Wednesday, October 9, 2013
Recent CMU PhD Graduate Kei Moriya's Paper Selected as Editors' Suggestion
A recent paper entitled "Differential photoproduction cross sections of the Σ(1385), Λ(1405) and Λ(1520)", by Kei Moriya and the CLAS Collaboration, in Physical Review C 88, 045201 (2013), has been selected as an Editors' Suggestion. Kei Moriya, a recent Ph.D. graduate from the CMU Physics Department working with Professor Reinhard Schumacher, is now a postdoc at Indiana University. His and his collaborators work at the Jefferson Laboratory in Newport News, Virginia. As a service to both readers and authors, Editors' Suggestions are a small number of papers published in Physical Review that the editors and referees find of particular interest, importance, or clarity.
Summary of the paper:
Hyperons are particles related to protons and neutrons but which contain one or more “strange” quarks in place of the basic “up” and “down” quarks. They are unstable, living only long enough to travel several centimeters in the laboratory before decaying to lighter particles. With suitable particle physics detectors, however, they can be studied in detail. Properties of “strange” particles are useful to understand in areas as diverse as CP violation, the collective behavior of baryons in nuclei, and the structure of neutron stars. In the paper suggested to readers by the Physical Review editors, the production cross sections for the first three “excited” hyperons, the Σ(1385), the Λ(1405), and the Λ(1520), were measured fully for the first time in the reaction where a photon (quantum of light) impinges upon a proton target. Note that the numbers in parentheses are the mass-energy of the respective particles in MeV, to be compared to the 938 MeV mass-energy of a proton. This production process can “blow the proton apart” and create out of the vacuum a new quark pair consisting of a strange quark and an anti-strange quark, using the energy of the photon. In the laboratory, the end result is appearance of a K meson and one of the given hyperons. Theoretical models for how this can happen are brought to bear on the data, and these models in turn probe questions of how Quantum Chromodynamics, the theory of the strong interaction, works in this energy regime.
The theoretical models were based on the effective-Lagrangian approach, and the accuracy of the predictions was seen to vary widely. The cross sections for the Λ(1405) region were seen to be strikingly different in the three available Σπ decay modes, in contradiction to what the simple quark model would demand. This indicated the effect of isospin interference in the production mechanism of the Λ(1405) that is not seen in the cases of the other two hyperons. This suggested that there may be “new” isospin-one baryonic structure lurking underneath the Λ(1405), which is something that had been tentatively suggested by so-called chiral unitary models and models based on a diquark ansatz of baryonic structure. This line of research has sparked community interest and is being followed up at accelerator laboratories in Germany (Bonn/ELSA) and Japan (Osaka/Spring-8).