FAIR next generation scientists - 8th Edition Workshop

The structure of $\mathrm{\Lambda }(1405)$ resonance challenges hadron physicists for more than forty years. Its structure is controversially debated as either: an antikaon-nucleon bound state, a dynamically generated baryon resembling a meson-baryon molecule, or a resonance with $\pi \mathrm{\Sigma }$ and $\overline{K}N$ poles. Experimental results vary in line shape and peak position, depending on reaction type, making their interpretation very challenging.

The ${\mathrm{\Sigma }}^{+}\pi$ and ${\mathrm{\Sigma }}^{-}{\pi }^{+}$ decay channels of the $\mathrm{\Lambda }(1405)$ had already been studied by HADES in $p+p$ collisions at 3.5 GeV, revealing a peak position below 1400 MeV/c${}^2$ [1]. The new HADES electromagnetic calorimeter (ECAL) enables separation of the $\mathrm{\Lambda }(1405)$ from $\mathrm{\Sigma }(1385)$ by measuring the decay channel ${\mathrm{\Sigma }}^0{\pi }^0$, which is not allowed for the latter. The exclusive analysis in the channel $p+K^{+}+\mathrm{\Lambda }(1405)\to {\mathrm{\Sigma }}^0(\mathrm{\Lambda }(p+{\pi }^{-})+\gamma )+{\pi }^0(\gamma +\gamma )$ with missing ${\pi }^0$ is the most promising for HADES because of the largest acceptance. The main goal is to measure the line shape of $\mathrm{\Lambda }(1405)$ and peak position. In my presentation, I will present the analysis strategy and show preliminary results. The strategy and its verifications are confronted with simulation results.

I will stress the importance of a new Forward Detector covering the very forward region (${0.5}^{\circ }<\theta <6^{\circ }$). Although it covers only a small fraction of the solid angle, it significantly increases acceptance for the hyperons.

An intermediate results of my analysis is inclusive ${\mathrm{\Sigma }}^0$ production reconstructed via ${\mathrm{\Sigma }}^0\to \mathrm{\Lambda }+\gamma$ and $\mathrm{\Lambda }\to p+{\pi }^{-}$. The results of this analysis will be presented, too.

[1] G. Agakishiev et al. Baryonic resonances close to the anty-KN threshold: The case of $\mathrm{\Lambda }$(1405) in pp collision. Phys. Rev. C87:025201, 2013.