Session E16: Hadronic Physics I

Polarization of Hyperons in Elementary Photoproduction

Recent measurements using the CLAS detector at Jefferson Lab of the reactions $\vec\gamma + p \to K^+ + \vec\Lambda$ and $\vec\gamma + p \to K^+ + \vec\Sigma^0$ have determined the spin transfer coefficients $C_x$ and $C_z$ for the first time. These observables quantify photon circular polarization that is transferred to the recoiling hyperons in the scattering plane. The unexpected result is that $\Lambda$ hyperons are produced ``100\% polarized'', as seen when combining $C_x$ and $C_z$ with the previously-measured induced transverse polarization, $P$. Furthermore, $C_x$ and $C_z$ seem to be linearly related. We present the experimental results and offer a hypothesis which can explain these observations: the produced strange quark, when subjected to a pure spin-orbit type of interaction, preserves its state of polarization throughout the hadronization process. This hypothesis suggests that quark dynamics are relevant to the formation of hadrons even in the ``nucleon resonance'' region of excitation.   

Search for $\Phi(1860)$ Pentaquark States with CLAS

Following the first observations of the possible $S=+1$ pentaquark state $\Theta^{+}(1540)$, there have been many experiments in various laboratories to confirm these results and to search for states which could be associated with other members of the pentaquark antidecuplet. NA49 collaboration reported the observation of narrow $S=-2$ states with masses about $1860$~GeV showing in the $\Xi \pi$ invariant mass spectra. These states were identified as isospin $3/2$ members of the pentaquark antidecuplet. However, other experiments have failed to reproduce these results. A new experiment has recently been performed at Jefferson Lab using the CLAS detector to search for the $\Phi(1860)$ state in photoproduction on a deuterium target. We will present the preliminary results of the analysis in this talk.   

First measurement of coherent $\phi$-meson photoproduction on deuteron

\newcommand{\sigphin}{$\sigma_{\phi N}$} \newcommand{\gevc}{~GeV$^2$/c$^2$} Coherent $\phi$-meson photoproduction on deuteron is studied in a high-statistics photo-deuteron experiment at CLAS with a tagged photon beam ($E_{\gamma}=$ 0.8 -- 3.6~GeV). The cross section and decay angular distributions have been measured for the first time up to a squared four-momentum transfer $t = (p_{\gamma}-p_{\phi})^2 =-2$\gevc. The cross sections are compared with predictions from a re-scattering model. In the framework of vector meson dominance (VMD), the data are consistent with the total $\phi$-N cross section \sigphin ~at about 10~mb. If vector meson dominance is violated, a larger \sigphin ~from the A-dependence experiment~[1] is possible by introducing a larger $t$-slope for the $\phi N\rightarrow\phi N$ process than that for the $\gamma N\rightarrow\phi N$ process. The decay angular distributions follow the prediction from helicity conservation. This measurement demonstrates a new approach to the study of the $\phi$-N interaction in the energy region where VMD may not be a good approximation.\\ ~[1]~ T.~Ishikawa, {\it et al.} Phys. Lett. {\bf B608}, 215 (2005).   

Preliminary Analysis to Extract the $\omega$-N and $\phi$-N Total Cross Sections

There is a scarcity of data for the $\omega$-N and $\phi$-N total cross sections. Such measurements are useful as inputs to theoretical models describing many-nucleon interactions as in the case of Relativistic Heavy Ion collisions. Furthermore, when applied to an optical model, the data can set limits on the broadening of the intrinsic width of the vector mesons in a nuclear medium. Due to the impracticality of building $\omega$- and $\phi$-meson beams, these cross sections can be accessed through secondary interactions. An experiment was completed in Hall B at Jefferson Lab with the proper conditions where the $\omega$- and $\phi$-mesons were produced from a tagged photon beam off various nuclear targets ranging from deuterium to Pb. The vector mesons were detected through their rare decay to $\rm{e^{+}e^{-}}$ in order to eliminate final state interactions of the decay particles. Preliminary normalized yields will be shown as a function of the target atomic number, A, from which the total cross sections will be extracted.   

Medium modifications of the $\rho$ vector meson in photoproduction off nuclei

Theoretical calculations predict the modification of properties of vector mesons, such as a shift in their masses and/or broadening of their widths in dense nuclear matter. These effects can be related to partial restoration of chiral symmetry at high density or temperature. To explore these, we performed an experiment at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). The data were taken with a beam of tagged photons with energies up to 4 GeV on various nuclear targets. The properties of the $\rho$ vector mesons were investigated via their rare leptonic decay to $e^+e^-$ . This decay channel is preferred over hadronic modes in order to eliminate final state interactions in the nuclear matter. The combinatorial background in the mass spectrum was removed by a self-normalizing mixed-event technique. The $\rho$ mass distributions were extracted for each of the targets. We obtained statistically significant results regarding medium modification of the $\rho$ in the nuclear medium that rule out large medium effects (mass shift parameter $\alpha > 0.1$) within the 99\% confidence level.   

New Measurements of the Proton Double-Spin Asymmetries $A_1$ and $A_2$ In and Above the Resonance Region

The CLAS (CEBAF Large Acceptance Spectrometer) EG1b experiment in Hall-B at Jefferson Laboratory utilized a polarized electron beam at various (1.6, 2.5, 4.2, 5.6 GeV) energies and polarized frozen NH$_3$ and ND$_3$ targets to measure target and double-polarization asymmetries of inclusive and exclusive electron-nucleon scattering events. The polarized proton double-spin asymmetries $A_1$ (for $0.05$ GeV$^2 < Q^2 < 4.0$ GeV$^2$) and $A_2$ (for $0.15$ GeV$^2 < Q^2 < 2.0$ GeV$^2$) have been extracted from a combined analysis of the data at the four beam energies. Preliminary results for $A_1$ and $A_2$ in the specified ranges will be shown.   

BigBite: A new large acceptance spectrometer for Jefferson Lab Hall A

A new spectrometer was recently added to the Jefferson lab Hall A experimental setup. This new device, nicknamed Bigbite for its large momentum and angular acceptance, brings new and exciting physics capabilities to Jefferson lab. Bigbite provides a solid angle acceptance of over 75 msr and a momentum acceptance of over 80\% and can be operated with luminocities upto 5 $\times 10^{36}$ cm$^2$s$^{-1}$. This newly commissioned spectrometer was recently used for Jefferson lab experiment E02-013; Measurement of the electric form factor of the neutron in the high $Q^{2}$ range from 1.2 to 3.5~$\mathrm{GeV} ^2$ through $\vec e(\vec{^3He},e'n)$. The new detector package including a set of Multiwire Drift Chambers, a scintillator plane, pre- shower and shower detectors was constructed for the spectrometer. This detector package allows the spectrometer to operate under high rate conditions with counting rates as high as 20 MHz/wire-chamber-plane while achieve a spatial resolution of $\sigma~\sim~200~\mu m$. Now there is a long list of approved Jefferson lab experiments accounting for more than 100 beam days waiting to use the Bigbite spectrometer.