Lineshape of the Lambda(1405) Hyperon Measured Through its Sigma0 pion0 Decay

TL;DR

This study measures the lineshape of the Lambda(1405) hyperon via its Sigma0 pion0 decay, providing insights into its structure and challenging models predicting it as a two-state resonance.

Contribution

First detailed measurement of Lambda(1405) lineshape in the Sigma0 pion0 decay channel, with separation of overlapping states at COSY-Juelich.

Findings

Lambda(1405) lineshape similar to other decay modes

No significant mass shift observed

Results challenge two-state resonance models

Abstract

The pp -> p K+ Y0 reaction has been studied for hyperon masses m(Y0)<1540 MeV/c2 at COSY-Juelich by using a 3.65 GeV/c circulating proton beam incident on an internal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrometer. Such configurations are sensitive to the production of the ground state Lambda and Sigma0 hyperons as well as the Sigma0(1385) and Lambda(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states could be well separated, though with limited statistics. The shape and position of the Lambda(1405) distribution, reconstructed cleanly in the Sigma0 pion0 channel, are similar to those found from other decay modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict Lambda(1405) to be a two-state resonance.