Erschienen in:135 p (2017). = Univeristy of Cologne, Diss., 2016
Sprache:
Englisch
Entstehung:
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Beschreibung:
Probing elementary symmetries and symmetry breaking tests our understanding of the theory of strong forces, Quantum Chromodynamics. The presented study concentrates on the charge symmetry forbidden reaction dd→4Heπ0. The aim is to provide experimental results for comparison with predictions from Chiral Perturbation Theory (χPT) to study effects induced by quark masses on the hadronic level, e.g., the proton-neutron mass difference. First calculations showed that in addition to the existing high-precision data from TRIUMF and IUCF, more data are required for a precise determination of the parameters of χPT. These new data should comprise the measurement of the charge symmetry forbidden dd→4Heπ0 reaction at sufficiently high energy, where the p-wave contribution becomes important. A first measurement with the WASA-at-COSY experiment at an excess energy of ε = 60 MeV was performed, but the results did not allow for a decisive interpretation because of limited statistics. This thesis reports on a second measurement of the dd→4Heπ0 reaction at ε = 60 MeV using an improved WASA detector setup aiming at higher statistics. A sample of 336 ± 43 event candidates have been extracted using a data set from an eight-week long beamtime, and total and differential cross sections have been determined. The angular distribution has been described with a function of the form dσ/dΩ = a + bcos^2θ*, where θ* is the scattering angle of the pion in the c.m. coordinate system. The obtained parameters a and b and the total cross section are: a = (1.75 ± 0.46(stat.) +0.31-0.8(syst.)) pb/sr, b = (13.6 ± 2.2(stat.) +0.9-2.7(syst.)) pb/sr, σ_tot = (79.1 ± 7.3(stat.) +1.2-10.5(syst.) ± 8.1(norm.) ± 2.0(lumi. syst.)) pb. For this experiment a modified detector setup optimized for a time-of-flight measurement of the forward going particles has been used. After detector calibration and track reconstruction, signal events have been selected using a chain of cuts and a kinematic fit. For absolute normalization the integrated luminosity has been ...