Session D15: Light Mesons and Baryons

Photoproduction of scalar mesons using CLAS at JLab

The standard quark model makes no allowance for the existence of gluons outside hadrons. However lattice QCD predicts bound states of pure gluons, called glueballs. According to lattice calculations, the lightest of these experimentally unverified particles is expected to have $J^{PC}=0^{++}$. There are three known mesons candidates with this spin and parity that can mix with the glueball: the $f_{0} $(1370), the $f_{0} $(1500) and the $f_{0} $(1710). All of these particles have been reported in various experiments with the exception of photoproduction. The ZEUS collaboration observed a resonance at 1.7 GeV in \textit{ep} collisions (with an exchanged virtual photon). To search for the presence of this resonance in photoproduction, the reaction $\gamma p\to f_{J} p\to K_{s}^{0} K_{S}^{0} p\to 2(\pi^{+}\pi^{-})p$ was analyzed using data from two experiments conducted using the CLAS detector at JLab. The $K_{s} K_{s} $channel was chosen because this fixes the parity of the parent $f_{J} $ particle to be positive. Preliminary results from analysis of these data will be presented   

Preliminary Results of T and F Asymmetries for KLambda Photoproduction from the Proton

The search for undiscovered excited states of the nucleon continues to be a focus of experiments at Jefferson Lab. A large effort has been launched using the CLAS detector to provide the database, which will allow nearly model-independent partial wave analyses 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 [1] 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 polarized photons incident on a transversely polarized frozen spin target (FROST) [2] comprising butanol, operated at the low temperature of 30mK. The reaction products were detected in CLAS using tagged photons. We will present preliminary data of the T and F asymmetries of the K-Lambda final state with comparisons to predictions of recent multipole analyses. There are very few published measurements of the T asymmetry and none of the F asymmetry for the K-Lambda channel. This work is the first of its kind and will significantly broaden the world database for this reaction.\\[4pt] [1] A.V. Anisovich et al., Eur. Phys. J. A48 (2012) 15.2] C.D. Keith et al., Nucl. Instr. Meth. A694 (2012) 27.   

Rotation averaged one-particle-exchange potential in Light Front time-ordered perturbation theory

We present a rotation average of the lowest order two-body interaction diagram in the Light Front time-ordered perturbation theory. By taking advantage of the boost invariance of the Light Front Formulation, and taking an additional rotational average, we restored the complete Lorentz invariance for each individual time-ordered diagram in the Light Front formulation. We show that approperiate expansion of the kernel before taking the rotation average can generate good analytical approximation to the complete Lorentz invariant kernel in a controlled way. The obtained kernel can be used in two body interaction model calculations.   

Nucleon Structure from Lattice QCD Using a Nearly Physical Pion Mass

We report the first lattice QCD calculation using the almost physical pion mass $m_\pi$=149 MeV that agrees with experiment for four fundamental isovector observables characterizing the gross structure of the nucleon: the Dirac and Pauli radii, the magnetic moment, and the quark momentum fraction. The key to this success is excluding the contributions of excited states. An analogous calculation of the nucleon axial charge governing beta decay fails to agree with experiment, and we discuss possible sources of error.   

Lattice study of quark distribution amplitudes in the pion and its excitations

Lattice QCD serves as a computational framework capable of predicting the spectrum of hadronic excitations from first principles. Our desire to describe the wealth of existing experimental data on the spectrum and to predict the outcomes of future experiments poses numerous challenges. Thus, obtaining an accurate resolution of excited states using methods of LQCD is complicated due to the faster decay of excited state correlation functions in Euclidean space in comparison with those of ground states, which we overcome through the use of anisotropic lattices with a finer temporal than spatial discretization. The aim of this project is to go beyond the spectrum to discern the structure of the states through the computation of the quark distribution amplitudes for both the ground and excited pion states on improved anisotropic lattices developed by the Hadron Spectrum Collaboration. Application of variational method allows us to extract the excited-state spectrum. When combined with undergoing parallel perturbative study of renormalization coefficients for quark bilinear operators, this work will enable us to explore the internal structure of the excited states, and to investigate the approach to a quark and gluon description of hadrons when probed at high-momentum transfers.   

Deeply Virtual Exclusive Reactions in a Polarized Electron Ion Collider

An Electron Ion collider is proposed as the next major project for hadronic physics studies at high energy. I briefly review the JLab MEIC proposal, including a full acceptance detector. I focus on the integration of a far-forward spectrometer into the accelerator lattice of the Interaction Point (IP), optimized for detection of the recoil nucleons or nuclei in Deeply Virtual Exclusive Scattering (DVES) reactions. The far forward design features 100\% neutron detection in a cone of 25 mrad around $0^\circ$, complete detection of beam ions at all angles (down to $0^\circ$ for momenta $|p-p_0|/p_0>0.05\%$, where $p_0$ is the beam momentum, and detection of beam ions at all momenta, for scattering angles at the IP greater than 3 mrad. The momentum resolution in the far forward region is $\le 3\cdot 10^{-4}$, limited by the intrinsic beam momentum spread. Spatial imaging of nuclei via DVES in the coherent peak requires measuring transverse momentum transfers to less than $1/R_A$, where $R_A$ is the nuclear rms radius. For all but the lightest nuclei, mapping the coherent peak requires a special high-$\beta^\ast$ tune. I discuss the trade-offs of luminosity, count rate, and spatial resolution for DVES studies on nuclei.   

Exclusive Meson Production in Hall C at JLab 12 GeV

Exclusive meson production plays an important role in probing the quark and gluon distributions in hadrons. The additional flavor degree of freedom in the $H(e,e'K^{+})\Lambda/\Sigma^{0}$ reactions provides a unique opportunity to study the mechanism underlying strangeness production and the transition from hadronic to partonic degrees of freedom. The 12~GeV upgrade at Jefferson Lab provides the energies, and Hall~C, with its heavily-shielded detector setup in a highly-focusing spectrometer, is optimal for precision kaon cross section measurements. A new threshold aerogel Cerenkov detection system provides a simple and economical option for kaon identification. Measurements of reactions with neutral final states allow one to probe universal features of the Generalized Parton Distribution and to verify their formalism in thus far unexplored regimes. The addition of a neutral particle spectrometer in Hall C augments its scientific capabilities to include photon detection from, e.g., $\pi^{0}$ decay or Deeply Virtual Compton Scattering. In this talk I will present the current status and discuss the outlook on future studies of probing quark and gluon distributions through exclusive reactions in Hall C as well as the particle identification requirements for each of these stages.