Helicity amplitudes of the Lambda(1670) and two Lambda(1405) as dynamically generated resonances

We determine the helicity amplitudes A(1/2) and radiative decay widths in the transition Lambda(1670) -> gamma Y (Y = Lambda or Sigma(0)). The Lambda(1670) is treated as a dynamically generated resonance in meson-baryon chiral dynamics. We obtain the radiative decay widths of the Lambda(1670) to gamma Lambda as 2 +/- 1 keV and to -gamma Sigma(0) as 120 +/- 50 keV. Also, the Q(2)-dependence of the helicity amplitudes A(1/2) is calculated. We find that the K Xi component in the Lambda(1670) structure, mainly responsible for the dynamical generation of this resonance, is also responsible for the significant suppression of the decay ratio Gamma(gamma A)/Gamma(gamma Sigma 0). A measurement of the ratio would, thus, provide direct access to the nature of the Lambda(1670). To compare the result for the Lambda(1670), we calculate the helicity amplitudes Lambda(1/2) for the two states of the Lambda(1405). Also, the analytic continuation of Feynman parameterized integrals of more complicated loop amplitudes to the complex plane is developed which allows for an internally consistent evaluation of A(1/2).D.J. wishes to acknowledge the hospitality of the University of Valencia, where part of this work was done. The work of M. D. is supported by the DFG (Deutsche Forschungsgemeinschaft, Gz: DO 1302/1-2). This work is partly supported by DGICYT Contract No. FIS2006-03438, the Generalitat Valenciana in the program Prometeo and the EU Integrated Infrastructure Initiative Hadron Physics Project under contract RII3- CT-2004-506078 and the DFG under contract No. GRK683. We also acknowledge the collaboration agreement between the JSPS of Japan and the CSIC of Spain, the Grant for Scientific Research (No. 20028004) and the Grant-in-Aid for the Global COE Program “The Next Generation of Physics, Spun from Universality and Emergence” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. This research was done under Yukawa International Program for Quark-Hadron Sciences. We would like to thank U.-G. Meißner for a careful reading of the manuscript.Peer reviewe