HADRON 2017

η- and π0-production in proton-deuteron fusion to 3HeX with WASA-at-COSY

26 Sept 2017, 19:00

2h

Auditorium Hall

Poster Spectroscopy of mesons Poster session

Speaker

Mr Nils Huesken (WWU Muenster)

Description

The production of mesons in proton-deuteron fusion has historically received considerable interest as a natural way to study the interaction between mesons and nuclear matter through final state interactions. Thus, the near threshold regions for the production of $\eta$ and ${\pi }^0$ mesons in $\text{p}\text{d}\to {}^3\text{He}X$ are already well explored, yet still subject of active research. Especially the ${}^3\text{He}\eta$ final state is of great interest, as a sharp rise of the total cross section within the first few MeV of excess energy has repeatedly been linked to the possible existence of a ${}^3\text{He}\eta$ (quasi-)bound state. However, at higher excess energies, the database for these reactions becomes much more sparse. Here, the $\text{p}\text{d}\to {}^3\text{He}\eta$ centre-of-mass angular distributions are forward peaked already at low excess energies, in contrast to theoretical calculations based on two-step models. This observation has raised questions about a change of production mechanism within an excess energy interval in which the $\text{p}\text{d}\to {}^3\text{He}\eta$ reaction can nicely be studied with the WASA-at-COSY experiment. For this reason, a measurement of this reaction at 15 excess energies between $Q_{\eta }\approx 13\text{MeV}$ and $Q_{\eta }\approx 81\text{MeV}$ with a step size of $\mathrm{\Delta }Q\approx 4.8\text{MeV}$ was performed in 2014. Precise angular distributions can be extracted, that allow for the first time to study their behavior with rising energy over a large excess energy region, without involving systematical uncertainties between different experimental setups, that have hindered detailed comparisons in the past.
In case of the ${}^3\text{He}{\pi }^0$ final state, there exists a sizable database for collinear production $\cos {\vartheta }_{{\pi }^0}^{\text{cm}}=\pm 1$, whereas not much information is available on angular distributions. While a smooth decrease of the cross section is observed for forward scattered pions, the database for $\cos {\vartheta }_{{\pi }^0}^{\text{cm}}=-1$ exhibits a minimum in the energy region covered by the dataset presented. With our measurement, a detailed extraction of the differential distributions between $\cos {\vartheta }_{{\pi }^0}^{\text{cm}}\approx -1$ and $\cos {\vartheta }_{{\pi }^0}^{\text{cm}}\approx -0.4$ is performed for excess energies between $Q_{{\pi }^0}\approx 426\text{MeV}$ and $Q_{{\pi }^0}\approx 494\text{MeV}$.
The current status of the analyses regarding both the ${}^3\text{He}\eta$, as well as the ${}^3\text{He}{\pi }^0$ final state will be presented.

Presentation materials

poster-salamanca.pdf