Properties of the Lambda(1405) Hyperon Studied at CLAS Using Photoproduction

Conference · Fri Jul 01 00:00:00 EDT 2011

OSTI ID:1994965

Moriya, Kei

The internal structure of the Lambda(1405) hyperon has remained an unresolved mystery in hadron physics for decades. Observed as a resonance in sigma pi just below the N-Kbar threshold, it is the first excited Lambda 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 1960's, this has lead to theoretical speculation on the nature of this resonance and the dynamics involved in forming it. Dalitz was among the first to point out that the strong attraction of the N-Kbar channel near threshold could produce a quasi-bound state that can only decay to the below-threshold Sigma pi mode. The otherwise highly successful constituent quark model has difficulties in assigning the Lambda(1405), since naively the mass is calculated to be almost degenerate with the Lambda(1520). In recent years, advances in chiral unitary models have made definite predictions for the line shape of the Lambda(1405), based on a strong coupling approach, to dynamically generate the Lambda(1405) from constituent mesons and baryons. Furthermore, the chiral unitary model predicts that there are two I = 0 poles for the Lambda(1405), one coupling strongly to the Sigma pi decay mode, and the other a narrower resonance that couples more strongly to N-Kbar. The composite nature of the Lambda(1405) has repercussions, such as to the nature of baryon resonances above 1.9 GeV in mass which may decay to Lambda(1405), and also the topic of kaon condensation in the nuclear medium. Past experiments have shown distortions of the line shape of the Lambda(1405), but had statistics too limited to draw strong conclusions. With the CLAS collaboration at Jefferson Lab, we have studied the Lambda(1405) via photoproduction 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 Sigma^+ pi^?,Sigma0pi0, and sigma? pi+ 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 Lambda(1405) are indeed quite different for each decay channel. This lends support to the idea that the Lambda(1405) has strong dynamical components that distinguish it from other known resonances. In addition, the cross sections of the nearby Sigma(1385) and Lambda(1520) resonances have also been measured, and comparisons to the Lambda(1405) cross section will be shown. Another basic property of the Lambda(1405) that has previously been experimentally unconfirmed is its spin and parity quantum numbers. Past experiments have not shown a conclusive result for the expected spin and parity of JP= 1/2?. In the CLAS experiment we found a substantial polarization of the photoproduced Lambda(1405), which is measured through the daughter Sigma+, and we determined the parity of the parent Lambda(1405) from how this polarization was transferred. Our spin-parity analysis shows that indeed the assignment of 1/2? is correct.

• Research Organization: Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Nuclear Physics (NP)
• DOE Contract Number: AC05-06OR23177
• OSTI ID: 1994965
• Report Number(s): JLAB-PHY-11-1596; DOE/OR/23177-6964
• Resource Relation: Conference: PANIC11 - 19th Particles & Nuclei International Conference, MIT, Cambridge, MA, USA, July 24, 2011
• Country of Publication: United States
• Language: English