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Register a new userInvestigation of the dd -> 3He n pi 0 reaction with the FZ Julich WASA-at-COSY facility
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An exclusive measurement of the dd -> 3He n pi 0 reaction was carried at a beam momentum of p = 1.2 GeV/c using the WASA-at-COSY facility. For the first time data on the total cross section as well as differential distributions were obtained. The data are described with a phenomenological approach based on a combination of a quasi-free model and a partial wave expansion for three-body reaction. The total cross section is found to be sigma(tot) = (2.89 +- 0.01(stat) +- 0.06(sys) +- 0.29(norm)) mu b. The contribution of the quasi-free processes (with the neutron being target or beam spectator) accounts for 38% of the total cross section and dominates the differential distributions in specific regions of the phase space. The remaining part of the cross section can be described within a partial wave decomposition indicating the significance of p-wave contributions in the final state.
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The search for 4He-eta bound states was performed with the WASA-at-COSY facility via the measurement of the excitation function for the dd->3He n pi0 and dd->3He p pi- processes. The beam momentum was varied continuously between 2.127 GeV/c and 2.422 GeV/c, corresponding to the excess energy for the dd->4He eta reaction ranging from Q=-70 MeV to Q=30 MeV. The luminosity was determined based on the dd->3He n reaction and quasi-free proton-proton scattering via dd->pp n_spectator n_spectator reactions. The excitation functions determined independently for the measured reactions do not reveal a structure which could be interpreted as a narrow mesic nucleus. Therefore, the upper limits of the total cross sections for the bound state production and decay in dd->(4He-eta)_bound->3He n pi0 and dd->(4He-eta)_bound->3He p pi- processes were determined taking into account the isospin relation between both the channels considered. The results of the analysis depend on the assumptions of the N* momentum distribution in the anticipated mesic-4He. Assuming as in the previous works, that this is identical with the distribution of nucleons bound with 20 MeV in 4He, we determined that (for the mesic bound state width in the range from 5 MeV to 50 MeV) the upper limits at 90% confidence level are about 3 nb and about 6 nb for npi0 and ppi- channels, respectively. However, based on the recent theoretical findings of the N*(1535) momentum distribution in the N*-3He nucleus bound by 3.6 MeV, we find that the WASA-at-COSY detector acceptance decreases and hence the corresponding upper limits are 5 nb and 10 nb for npi0 and ppi- channels respectively.
Charge symmetry breaking (CSB) observables are a suitable experimental tool to examine effects induced by quark masses on the nuclear level. Previous high precision data from TRIUMF and IUCF are currently used to develop a consistent description of CSB within the framework of chiral perturbation theory. In this work the experimental studies on the reaction dd->4He{pi}0 have been extended towards higher excess energies in order to provide information on the contribution of p-waves in the final state. For this, an exclusive measurement has been carried out at a beam momentum of p=1.2 GeV/c using the WASA-at-COSY facility. The total cross section amounts to sigma(tot) = (118 +- 18(stat) +- 13(sys) +- 8(ext)) pb and first data on the differential cross section are consistent with s-wave pion production.
The excitation function for the $pdrightarrow$ $d p pi^{0}$ reaction has been measured by WASA-at-COSY experiment with the aim of searching for $^{3}hspace{-0.03cm}mbox{He}$-$eta$ mesic nuclei. The measurement in the vicinity of $eta$ meson production was performed using a ramped proton beam. The data analysis and interpretation was carried out with the assumption that the $eta$-mesic Helium decays via the formation of an intermediate N$^{*}$(1535) resonance. No direct signal of the $eta$-mesic nucleus is observed in the excitation function. We determine a new improved upper limit for the total cross section for the bound state production and decay in the $pdrightarrow$ $(^{3}hspace{-0.03cm}mbox{He}$-$eta)_{bound} rightarrow$ $d p pi^{0}$ process. It varies between 13 nb to 24 nb for the bound state with width in the range $Gamma in (5,50)$ MeV.
A series of publications of the WASA collaboration culminates in a recent paper of Pricking, Bashkanov and Clement [1] claiming a dibaryon resonance at 2370 MeV. However, as explained here, there are logical flaws in this result. A natural alternative arises from the reaction pd -> N N*(1440)p_s where p_s is a spectator proton. There is supporting evidence from a recent experiment of Mielke et al. on dp -> 3He-piplus-piminus
New data on the production of single neutral pions in the $pdrightarrow{}^3textrm{He},pi^0$ reaction are presented. For fifteen proton beam momenta between $p_p=1.60;textrm{GeV}/c$ and $p_p=1.74;textrm{GeV}/c$, differential cross sections are determined over a large fraction of the backward hemisphere. Since the only previous systematic measurements of single-pion production at these energies were made in collinear kinematics, the present work constitutes a significant extension of the current knowledge on this reaction. Even this far above the production threshold, significant changes are found in the behaviour of the angular distributions over small intervals in beam momentum.
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