PoS - Proceedings of Science
Volume 281 - The 26th International Nuclear Physics Conference (INPC2016) - QCD: Partonic phenomena - Thursday 15
Charge Symmetry Breaking In The $dd \to {}^4$He$\pi^0$ Reaction With WASA-at-COSY
M.K. Zurek* on behalf of the WASA-at-COSY Collaboration
*corresponding author
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Pre-published on: May 04, 2017
Published on: May 09, 2017
Abstract
The presented study concentrates on the charge symmetry breaking reaction $dd \rightarrow {}^4\text{He}\pi^0$. The aim is to provide experimental results for comparison with predictions from chiral perturbation theory (ChPT) to study effects induced by quark masses on the hadronic level, e.g., the proton-neutron mass difference. First calculations showed that more data are required, in addition to the existing high-precision measurements from TRIUMF and IUCF, for a precise determination of the parameters of ChPT. These new data should comprise the measurement of the charge symmetry forbidden $dd \rightarrow {}^4\text{He}\pi^0$ reaction at sufficiently high energy to get access to the higher partial waves contributions, especially $p$-wave. A first result from the WASA-at-COSY experiment at an excess energy of $Q = 60$ MeV did not allow for a decisive interpretation because of limited statistics. In this paper the second measurement of the $dd \rightarrow {}^4\text{He}\pi^0$ reaction at $Q = 60$ MeV using an improved WASA detector setup aiming at higher statistics is described. The total and differential cross sections have been determined. The preliminary result of the total cross section is $\sigma^{\text{tot}}_{\text{prel}} = \left(79.1 \pm 7.3(\text{stat.}) ^{+1.2}_{-10.5}(\text{syst.}) \pm 8.1 (\text{norm.}) \pm 2.0 (\text{lumi. syst.})\right) \text{pb}$. The angular distribution has been described up to second order in $\cos\theta^*$ with a function of the form $\text{d}\sigma/\text{d}\Omega = a + b\cos^{2}\theta^*$, where $\theta^*$ is the scattering angle of the pion in the c.m. coordinate system. The obtained preliminary parameters $a$ and $b$ are $a_{\text{prel}} = \left(1.75 \pm 0.46 (\text{stat.}) ^{+0.31}_{-0.8} (\text{syst.})\right)\text{pb/sr}$ and $b_{\text{prel}} = \left(13.6 \pm 2.2 (\text{stat.}) ^{+0.9}_{-2.7} (\text{syst.})\right)\text{pb/sr}$.
DOI: https://doi.org/10.22323/1.281.0310
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