Session MC: Mini-Symposium on Baryons with Spatial and Flavor Excitations II

Why We Need Neutron Data for Pion Photoproduction

An overview of the GW SAID group effort to analyze new pion photoproduction on neutron-target. The main database contribution came from the recent CLAS and MAMI unpolarized and polarized measurements. The differential cross section for the processes $\gamma $n$\to \pi $N was extracted from new measurements accounting for Fermi motion effects in the impulse approximation (IA) as well as NN and $\pi $N effects beyond the IA. The EM resonance coupling results are compared to other recent studies.   

The A2 Experiment Program at MAMI

The Mainz Microtron MAMI is an accelerator for electron beams run by the Institut f\"ur Kernphysik of the Johannes Gutenberg-Universit\"at Mainz used for hadron physics experiments. Of it's three active experimental halls, the A2 facility, which features the presence of the SLAC Crystal Ball detector, has produced a plethora of experimental results, which has contributed to the understanding of the structure of the nucleon. An overview and update of the current A2 program will be presented.   

Pion Electroproduction and VCS in the $\Delta $ Resonance Region

The study of the N to $\Delta $ transition has been a subject of intense scientific interest for more than two decades. The pion electroproduction and VCS channels of the transition allow, through the measurement of the transition quadrupole amplitudes, the exploration for non-spherical angular momentum amplitudes in hadrons while the VCS channel also provides access to the nucleon polarizabilities. Results from the recent JLab/Hall-A and MAMI experiments will be presented and future prospects will be discussed.   

Electroproduction of $\pi^0$ in the resonance region at high $Q^2$ with CLAS

An extensive program is underway at Jefferson Lab to study the eletromagnetic excitations of baryon states. We report the analysis of exclusive single $\pi^0$ electroproduction in the resonance region at Jefferson Lab in the $Q^2$ range of 2 to 6 $GeV^2/c^2$. A longitudinally polarized $5.75$ $GeV$ electron beam was incident on a 5 cm long liquid Hydrogen target. The CLAS spectrometer at Jefferson Lab was used to detect the final state particles. The data was taken between October 2001 and January 2002. Preliminary results for differential cross sections over the entire $4\pi$ $c.m.$ solid angle will be presented. This high precision measurement will allow us to access the structure and dynamics of nucleon excitations with masses up to 2 $GeV$.   

The Beam-Helicity Asymmetries for the Reactions $\gamma p \rightarrow p K^{+} K^{-}$, $\gamma p \rightarrow \Xi^{-} K^{+} K^{+}$, and $\gamma p \rightarrow p \pi^{+} \pi^{-}$

The first time measurement of the angular dependence of the beam-helicity asymmetry is demonstrated for the reactions $\gamma p \rightarrow p K^{+} K^{-}$ and $\gamma p \rightarrow \Xi^{-} K^{+} K^{+}$. Both results are compared to the beam-helicity asymmetry of the reaction $\gamma p \rightarrow p \pi^{+} \pi^{-}$. The asymmetry for $\gamma p \rightarrow p K^{+} K^{-}$ is further studied as a function of the kinematical variables photon beam energy, invariant mass of the $K^{+} K^{-}$ system, and invariant mass of the $p K^{-}$ system. It is shown that the asymmetry is sensitive to these variables and thus are key observables in the modeling of the reaction's dynamics. The data obtained was from the CLAS g12 experiment.   

Cross Sections of K$^{0}\Lambda $ Photoproduction off the Deuteron

There has been great effort in mapping out the spectroscopic decay of proton resonances. Similar experiments of neutron resonances must also be investigated since the photocoupling will be different. The decay to K$^{0}\Lambda $ from a resonant neutron state was explored using g10 data from Jefferson National Laboratory. The data were obtained from a liquid deuterium target placed in the CEBAF Large Acceptance Spectrometer (CLAS). The photon beam energies analyzed range from 1.0 to 3.0 GeV. The hadrons K$^{0}$ and $\Lambda $ were identified by their corresponding decay products $\pi^{+}\pi^{-}$ and p$\pi^{-}$ respectively. The yield from this channel was determined using invariant mass cuts and missing mass techniques. The differential and total cross sections are now computed.   

The $\gamma n \to K^0 \Lambda$ photoproduction studied with an electromagnetic calorimeter complex FOREST

Nucleon resonance have been experimentally studied by means of meson production reactions for understanding low-energy scale QCD. Photoproduction is one of the useful tools to reveal properties of excited nucleons. Indeed, the $\pi$ and $\eta$ photoproduction reactions have been intensively investigated until now. Kaon photoproduction is the best probe to study highly excited nucleons, which hardly couple to $\pi N$ and $\eta N$. Simultaneous $K^0 \Lambda$ production is more advantageous than $K^+ \Lambda$ production which is reported by many experimental groups. It is because the kaon exchange is forbidden in the $K^0 \Lambda$ photoproduction, and because Born term contribution is small. The $\gamma d \to (K^0 \Lambda) p \to (\pi^0 \pi^0 p \pi^-)p$ reaction is experimentally investigated with an electromagnetic calorimeter FOREST at Research Center for Electron Photon Science, Tohoku University. The $K^0$ and $\Lambda$ particles are clearly observed in $\pi^0 \pi^0$ and $\pi^- p$ invariant mass distributions. We will present the current status of the exclusive $\gamma n \to K^0 \Lambda$ reaction.   

Cascade Photoproduction at CLAS

The doubly-strange baryon spectrum has received some attention recently with the advent of using an unflavored photon beam to excite protons to the ``Cascade'' ($\Xi$) states. There are several potential advantages with such an experiment when one considers the narrowness of the signals claimed from strangeness-beam experiments carried out in the 60's and 70's. I will present recent results from CLAS at Jefferson Lab including cross sections and upper-limits, after which I will discuss physical and theoretical implications.   

Study of the Hyperon-Nucleon Interaction in Exclusive $\Lambda$ Photoproduction off the Deuteron

Understanding the nature of the nuclear force in terms of the fundamental degrees of freedom of the theory of strong interaction, Quantum Chromodynamics (QCD), is one of the primary goals of modern nuclear physics. While the nucleon-nucleon (NN) interaction has been studied for decades, a systematic description of the NN potential has been achieved only recently with the development of low-energy Effective Field Theories (EFT). To obtain a comprehensive understanding of the strong interaction, dynamics involving strange baryons must be studied. Currently, little is known about the properties of the hyperon-nucleon (YN) and the hyperon-hyperon (YY) interactions. In this talk I will describe our current research of the $\Lambda n$ interaction using the E06-103 experiment performed with the CLAS detector in Hall B at Jefferson Lab. The large kinematic coverage of the CLAS combined with the exceptionally high quality of the experimental data allows to identify and select final-state interaction events in the reaction $\gamma d\rightarrow K^+\Lambda n$ and to establish their kinematical dependencies. The large set of observables we aim to obtain will provide tight constraints on modern YN potentials. I will present the current status of the project and will discuss future incentives.   

Nucleon Structure in realistic QCD on a lattice

I will present recent advances in computing nucleon structure from first principles on a lattice and discuss their connection to current experiments. Making reliable predictions from Quantum Chromodynamics requires numerical methods. After decades of development of theory and computing, lattice QCD is now capable of calculations with realistic parameters. Calculating structure of the proton and the neutron is critical to both validating lattice QCD methods and providing theory counterpart to modern experimental efforts to study hadron structure, with proton charge radius, electromagnetic form factors, and proton spin origin being the most notable. In addition, searches for deviations from the Standard Model, such as neutron-antineutron oscillation, require specific nucleon matrix elements, which can be computed only on a lattice without model assumptions.   

Construction of antikaon-nucleon potential based on chiral unitary approach

We study the single-channel local potential for the antikaon-nucleon system focusing on the behavior of the scattering amplitude in the complex energy plane. In previous works, the local potential has been constructed to reproduce the scattering amplitude derived from the coupled-channel chiral unitary approach on the real energy axis. In this work, analyzing the scattering amplitude in the complex energy plane, we find that the scattering amplitude is not completely reproduced away from the real axis. Especially, the pole structure of the Lambda(1405) is qualitatively different from the original one in the chiral unitary approach. It is considered that the difference of the pole structure affects the physical observables in few-body systems. Therefore, we discuss the method to improve the local potential for a better description of the scattering amplitude in the complex energy plane.