Session PE: Excited Baryons

Polarization Observables for Kaon Photoproduction from Transversely Polarized Protons

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]. 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. In 2010, double-polarization data were taken at JLab using circularly and linearly polarized photons incident on a transversely polarized frozen spin butanol target [3]. The photons were energy-marked using the Hall B photon tagging system and the reaction products were detected in CLAS. We will present preliminary data of the T, F, and H asymmetries of the K$+$ Lambda and K$+$ Sigma final states with comparisons to predictions of recent multipole analyses. There are very few published measurements of the T asymmetry and none of the F and H asymmetries for the K$+$ Lambda channel. K$+$ Sigma has no published data for these asymmetries. This work is the first of its kind and will significantly broaden the world database for these reactions.\\[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.\\[0pt] [3] C.D. Keith et al., Nucl. Instr. Meth. A694 (2012) 27.   

Double-polarization E and G observables in $\eta$ photo-production on the CLAS Frozen Spin Target

Exclusive $\eta$ photo-production is an important tool in baryon spectroscopy, allowing isolation of N* (I=1/2) resonances. Furthermore, it has long been recognized that due to significant overlapping of resonances in the baryon spectrum, differential cross-sections are not enough; measuring polarization observables is vital. Extensive data have been collected at Jefferson Lab in Hall B with circularly and linearly polarized tagged photon beams incident on longitudinally polarized protons provided by the Frozen Spin Target (FROST). The presentation will cover analysis and preliminary results of double-polarization observables E and G in $\eta$ photo-production - data entirely missing from the world database.   

$\Xi^{-}$ Polarization in Photoproduction with CLAS

The weak decay of hyperons offers a valuable means of measuring their polarization, providing insight into their production mechanisms. Jefferson Lab's CLAS collaboration has utilized this property of weak decays and has published the most precise polarization measurements of $\Lambda$ to date in both photo and electroproduction. No such study however has been published for the $\Xi$ baryon in either photo or electroproduction. High statistics CLAS data was collected in 2008 with a luminosity $68 pb^{-1}$ using a circularly polarized photon beam with energies up to 5.45 GeV and a liquid hydrogen target. This dataset known as ``g12'' is the world's largest for meson photoproduction and provides a first time opportunity to study various aspects of the $\Xi$ baryon in photoproduction. By analyzing the angular distribution of the $\Xi^{-} \to \pi^{-} (\Lambda^{0}) $ decay in the $ \gamma p \to K^{+} K^{+} \Xi^{-} \to K^{+} K^{+} \pi^{-} (\Lambda^{0}) $ reaction, the induced and transferred polarization of the $\Xi^{-} $ can be measured. The data has a nearly background free signal with approximately 4500 events, a globally unprecedented yield for detecting $\Xi^{-} \to \pi^{-} (\Lambda^{0})$ in photoproduction. Preliminary results displaying polarization as a function of beam energy and center of mass $\Xi^{-}$ angle will be presented. Additionally basic features of the data and simulation will be shown including mass spectra and important aspects of the phase space.   

Polarization observables from the photoproduction of $\omega$-mesons using Linearly Polarized Photons

We report on the Spin Density Matrix Elements (SDMEs), for the $\omega$ meson using a beam of linearly polarized photons in the photon energy region of $E_{\gamma} = 1.3$ to $1.9$~ GeV. The angular distributions of the daughter pions from $\omega$ decay give critical information on the mechanisms for the photoproduction of $\omega$ meson. Other observables are easily obtained using the SDMEs, such as the photon beam asymmetry $\Sigma$ or the parity asymmetry $\Sigma_{P}$, which calculated as function of the Mandelstam variables $s$ and $t$ ($\Sigma = \Sigma(s,t)$), constraints on the partial wave analyses used to extract the nucleon excitation spectrum from the available data and help distinguishing between pion exchange and pomeron exchange (VDM model). The SDMEs, $\rho_{MM'}^0$ that parametrize the $\omega$ meson decay angular distribution, provide an understanding in determining the spin and parity of the exchanged particle in the $t$-channel production process and deviations from the Vector Dominance Model (VDM). Since the $\omega$ is an isoscalar, it couples only to $N^*$ states, i.e.~$I=\frac{1}{2}$. This eliminates $\Delta$ resonance production. These preliminary results are from the g8b dataset collected in 2005 with CLAS detector at Jefferson Lab.   

Experimental studies of hadronization and parton propagation in SIDIS

A wealth of new data has become available over the past decade from DESY, Jefferson lab, Fermilab, RHIC and LHC that connect hadron propagation and hadron formation. While Drell-Yan reaction and heavy-ion collions contribute different kinds of information of parton propagation in nuclear environment, the most direct information comes from DIS. Unique feature of semi-inclusive DIS is its ability to investigate time-dependence of handronization by embedding it in the nuclei of varying size. Such studies begun in late 70s in SLAC, and continued with HEMRES and Jefferson lab program. A series of measurements of neutral pion electroproduction, performed on deuterium, carbon, iron and lead targets exposed to a 5.014 GeV electron beam will be presented. The data were collected using the CEBAF Large Acceptance Spectrometer at Jefferson Lab. The goal of the experiment was to measure attenuation of hadrons in a medium of varying size normalized to deuterium. We present preliminary three-fold multiplicity ratios of neutral pions in ($\nu$, Q$^{2}$, $z$) and (Q$^{2}$, z, p$_{T}^{2}$) bins. Combined with extensive data on charged pion states, these data are providing new insights into hadronization mechanism.   

Investigation of the In-Medium Kaon-Nucleon Interaction

One method to study the strong interaction inside of the nucleus is with the absorption of hadrons. The E01-112 experiment in Hall B at the Thomas Jefferson National Accelerator provided data on the photo-production of the $\rm{K^{0}_{s}}$ in nuclei of deuterium, carbon, iron, and lead. The kaon is interesting since the antikaon-nucleon potential is attractive, leading to predictions of strangeness in a dense environment like a neutron star. On the other hand, the kaon-nucleon potential is repulsive, indicating that kaons should traverse the medium with fewer interactions. The absorption of the $\rm{K^{0}_{s}}$ by a bound nucleon inside a nucleus will indicate how the potential changes; is it strengthened or weakened in the medium. In this talk, I will present preliminary transparency ratios versus mass number.   

A Study of the $\rho$--$\omega$ Interference in the Leptonic Decay Channel from Photoproduction off of a $^1$H target at JLab

Although the phenomena of $\rho$--$\omega$ interference has been studied at great length in pionic decay channel over the past 50 years, a study of the interference in a purely electromagnetic production and decay channel has never been performed on an elementary proton target until now. Preliminary results and comparisons with previous studies on heavier nuclear targets will be discussed, as well as how these results can help interpret recent discrepancies in medium modification studies of the $\rho$ and $\omega$ from JLab/CLAS and CBELSA/TAPS.   

Dalitz Decay of Pseudoscalar Mesons from Photoproduction on Hydrogen Target with CLAS

Experimental results on the Dalitz decay of pseudoscalar mesons P($\pi^{0}, \eta, \eta^{\prime} $) $\rightarrow e^{+} e^{-} \gamma$ produced in the photoproduction reaction $\gamma$ + p $\rightarrow$ P($\pi^{0}, \eta, \eta^{\prime} $) + p on a Hydrogen target, with CLAS setup at Thomas Jefferson National Accelerator Facility (TJNAF), are presented. The total statistics collected for $\pi^{0}, \eta$ Dalitz decay exceeds the world's published statistics by an order of magnitude, while the Dalitz decay of $\eta^{\prime} \rightarrow e^{+} e^{-} \gamma$ is observed and measured for the first time. The data obtained will allow for measurements of transition form factors that are encoded in the mass spectrum of the $e^{+} e^{-}$ system. The current status of analysis of the $\pi^{0}, \eta$ transition form factors and the search for the heavy photon from the $e^{+} e^{-}$ mass system will be discussed.   

Baryons at Varying Nc: a study with Lattice QCD and Effective Theory

Recent Lattice QCD (LQCD) calculations of baryon masses with the number of colors $N_c=3,$ 5 and 7 [1] have opened the opportunity for exploring quantitatively the $1/N_c$ expansion of QCD in the baryon sector. While similar studies have been carried out for glueballs and mesons, which show a remarkably well behaved $1/N_c$ expansion down to the real world's $N_c=3$, the results in [1] are the first ones of their kind. The calculations were performed in quenched LQCD and with quark masses giving $M_\pi >400$ MeV. The results are analyzed using an Effective Theory based on the combination of Baryon Chiral Perturbation Theory and the $1/N_c$ expansion [2]. A detailed discussion of the analysis and its implications will be presented, along the lines of a current work in progress [3].\\[4pt] [1] T. DeGrand, Phys. Rev. D 86, 034508\\[0pt] [2] A. Calle Cordon and J. L. Goity, Phys. Rev. D87, 016019\\[0pt] [3] A. Calle Cordon, T. DeGrand and J. L. Goity, work in progress.