The radiative decay of the $\Lambda(1405)$ and its two-pole structure

Abstract

We evaluate theoretically the radiative decay widths into $\gamma\Lambda$ and $\gamma\Sigma^0$ of the two poles of the $\Lambda(1405)$ found in chiral unitary theories and we find quite different results for each of the two poles. We show that, depending on which reaction is used to measure the $\Lambda(1405)$ radiative decays, one gives more weight to one or the other pole, resulting in quite different shapes in the $\gamma\Lambda(\Sigma^0)$ invariant mass distributions. Our results for the high-energy pole agree with those of the empirical determination of the $\gamma\Lambda$ and $\gamma\Sigma^0$ radiative widths (based on an isobar model fitting of the $K^-p$ atom data), which are sometimes referred to as ``experimental data''. We have made a detailed study of the $K^-p\to\pi^0\gamma\Lambda(\Sigma^0)$ and $\pi^-p\to K^0\gamma\Lambda(\Sigma^0)$ reactions and have shown that they, indeed, lead to different shapes for the $\gamma\Lambda(\Sigma^0)$ invariant mass distributions.