a kinetic energy of 1.25 GeV measured with HADES. Exclusive dilepton decay channels $ppe^{+}e^{-}$ and $ppe^{+}e^{-}gamma$ have been studied and compared with the partial wave analysis of the hadronic $pppi^{0}$ channel. They allow to access both $Delta^+ to ppi^0(e^+e^-gamma)$ and $Delta^+ to pe^+e^-$ Dalitz decay channels. The perfect reconstruction of the well known $pi^0$ Dalitz decay serves as a proof of the consistency of the analysis. The $Delta$ Dalitz decay is identified for the first time and the sensitivity to N-$Delta$ transition form factors is tested. The $Delta$(1232) Dalitz decay branching ratio is also determined for the first time; our result is (4.19 $pm$ 0.62 syst. $pm$ 0.34 stat.) $times $ 10$^{-5}$, albeit with some model dependence.
The thermally activated proton diffusion in BaZr0.9Y0.1O3-{delta} was studied with electrochemical impedance spectroscopy (IS) and quasi-elastic neutron scattering (QENS) in the temperature range from 300 K to 900 K. The diffusivities for the bulk material and the grain boundaries as obtained by IS obey an Arrhenius law with activation energies of 0.46 eV and 1.21 eV, respectively. The activation energies obtained by IS for the bulk are 0.26 eV above 700 K and 0.46 eV, below 700 K. The total diffusivity as obtained by IS is by one order of magnitude lower than the microscopic diffusivity as obtained by QENS. The activation energies obtained by QENS are 0.13 eV above 700 K and 0.04 eV, below 700 K. At about 700 K, the diffusion constants for IS and QENS have a remarkable crossover, suggesting two processes with different activation energies.
The momentum spectra of K+ produced at small angles in proton-proton and proton-deuteron collisions have been measured at four beam energies, 1.826, 1.920, 2.020, and 2.650 GeV, using the ANKE spectrometer at COSY-Juelich. After making corrections for Fermi motion and shadowing, the data indicate that K+ production near threshold is stronger in pp- than in pn-induced reactions. However, most of this difference could be made up by the unobserved K0 production in the pn case.
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