Session JC: Hadron Structure Theory

CTEQ-Jefferson Lab (CJ) global analysis of parton distributions

We present new sets of next-to-leading order parton distribution functions (PDFs) determined by global fits to a wide variety of data for hard scattering processes. The analysis includes target mass and higher twist corrections needed for the description of deep-inelastic scattering data at large $x$ and low $Q^2$, and nuclear corrections for deuterium targets. The PDF sets correspond to three different models for the nuclear effects, and provide a more realistic uncertainty range for the $d$ quark PDF compared with previous fits. Applications to particle production cross sections at colliders are also discussed.   

New formulation of target mass corrections in deep-inelastic lepton-nucleon scattering

The description of deep-inelastic lepton-nucleon scattering at finite values of $Q^2$ requires subleading corrections such as those arising from target mass corrections (TMCs) to be accounted for, particularly at large values of the parton momentum fraction $x$. The standard method of incorporating TMCs via Nachtmann moments of structure functions, constructed from local operators within the operator product expansion, is known to have practical difficulties reproducing the corresponding Cornwall-Norton moments of the structure functions without TMCs. We consider several potential resolutions of this paradox, and discuss consequences for the interpretation of parton distributions at finite values of $Q^2$.   

Comparative study of nonperturbative heavy quarks in the nucleon

We perform an analysis of the role of nonperturbative (or intrinsic) charm in the nucleon. Charm is generated nonperturbatively through Fock state expansions of the nucleon wave function to include five-quark virtual states involving charmed mesons and baryons. We consider contributions from a variety of charmed meson-baryon states and find surprisingly dominant effects from the $\bar{D}^{* 0}\, \Lambda_c^+$ configuration. Particular attention is paid to the existence and persistence of high-$x$ structure for intrinsic charm, and the $x$ dependence of the $c-\bar c$ asymmetry predicted in meson-baryon models. We also discuss efforts to constrain intrinsic charm via a forthcoming global QCD analysis, and the possibility of extracting intrinsic strangeness using a similar approach.   

Systematics of Nuclear Effects in Polarized $^3$He Structure Functions and Asymmetries

We present a detailed analysis of nuclear effects in inclusive electron scattering from polarized $^3$He nuclei, including for polarization asymmetries and structure function moments, in both the nucleon resonance and deep-inelastic scattering regions. We compare the results of calculations within the weak binding approximation (WBA) at finite $Q^2$ with several commonly used ansaetze for simplifying the nuclear corrections, and assess their impact on extractions of the free neutron structure. In addition, we make predictions for the $Q^2$ dependence of quasielastic (QE) scattering from polarized $^3$He, data on which can be used to constrain the nucleon smearing functions in $^3$He. As a check of the formalism, we compare the WBA calculations in the QE region with the world's data on QE electron-deuteron cross sections.   

Relativistic pion loop corrections to the electromagnetic nucleon coupling

We present a relativistic formulation of pion loop corrections to the coupling of photons with nucleons on the light-front. Vertex and wave function renormalization constants are computed to lowest order in the pion field, including their nonanalytic behavior in the chiral limit, and studied numerically as a function of the ultraviolet cutoff. Particular care is taken to explicitly verify gauge invariance and Ward-Takahashi identity constraints to all orders in the $m_\pi$ expansion. The results are used to compute the chiral corrections to matrix elements of local operators, related to moments of deep-inelastic structure functions. Finally, comparisons of results for pseudovector and pseudoscalar coupling allows the resolution of a long-standing puzzle in the computation of pion cloud corrections to structure function moments.   

Cross section and nucleon analyzing power of \textit{nd} breakup scattering in FSI, QFS and Space Star kinematic conditions at $E_{lab} =$14.1 MeV

We study inelastic neutron-deuteron scattering on the basis of the configuration-space Faddeev-Noyes equations [1]. The Merkuriev-Gignoux-Laverne approach [2] is generalized for arbitrary nucleon-nucleon potentials and an arbitrary number of partial waves. Neutron-deuteron breakup amplitudes are calculated using the Argonne AV14 nucleon-nucleon potential at the incident neutron energy 14.1 MeV. To calculate breakup amplitudes we take into account all orbital angular momenta of subsystems $\ell $ and $\lambda \le $ 4, the total angular momentum of a pair nucleons $j \le $ 3, and the total three-body angular momentum $M$ up to 13/2$.$ The angular distribution and nucleon analyzing power A$_{y} $have been calculated for Final State Interaction, Quasi Free Scattering, and Space Star configurations. The results are compared with the experimental data and the prediction of the Bochum group [3]. 1. S.P. Merkuriev, Ann. Phys. (N.Y.) \textbf{130}, 3975 (1980); S.P. Merkuriev, Acta Physica (Austriaca) Suppl. XXIII, 65 (1981). 2. S.P. Merkuriev, C. Gignoux and A. Laverne, Ann. Phys. \textbf{99}, 30 (1976). 3. W. Gl\"ockle, H. Wita\l a, D. H\"{u}ber, H. Kamada, J. Golak, Phys. Rep. 274 (1996) 107-285.   

Measurement of the Isovector Giant Quadrupole Resonance of $^{124}$Sn at HI$\gamma$S

The isovector giant quadrupole resonance (IVGQR) is challenging to characterize due to its broad width and low cross section relative to the nearby giant dipole resonance. We have measured the IVGQR of $^{124}$Sn via Compton scattering of nearly 100\% linearly polarized photons at the High Intensity $\gamma$-ray Source (HI$\gamma$S). The term representing the interference of the E1 and E2 amplitudes in the ratio of the out-of-plane to in-plane scattering cross sections changes sign when going from forward to backward angles [1]. This provides enhanced sensitivity to the observation of the IVGQR, significantly reducing the errors in the extracted resonance parameters. The out-of-plane to in-plane scattering ratio was measured as a function of beam energy from 20-34 MeV at angles of 55$^{\circ}$ and 125${^\circ}$ using the 8-element HI$\gamma$S NaI Detector Array (HINDA). The IVGQR parameters were then obtained by simultaneously fitting the scattering ratios at both angles. This measurement continues our effort to establish the A-dependence of the IVGQR parameters across a wide range of masses. The new results will be presented, and the current status of our ongoing program will be summarized. \\[4pt] [1] S. Henshaw \textit{et al}, Phys. Rev. Lett. \textbf{107}, 222501 (2011).   

A New EM CKM Matrix: Implications of the Nucleon Strange Quark Content, Anomalous Magnetic Moments of Nucleons and Electric and Magnetic Nucleon Form Factors

A new electromagnetic neutral-current quark mixing matrix, analog to the well-known Cabibbo-Kobayashi-Maskawa (CKM) weak charge-current matrix, is proposed to account for the strange quark content of the neutron and proton and part of the anomalous axial vector magnetic moments. The EM-CKM matrix is shown to be equivalent to the weak-CKM matrix following an EM to weak gauge symmetry transformation, demonstrating the universality of the Standard Model (SM) CKM quark mixing matrix. The electric and magnetic form factors are reformulated using a new QCD three quark nucleon gyromagnetic factor, Dirac and Pauli form factors and anomalous kappa factors. The old 1943 Jauch form factors which have been systematically used and developed for many years is shown to be in stark disagreement with the new global set of experimental polarized electron-proton scattering data whereas the reformulated SM parameter set of this study is shown to agree very well, lending strong support for this new EM SM approach.   

Meson Production off the Deuteron

An accurate evaluation of the electromagnetic couplings N$^{\ast}$ ($\Delta^{\ast}) \to \gamma $N from meson photoproduction data remains a paramount task in hadron physics. Here we focus on the single-pion production data and note that a complete solution requires couplings from both charged and neutral resonances, the latter requiring $\pi^{-}$p and $\pi^{\mathrm{0}}$n photoproduction off a neutron target (where the neutron is bound in the deuteron.) Experimental data for neutron-target photoreactions are much less abundant than those utilizing a proton target, constituting only about 15{\%} of the present World database. As a result, our knowledge of the neutral resonance couplings is less precise as compared to the charged values. Extraction of the two-body ($\gamma $n $\to \pi^{-}$p and $\gamma $n $\to $ $\pi^{\mathrm{0}}$n) cross sections requires the use of a model-dependent nuclear correction, which mainly comes from final-state interactions (FSI). We recently applied our FSI corrections to CLAS $\gamma $d $\to \pi ^{-}$pp data to get elementary cross sections for $\gamma $n $\to \pi^{-}$p for a broad energy range, E$_{\gamma}$ \textgreater\ 1 GeV. Then, we did the same for a MAMI-B GDH experiment to get $\gamma $n $\to \pi^{-}$p about the $\Delta $-isobar.   

Analysis of Lattice Baryon Masses in $1/N_c$ expansion

The $1/N_c$ expansion of QCD is a perturbative and successful framework for the phenomenology of ground state and excited state baryons. As proposed in [1], it is a systematic expansion in powers of $1/N_c$ and a good approximation, qualitatively and quantitatively, when $N_c=3$. The $1/N_c$ expansion has been applied to the ground state baryons including studies of $SU(6)$ spin-flavor symmetry in [2]. A framework was suggested in [3] for excited baryons based on the $1/N_c$ expansion was implemented and analyzed with the physical masses from the Particle Data Group. And a successful calculation of the spin identified spectrum of Nucleons and Deltas has been carried out in lattice QCD [4]. Using mass operators organized according to the $1/N_c$ expansion, the analysis of lattice baryon masses has been completed for the negative parity baryons in the multiplet $[70,1]^-$ of the $SU(6)\times O(3)$ representation, and results for that multiplet and the rest of the multiplets will be presented in this talk.\\[4pt] [1] G. 'tHooft Nucl.Phys.B72, 461(1974).\\[0pt] [2] R.F. Dashen, E.E. Jenkins, and A.V. Manohar Phys.Rev.D51 (1995).\\[0pt] [3] C.E. Carlson, C.D. Carone, J.L. Goity, R.F. Lebed Phys.Rev.D59 (1999).\\[0pt] [4] R.G. Edwards, J.J. Dudek, D.G. Richards, S.J. Wallace Phys.Rev.D84 (2011).