Session NH: Hadron Physics I

Electroexcitation of the $\Delta^{+}$(1232) at low momentum transfer

The study of the N to $\Delta $ transition has been a subject of intense scientific interest for many years. Measurements of the pion electroproduction and VCS channels allow the exploration for non-spherical angular momentum amplitudes in hadrons through the measurement of the transition quadrupole amplitudes, while the VCS channel also provides access to the generalized polarizabilities of the nucleon. Results from the recent JLab/Hall-A and MAMI experiments will be presented and future prospects will be discussed.   

Spin-flavor composition of excited baryons

The excited baryon masses are analyzed in the framework of the $1/N_c$ expansion using the available physical masses and also the masses obtained in lattice QCD for different quark masses. The baryon states are organized into irreducible representations of $SU(6)\times O(3)$, where the $[{\bf{56}},\ell^P=0^+]$ ground state and excited baryons, and the $[{\bf{56}},2^+]$ and $[{\bf{70}},1^-]$ excited states are analyzed. The analyses are carried out to $O{1/N_c}$ and first order in the quark masses. The issue of state identifications is discussed. Numerous parameter independent mass relations result at those orders, among them the well known Gell-Mann-Okubo and Equal Spacing relations, as well as additional relations involving baryons with different spins. It is observed that such relations are satisfied at the expected level of precision. Predictions for physically unknown states for each multiplet are obtained. From the quark-mass dependence of the coefficients in the baryon mass formulas an increasingly simpler picture of the spin-flavor composition of the baryons is observed with increasing pion mass (equivalently, increasing $m_{u,d}$ masses), as measured by the number of significant mass operators.   

Lattice input on the inclusive flavor-breaking $\tau$ V$_{us}$ puzzle

Recent versions of the standard approach to implementing the flavor-breaking finite-energy sum rule determination of V$_{us}$ using spectral data obtained from hadronic tau decays produce values of V$_{us}$ more than 3 sigma low relative to the expectations of 3-family unitarity. We revisit this problem, focusing on systematic issues in the treatment of OPE contributions, employing lattice data for the relevant flavor-breaking correlator combinination to help in understanding how to treat the slowly converging D=2 series and investigate potential D $>$ 4 non-perturbative contributions. The results, in combination with observations from additional flavor-breaking continuum sum rules, are shown to suggest an alternate implementation of the flavor-breaking sum rule approach. This alternate analysis approach is shown to produce significantly higher V$_{us}$ than obtained using the assumptions of the conventional implementation, for reasons that will be explained in detail. We also show that, when this approach is implemented using new preliminary results for the tau K pi branching fractions, the V$_{us}$ obtained is in excellent agreement with that obtained from recent analyses of K$_{ell 3}$ using lattice input for f$_+$(0).   

Exploring the potential for studies of the electromagnetic structure of the kaon at 12 GeV JLab

The measurement of form factors plays a pivotal role in the study of hadron structure. Pions and kaons are the simplest strongly bound quark-gluon systems in nature. Pions are the lightest QCD quark systems, having a key role in our understanding of the dynamic generation of mass. Kaons also contain strangeness. The pion form factor has been measured over a wide range of $Q^2$. The range of kaon form factor data beyond the dynamic range reachable with elastic scattering is much smaller. A key issue in reaching higher values of $Q^2$ is the need to quantify the role of the kaon pole in order to extract the form factor from kaon electroproduction data. Understanding the relative contribution of the longitudinal cross section and its kinematic dependencies is important in this effort. Arguably the best way to access data in the higher $Q^2$ regime is with dedicated kaon experiments at 12GeV Jlab. However, information can be gained from existing data. In this talk I will discuss the analysis of existing data from exclusive and semi-inclusive scattering experiments optimized for pions which contained kaons in their acceptance. Preliminary kaon cross sections will be shown and an outlook for kaon form factor measurements at 12GeV JLab will be discussed.   

Physics Opportunities with the Neutral Particle Spectrometer in Hall C

The two-arm combination of neutral-particle detection and a high-resolution magnetic spectrometer offers unique scientific capabilities to push the energy scale for studies of the transverse spatial and momentum structure of the nucleon through reactions with neutral particles requiring precision and high luminosity. It enables precision measurements of the deeply-virtual Compton scattering cross section at different beam energies to extract the real part of the Compton form factor without any assumptions. It allows measurements to push the energy scale of real Compton scattering, the process of choice to explore factorization in a whole class of wide-angle processes, and its extension to neutral pion photo-production. It further makes possible measurements of the basic semi-inclusive neutral-pion cross section in a kinematic region where the QCD factorization scheme is expected to hold, which is crucial to validate the foundation of this cornerstone of 3D transverse momentum imaging. Adding the option of polarized targets to such a setup, allows for exploration of further scientific directions, e.g., timelike Compton scattering. We describe the unique science program as enabled by the Neutral-Particle Spectrometer and the magnetic spectrometer pair in Hall C at JLab.   

Measurement of the Neutral Pion Lifetime

The QCD chiral anomaly is the dominant contribution to the $\pi ^{0}\to \gamma \gamma $ decay. Therefore, a precision measurement of the $\pi^{0}$ decay width and its comparison with prediction of $\pi^{0}$ lifetime can be used as a test of QCD at the confinement scale. Recent theoretical activities have demonstrated high precision (1{\%} level) calculations of the decay width of the $\pi ^{0}$ into two photons. An experimental determination with comparable precision will be critical to test these predictions. At Jefferson Lab, the PrimEx Collaboration has performed high precision experiments to measure the $\pi^{0}$ lifetime using the Primakoff effect. The first (PrimEx-I) experiment resulted in a published 2.8{\%} total uncertainty in the $\pi^{0}$ decay width. PrimEx-II was carried out in the fall of 2010 with the final goal of 1.4{\%} precision. The preliminary result of this experiment will be presented. This work is supported in part by the US Department of Energy under contract number DE-FG02-03ER41231.   

Exploring hadron structure through exclusive kaon electroproduction from JLab 6GeV to 12GeV

Exclusive reactions have been successfully used to probe hadrons at long and short distance scales, allowing us to study the interaction of elementary particles and their dynamics on the basis of Quantum Chromodynamics (QCD). The electroproduction of mesons has shown to be a powerful tool for these studies. High precision data for the pion taken at the 6 GeV Jefferson Lab provided important information about the pion form factor and brought us puzzles regarding the applicability of hard-soft QCD factorization. The kaon provides an interesting way to expand these studies, opening the possibility to access the production mechanism involving strangeness physics and also search for the onset of factorization on systems containing heavier quarks. Most of the precision cross section measurements at the 6 GeV Jefferson Lab were primarily designed for pions, but some of these experiments also captured kaons in their acceptance. In this talk, I will show preliminary kaon cross section results from such experiments. I will also discuss plans to explore the extended $Q^2$ range capability with dedicated kaon experiments at the 12 GeV Jefferson Lab to study the onset of factorization for mesons including strangeness and the meson electroproduction mechanism in general.   

Heavy quarkonium in the basis light-front quantization approach

I present a study of the charmonium and bottomonium spectra using the basis light-front quantization. We implement a one-gluon exchange interaction in the leading Fock sector following Ref.~[1]. We also adopt a phenomenological confining interaction based on the AdS/QCD and light-front holography. The results are compared with the experimental data.\\[4pt] [1] Paul Wiecki \textit{et al}, Phys. Rev. D \textbf{91}, 105009 (2015)   

The Reliable Determination of $F_\pi$ beyond $Q^2$=6 GeV$^2$

The charged pion form factor, $F_{\pi}(Q^2)$, is an important quantity which can be used to advance our knowledge of hadronic structure. However, the extraction of $F_{\pi}$ from electroproduction data requires a model of the $^1$H$(e,e^{\prime}\pi^+)n$ reaction, and thus is inherently model dependent. Furthermore, one is either $(a)$ limited to the kinematic regime where the pion pole term dominates the longitudinal cross section ($-t_{min}<$0.20 GeV$^2$), or $(b)$ required to have some other reliable means to identify the non-pole backgrounds expected to dominate at higher $-t$. The E12-06-101 pion form factor experiment planned to run at Jefferson Lab Hall C in a few years respects constraint $(a)$, and is expected to provide reliable $F_{\pi}$ values of unprecedented quality up to $Q^2$=6 GeV$^2$. Measurements using the same Jefferson Lab apparatus above $Q^2>$8 GeV$^2$ are possible, provided one has a means to address constraint $(b)$. I will discuss some of the issues involved if one is to make these measurements a successful reality.