Kei Moriya IU Properties of the Λ(1405) Hyperon Studied at CLAS Using Photoproduction

The internal structure of the Λ(1405) hyperon has remained an unresolved mystery in hadron physics for decades. Observed as a resonance in Σπ just below the N-K threshold , it is the first excited Λ state; yet the nature of this resonance, where it belongs in the hierarchy of hyperons, and some of its basic properties, remain unsettled. In previous experiments, the invariant mass spectrum (lineshape) has been observed to be distorted from a simple Breit-Wigner form, and since the 1960s, this has lead to theoretical speculation on the nature of this resonance and the dynamics involved in forming it. The nature of the Λ(1405) has repercussions, such as to the nature of baryon resonances above 1.9 GeV in mass which may decay to Λ(1405), and also the topic of kaon condensation in the nuclear medium. In recent years, advances in chiral unitary models have made definite predictions for the lineshape of the Λ(1405), based on a strong coupling approach, to dynamically generate it from constituent mesons and baryons. With the CLAS collaboration at Jefferson Lab, we have studied the Λ(1405) via pho-toproduction with unprecedented statistics from a proton target. The large multi-particle acceptance of the CLAS detector allows us to measure and compare the lineshapes and differential cross sections of all three decay modes Σ + π − , Σ 0 π 0 , and Σ − π + from near the production threshold up to center-of-mass energies of 2.84 GeV. As predicted in the chiral unitary models, the line shapes and differential cross sections of the Λ(1405) are indeed quite different for each decay channel. This lends support to the idea that the Λ(1405) has strong dynamical components that distinguish it from other known resonances. In addition, the cross sections of the nearby Σ(1385) and Λ(1520) resonances have also been measured, and comparisons to the Λ(1405) cross section will be shown. Another basic property of the Λ(1405) that has previously been experimentally un-confirmed is its spin and parity quantum numbers. Past experiments have not shown a conclusive result for the expected spin and parity of J P = 1/2 −. In the CLAS experiment we found a substantial polarization of the photoproduced Λ(1405), which is measured through the daughter Σ + , and we determined the parity of the parent Λ(1405) from how this polarization was transferred. Our spin-parity analysis shows that indeed the assignment of 1/2 − …