Session Q11: Photoproduction and Dynamics of Light Mesons

Measurement of the Beam Asymmetry, I$^{\,\odot}$ and the Helicity Difference, $P_z^{\odot}$ in $\vec{\gamma} \vec{p}\to p\pi^+\pi^-$ with CLAS spectrometer at JLab

The study of baryon resonances helps provide a deeper understanding of the strong interaction since the properties of resonance states give insight as to the dynamics and relevant effective degrees of freedom for a baryonic QCD system. Higher-lying excited states at and above 1.9 GeV/$c^2$ are generally predicted to have strong couplings to the $\pi \pi N$ final states via $\pi\Delta$ or $\rho N$ intermediate states. Double-pion photoproduction is therefore important tool to investigate properties of higher-mass resonances. The CLAS g9a (FroST) experiment, as part of the $N^{\star}$ program at Jefferson Laboratory, has accumulated photoproduction data using linearly- and circularly-polarized photons incident on a longitually-polarized butanol target in the photon energy range 1.20 to 2.32 GeV. In this contribution, the extraction of the beam asymmetry using circularly polarized photons and the helicity difference for the reaction $\vec{\gamma} \vec{p}\to p\pi^+\pi^-$ will be described. Our preliminary results for the beam asymmetry are in overall good agreement with previous CLAS data.   

Gauge-invariant theory of two-pion photoproduction off the nucleon

A field-theoretical description of the photoproduction of two pions off the nucleon is derived that applies to real as well as virtual photons. The Lorentz-covariant theory is complete at the level of all explicit three-body mechanisms of dressed interacting hadrons. The full three-body reaction dynamics of the interacting $\pi\pi N$ system is accounted for by the Faddeev-type ordering structure of the Alt-Grassberger-Sandhas equations. The formulation is valid for hadronic two-point and three-point functions dressed by arbitrary internal mechanisms provided all associated electromagnetic currents are constructed to satisfy their respective (generalized) Ward-Takahashi identities. It is shown that coupling the photon to the Faddeev structure of the underlying hadronic two-pion production mechanisms results in a natural expansion of the full two-pion photoproduction current $M_{\pi\pi}^\mu$ in terms of multiple loops involving two-body subsystem scattering amplitudes of the $\pi\pi N$ system that preserves gauge invariance as a matter of course order by order in the number of loops. A closed-form expression is presented as well for the entire gauge-invariant current $M_{\pi\pi}^\mu$ with complete three-body dynamics.   

Photoproduction of $\Phi$(1020) meson in neutral decay mode $\gamma p \to \phi p \to p K_S K_L$ from CLAS

Using photoproduction data on hydrogen target collected with CLAS detector at Thomas Jefferson National Accelerator Facility the $\phi$(1020) meson production cross-sections and spin density matrix elements in the neutral decay mode $\phi \to K_S K_L$ are obtained for the first time. The preliminary results will be presented for photon energy range $E_{\gamma} = 1.6-2.6$ GeV. The measurements show that the Pomeron exchange mechanism is dominating at low momentum transfer, but this mechanism alone is not sufficient to describe data at high t. Here other processes, such as intermediate resonance exchanges, are changing the cross-section behavior.   

Phi meson photoproduction near threshold energies

$\phi$ meson photoproduction is discussed within an effective Lagrangian approach, as a part of a combined analysis of meson production reactions. Particular emphasis is given to the analysis of the recent photoproduction data from the LEPS Collaboration, where a bump structure at $\sqrt s \approx$ 2.15 GeV has been observed. Effects of the possible spin-1/2 and -3/2 resonances are investigated as well as the various background contributions, such as the pseudo-scalar and scalar $t$-channel meson exchanges, in addition to the pomeron exchange.   

Meson Spectroscopy in Coherent Production off $^4$He with CLAS

One of the main goals of the CLAS experiment eg6 at Jefferson Lab is to study the mesonic states in the coherent quasi-real photo production on $^4$He with the CEBAF 6 GeV electron beam. Studying the $\pi^0\eta$ and $\pi^0\eta'$ final states in the $t$-channel on hydrogen is challenging because the same final state particles can be produced by baryon resonances. However, the background from baryon resonances can be suppressed when the scattered nucleus is detected intact. In addition, production on a spin and isospin zero target simplifies PWA (Partial Wave Analysis). The final states $\pi^0\eta$ and $\pi^0\eta'$ have $C$=+1 and $P$=(-1)$^L$, where $L$ is the angular momentum of the system. The $L$ can be determined by analyzing the angular distribution of the decay products of the final states. In case of $L$=1, the produced system will have exotic quantum numbers $J^{PC}$=1$^{-+}$, which are not possible for ordinary $q\bar{q}$-bar mesons. To study these final states, the CLAS detector, together with a new radial time projection chamber for detection of low energy recoil $^4$He nuclei and the CLAS inner calorimeter, were used. The status of the calibration and the analysis of the data, taken in 2009, will be presented.   

The Search for a $\pi_1(1400)$ Exotic Meson in the $\gamma p\to\Delta^{++}\eta\pi^-$ System with CLAS

Over twenty years ago QCD-inspired models of hadronic states suggested the existence of mesons beyond the Naive Quark Model (NQM), which motivated a rigorous search for exotic mesons. The lightest of these states is the $\pi_1(1400)$ decaying to $\eta\pi^-$, this was observed in E852 at Brookhaven and confirmed in data previously taken for VES at IHEP. Photoproduction is predicted to favor production of the $J^{PC}=1^{-+}$ state resulting in the increase of the ratio of $\pi_1$ to $a_2$ mesons. A Partial Wave Analysis was conducted on the reaction $\gamma p\to\Delta^{++}X\to p\pi^+\pi^-(\eta)$, using the $\Delta^{++}$ to select the $\pi$ exchange. The analysis has shown the final spectra of the resonance decaying to $\eta\pi^-$ to be dominated by the quantum state of $J^{PC}=2^{++}$ corresponding to the presence of the $a_2(1320)$. The $J^{PC}=1^{-+}$ state, shows no structure in the intensity distribution. However, the phase difference between the $J^{PC}=1^{-+}$ and $J^{PC}=2^{++}$ amplitudes show the interference between the two states. The PWA mass independent fits, along with the mass dependent fits of the partial wave results, will be shown.   

Compton Scattering and Photo-absorption Sum Rules on Nuclei

We revisit the photo-absorption sum rule for real Compton scattering from the proton and from nuclear targets. In analogy with the Thomas-Reiche-Kuhn sum rule appropriate at low energies, we propose a new ``constituent quark model'' sum rule that relates the integrated strength of hadronic resonances to the scattering amplitude on constituent quarks. We study the constituent quark model sum rule for several nuclear targets. In addition we extract the $J=0$ pole contribution for both proton and nuclei. Using the modern high energy proton data we find that the $J=0$ pole contribution differs significantly from the Thomson term, in contrast with the original findings by Damashek and Gilman. We discuss phenomenological implications of this new result.   

A Gaussian Sum-Rules Analysis of Scalar Meson-Glueball Mixing

Gaussian QCD sum-rules (GSRs) are well-suited to a study of meson-glueball mixing due to their balanced sensitivity to ground and excited states. The GSRs corresponding to both diagonal and non-diagonal (crossed) two-point correlators between scalar-isoscalar (non-strange) quark currents and gluonic currents will be presented. For the crossed correlator, we show that perturbative and gluon condensate contributions are chirally-suppressed as compared to non-perturbative effects of the quark condensate, mixed condensate, and instantons, implying that the mixing of quark mesons and gluonium is of a non-perturbative origin. Independent predictions of the masses and relative coupling strengths stemming from the three GSRs (two diagonal and one non-diagonal) are remarkably consistent with a scenario of two states with masses of approximately $ 1\;{\rm GeV}$ and $1.4\;{\rm GeV}$ that couple to significant mixtures of quark and gluonic currents. The mixing is nearly maximal with the heavier mixed state having a slightly larger gluonium content than the lighter state.   

Generalized Linear Sigma Model Framework for Scalar and Pseudoscalar Mesons Below 2 GeV

Within the framework of a generalized linear sigma model that includes two nonets of scalar mesons (a two-quark nonet and a four-quark nonet) and two nonets of pseudoscalar mesons (a two-quark nonet and a four-quark nonet) a collective description of scalar and pseudoscalar mesons below 2 GeV will be presented. Particularly, the quark contents of these states as well as their roles in various low-energy processes (such as various scattering amplitudes as well as decays) will be discussed.