Photo- and electroproduction of $\Lambda(1405)$ via $\gamma^{(*)}p o K^+\pi^+\Sigma^-$

摘要

In this work, we perform a \textit{toy-model} analysis for the unpolarizedphoto- and electroproduction of $\Lambda(1405)\equiv\Lambda^*$ via$\gamma^{(*)}p\to K^+\pi^+\Sigma^-$ by employing the effective Lagrangianapproach at the tree level only. We consider that $\Lambda^*$ consists of thehigh-mass ($H$) and low-mass ($L$) poles as suggested by the chiral unitarymodel (ChUM). We determine all the model parameters including couplingconstants and cutoff parameters for the phenomenological strong form factors byusing the theoretical information from ChUM and available experimental data.The electromagnetic (EM) form factors for the two poles are parameterizedappropriately being similar to that for the neutron by employing the ChUMestimates for the EM root-mean-squared (rms) radii for the two poles. Weobserve from the numerical calculations that, for the photoproduction of$\Lambda^*$, the interference between the two poles turns out to bedestructive, resulting in the single-peak line at$M_{\pi^+\Sigma^-}\approx1405$ MeV in the invariant-mass distribution. On thecontrary, there appear two peaks in the distribution for the electroproductionas observed in the CLAS/Jlab electroproduction data, due to the constructiveinterference, which is mainly caused by the Dirac form factors for the twopoles. From this observation, we conclude that, in order to explain the photo-and electroproduction data simultaneously, i.e., \textit{single} and\textit{double} peaks, respectively, in the invariant-mass distribution, theinterference patterns for the two poles should follow those suggested by ChUM.In turn, it is a strong theoretical and experimental supports for the two-polestructure scenario for $\Lambda^*$.