Session CC: Mini Symposium on Meson Photoproduction and Nucleon Resonance Spectroscopy

The study of excited nucleons and their structure

The nucleon excitation spectrum reflects properties of the quark-gluon system and their interaction. Quark models and Lattice QCD make predictions of masses and quantum numbers of the excited states and their internal structure according to radial, spin, and orbital transitions of the 3-quark and gluon system. Pion induced transitions revealed many nucleon states consistent with these predictions, but many predicted states have not been observed, especially at higher masses. The quest for a better understanding of the underlying symmetries and the internal structure of baryons has led to a worldwide effort to measure the nucleon excitation spectrum using electromagnetic induced processes. Jefferson Lab is playing a key role in this effort. Differential cross sections and polarization observables have been measured with unprecedented precision. Some of the data have been used in coupled-channel resonance analyses that led to new evidence for a number of excited states that were previously unobserved or lacked strong evidence. In this talk, I will discuss two directions of experimental research, the search for new nucleon states using meson photoproduction and the study of transitions form factors to understand their internal structure and the nature of excited states.   

Recent Results of Polarization Observables for Kaon Photoproduction at CLAS

The search for undiscovered excited states of the nucleon continues to be a focus of experiments at Jefferson Lab. Recent LQCD calculations are consistent with the long standing quark model predictions of many more states than have so far been identified [1]. A large effort for the N* program has been launched using the CLAS detector to provide the database that will allow nearly model-independent extraction of the pseudo scalar meson production amplitudes to be carried out in the search for such states. Polarization observables play a crucial role in this effort, as they are essential in disentangling overlapping resonant and non-resonant amplitudes. Recent coupled-channel analyses [2] have found strong sensitivity of the K$+$ Lambda channel to several higher mass nucleon resonances. An overview of the collected experimental data from various experiments (unpolarized hydrogen or polarized butanol targets) using circularly and linearly polarized photon beams with comparisons to predictions of recent multipole analyses and the latest analysis results will be presented. Results from CLAS significantly broaden the world database in many regards and have an impact on multipole analyses.\\[4pt] [1] R.G. Edwards et al., Phys Rev D84 074508 (2011).\\[0pt] [2] A.V. Anisovich et al., Eur. Phys. J. A48 (2012) 15.   

Polarization Observables for $\gamma p\rightarrow K^{+}\Lambda$ using polarized photons on a longitudinally polarized target

The latest highlights of the $N^\ast$ program at Jefferson Lab were photo-production experiments using frozen-spin targets inside the CLAS detector. All combinations of beam-target double-polarization observables were obtained for multiple reactions. Thanks to the self-analyzing properties of $\Lambda$ all possible polarization observables for $K^{+}\Lambda$ are being extracted from the data resulting in a complete determination of the $K\Lambda$ amplitude, thus a least model-dependent determination of any resonances coupling to this channel. This talk will discuss the results of the first run period in 2007/2008 with longitudinally polarized frozen-spin target for this reaction: the beam-target asymmetries $E$ and $G$ as well as the target-recoil asymmetries $L_{x'}$ and $L_{z'}$.   

Meson Photoproduction on Polarized Deuterium

A successful description of the nucleon excitation spectrum is a basic test of how well the underlying forces are understood. Recent Lattice-QCD calculations have supported the predictions of $SU(6)\times0(3)$ quarks models for many ``missing'' levels which have yet to be observed. Most of these are predicted to couple weakly to the well-studied $\pi N$ channel. Stronger couplings are predicted to other decay channels such as $\pi\pi N$, $\rho N$ and $K\Lambda$, which can be studied in photo-production. Polarization observables on both polarized proton and neutron targets are needed to disentangle reaction mechanisms at the amplitude level. While polarized-proton data is comparatively abundant, data on the polarized neutron has been essentially non-existent. During 2011-2012, JLAB carried out experiment E06-101 (g14 run with CLAS) to measure pseudoscalar-meson photo-production reactions using circularly and linearly polarized beam on an HD-Ice target, where quasi-free kinematics of the ($\sim 25\%$ polarization) longitudinally polarized deuterium may be used to approximate a polarized neutron. This talk summarizes the status of on-going analyses of the $\pi p$, $K\Lambda$, $K^0\Sigma^0$, $\pi\pi n$, $\pi\Delta$, and $\rho n$ channels to extract relevant polarization observables.   

Studies of $\eta$ photo-production in the baryon resonance excitation region with CLAS

Because the pseudoscalar $\eta$ meson has zero isospin, exclusive $\eta$ photo-production offers the feature of isolating N* (I=1/2) resonance states. This ``isospin'' filter property can be very useful in helping disentangle the broad and overlapping excitations that make up the nucleon resonance spectrum. For that reason, a program of measurements on $\eta$ photo-production for the reaction $\gamma p \rightarrow \eta p$ has been conducted using the Hall B CEBAF Large Acceptance Spectrometer at Jefferson Lab. Results for the differential cross-section and for single- and double-polarization observables will be summarized. The general outlook for the ``complete experiment'' program for disentangling nucleon resonances decaying to the $p \eta$ channel will be discussed.   

Baryon Resonance determination from Lattice QCD

There has been a resurgence of interest in spectroscopy with a new generation of experiments worldwide, for example BES III, COMPASS, GSI/Panda, and Jefferson Lab's GlueX project as well as CLAS12. Spectroscopy reveals fundamental aspects of hadronic physics. However, the excited spectrum of light quark mesons and baryons is not well determined nor understood. Lattice QCD is quite amenable to such non-perturbative studies. Recent calculations have determined a version of the highly excited spectrum of baryons with zero and non-zero strangeness. Current efforts are aimed at determining the resonance structure of such states. I will report on these recent calculatuions and outline methods that are used for resonance parameter determination from the lattice.   

Recent developments in meson production analyses within a dynamical coupled-channels approach of J\"ulich-Athens-Washington model

The recent developments in the excited-baryon-analysis program by the J\"ulich-Athens/GA-Washington/DC Collaboration will be presented. The program's analysis is based on a dynamical coupled-channels approach developed by the Collaboration [1,2], where the basic symmetries, such as the two-body unitarity, analyticity, and gauge invariance are respected. In the hadronic reactions sector, the $\pi N$, $\eta N$, $K\Lambda$, and $K\Sigma$ channels are included, in addition to the effective $\pi\pi N$ channels $\sigma N$, $\rho N$, and $\pi\Delta$. Energies up to $\sqrt{s} = 2.2$ GeV are considered in the newest version. In the photon-induced reactions sector, the neutral and charged pion photoproduction processes are considered so far [2]. These are currently being extended to higher energies including the $\eta N$, $K\Lambda$, and $K\Sigma$ channels. Here, in particular, gauge invariance is enforced as dictated by the generalized Ward-Takahashi identity.\\[4pt] [1] D. R\"onchen \textit{et al.}, Eur.\ Phys.\ J.\ A \textbf{49}, 44 (2013).\\[0pt] [2] F. Huang \textit{et al.}, Phys.\ Rev.\ C \textbf{85}, 054003 (2012).