25-29 September 2017
Salamanca, Spain
Europe/Zurich timezone

## $\eta$- and $\pi^0$-production in proton-deuteron fusion to ${}^3\text{He}X$ with WASA-at-COSY

26 Sep 2017, 19:00
2h
Auditorium Hall

#### Auditorium Hall

Poster Spectroscopy of mesons

### 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 $\text{p}\text{d} \rightarrow {}^3\text{He}X$ 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 $\text{p}\text{d} \rightarrow {}^3\text{He}\eta$ 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 $\text{p}\text{d} \rightarrow {}^3\text{He}\eta$ 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 $\Delta Q \approx 4.8\;\text{MeV}$ 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.

### Primary author

Mr Nils Huesken (WWU Muenster)

### Co-authors

Prof. Alfons Khoukaz (WWU Muenster) Mr Florian Bergmann (WWU Muenster) Mr Kay Demmich (WWU Muenster)