Speaker
Description
The structure of the $\Lambda(1405)$ resonance has challenged 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 $\pi\Sigma$ and $\bar{K}N$ poles. Experimental results vary in line shape and peak position, depending on reaction type, making their interpretation very challenging.
The $\Sigma^+\pi^-$ and $\Sigma^-\pi^+$ decay channels of the $\Lambda(1405)$ 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 $\Lambda(1405)$ from $\Sigma(1385)$ by measuring the decay channel $\Sigma^0\pi^0$, which is not allowed for the latter. The exclusive analysis in the channel $p(4.5\;\mathrm{GeV}) + p \to p + K^+ + \Lambda(1405) \to \Sigma^0(\Lambda(p + \pi^-) + \gamma) + \pi^0(\gamma + \gamma)$ 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 $\Lambda(1405)$ and its peak position. In my presentation, I will discuss 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 result of my analysis is inclusive $\Sigma^0$ production reconstructed via $\Sigma^0 \to \Lambda + \gamma$ and $\Lambda \to p + \pi^-$. The results of this analysis will be presented, too.
[1] G. Agakishiev et al., "Baryonic resonances close to the anti-KN threshold: The case of Λ(1405) in pp collision," Phys. Rev. C 87, 025201 (2013).
