Session KC: Hadrons I

The Generalized Polarizabilities of the proton

The Generalized Polarizabilities (GPs) are fundamental quantities of the nucleon and they represent an essential part for a complete understanding of the nucleon structure and dynamics. The GPs can be explored through measurement of the Virtual Compton Scattering reaction. Similar to how the elastic form factors describe the charge and magnetization distributions, the GPs give access to the polarization densities inside the nucleon. Recent results from JLab and MAMI and future prospects will be discussed in this talk.   

Nucleon transverse densities from Dispersively Improved Chiral EFT

The transverse densities describe the distribution of charge and current at fixed light-front time and connect the nucleon's elastic form factors with its partonic structure. We compute the transverse densities at peripheral distances $b = O(M_\pi^{-1})$ in a novel approach combining dispersion theory and chiral EFT. The densities are obtained from the dispersive representation of the electromagnetic form factors and expressed as a superposition of $t$-channel hadronic exchanges. The spectral functions on the two-pion cut are constructed using elastic unitarity, $\pi N$ amplitudes from chiral EFT, and the pion timelike form factor data. Our formulation incorporates $\pi\pi$ rescattering in the $t$-channel and gives realistic spectral functions including the $\rho$ resonance. Accurate transverse densities are obtained down to distances $b \sim 0.5$ fm with controlled uncertainties. Our results allow us to identify the region of distances where transverse nucleon structure is governed by two-pion exchange, and to predict the spin and flavor structure of the peripheral densities. The methods can be extended to generalized parton distributions and other nucleon form factors.   

Loop Corrections to Baryon Properties in Relativistic Chiral SU(3) Effective Theory

We calculate the pseudoscalar meson loop contributions to the properties of flavor SU(3) octet and decuplet baryons, using a relativistic chiral effective theory framework consistent with Lorentz and gauge invariance. A finite range regularization prescription is applied and compared with dimensional regularization to show better convergence at higher meson masses. Results showing loop contributions to baryon masses, electromagnetic elastic and transition form factors, and flavor asymmetries in parton distribution are presented. The results are also compared with lattice QCD data to compute sigma terms and other relevant quantities.   

Pion-Proton Scattering with MUSE Apparatus

The Muon Proton Scattering Experiment (MUSE) was built to address the proton radius puzzle by simultaneously measuring the proton radius from both electron-proton and muon-proton elastic scattering in a momentum transfer range sensitive to the radius extraction. The experiment is carried out at the Paul Scherrer Institute (PSI) and uses a mixed e, $\mu$, and $\pi$ beam, alternating between positive and negative polarities, in the PiM1 secondary beam line. This, in combination with a large-acceptance, non-magnetic detector system also allows the MUSE apparatus to measure the absolute $\pi^\pm$p elastic cross sections and cross section ratios over a wide pion momentum range (115 to 210 MeV/c). In this talk I will present an opportunity to use the MUSE apparatus to fill gaps in the existing pion-proton elastic scattering data as well as a search for the proposed missing baryon resonances, below the $\Delta(1232)$.   

Status of the PrimEx-$\eta$ Experiment in Hall D at Jefferson Lab

The $\eta\rightarrow\gamma\gamma$ decay proceeds primarily via the chiral anomaly. A precise measurement of this decay width will provide critical input to extract the $\eta$-$\eta'$ mixing angles and the light quark mass ratio. Previous experimental results through $e^{+}e^{-}$ collisions are different from the result via the Primakoff effect by more than 4$\sigma$. The PrimEx-$\eta$ fixed-target experiment in Hall D at Jefferson Lab (E12-10-011\footnote{Spokespersons: A. Gasparian (contact), L. Gan, I. Larin, A. Somov}) will measure the $\eta\rightarrow\gamma\gamma$ decay width using the Primakoff method with a projected uncertainty of 3.2$\%$. This would be the most precise measurement of the decay width to date, and would address the discrepancy between the results of the two types of experiments. In conjunction with the $\eta\rightarrow\gamma\gamma$ decays, the total cross section for Compton scattering from the atomic electrons inside the targets (beryllium and helium) will be measured to help control the systematic uncertainties. The first phase of the data was collected in 2019 on a liquid He-4 target, corresponding to roughly half of the total proposed statistics. This talk will discuss the physics impacts of this experiment and present some of the initial results.   

$J/\psi-007$ Experiment: A Threshold $J/\psi$ Photoproduction Measurement in Hall C at Jefferson Lab

We report on the Jefferson Lab experiment E12-16-007 ($J/\psi-007)$ where we made a measurement of the elastic doubly differential $J/\psi$ photo-production cross section as a function of proton momentum transfer $t$ and photon energy $E_{\gamma}$ in the region corresponding to the discovered charm pentaquark states reported by the LHCb collaboration. The experiment was performed using a high-intensity real photon beam generated by a 10.6 GeV incident electron beam traversing a copper radiator upstream of a liquid hydrogen target in Hall C. The two high momentum spectrometers of Hall C, HMS, and SHMS were used to detect in coincidence the $e^{+} e^{-}$ di-lepton $J/\psi$ decay pair. Both the $t$-channel and $s$-channel resonant $J/\psi$ production were explored using a high incident photon flux beam combined with spectrometers angular and momentum settings scanning a range from $t_{min}$ to large $t$, for every accessible photon energy above threshold. The sensitivity of the measurements to the $s$-channel resonant pentaquarks signals, if they exist, is maximal at large $t-t_{min}$. We will present preliminary results of the doubly differential $J/\psi$ cross-section measurement as well as the total photo-absorption cross-section.   

Spin-Density Matrix Elements for Vector Meson Photoproduction at GlueX

The GlueX experiment at Jefferson Lab aims to study the light meson spectrum with an emphasis on the search for hybrid mesons. A linearly-polarized, 9~GeV photon beam is incident on a hydrogen target inside a detector with near-complete neutral and charged particle coverage. The experiment completed its first phase of data taking in 2018, producing orders of magnitude more data than previous polarized photoproduction experiments in this energy regime. Polarization observables, such as spin-density matrix elements, are important input for the theoretical description of the production mechanism---which will be required for the interpretation of any potential signals for exotic mesons. We present new results for the photoproduction of vector mesons, focusing on spin-density matrix elements for the $\omega(782)$ meson with dramatically improved statistical precision.   

The decay of the $b_1(1235)$ meson through the $\omega \pi$ channel at GlueX

A long-standing goal of hadron physics has been to understand how the quark and gluon degrees of freedom that are present in the fundamental QCD Lagrangian manifest themselves in the spectrum of hadrons. The GlueX experiment at Jefferson lab contributes to the global spectroscopy program using 8-9~GeV linearly polarized photons. This experiment focuses on the exploration of the light-quark domain, potentially accessing hybrid mesons with exotic $J^{PC}$ quantum numbers in photoproduction reactions. The decay of several exotic mesons (e.g.\ $\pi_1(1600)$, etc.)\ to $b_1 \pi$ can be accessed through the dominant decay $b_1 \rightarrow \omega \pi$. In this talk we discuss the channel $\gamma p \rightarrow p \pi^+ \pi^- 4 \gamma$ which can reconstruct to the reaction $\gamma p \rightarrow p b_1$, where the axial-vector meson $b_1 (1235)$ decays to $\omega \pi$. Understanding this channel is important, particularly in the context of Partial Wave Analysis and being able to extract the D/S wave ratio, which is of interest to validate predicted couplings to this axial-vector resonance from Lattice QCD calculations.   

Threshold pi- Photoproduction on the Neutron near Threshold

Recent data from the PIONS@MAX-lab Collaboration, measuring the total cross sections of the pion incoherent photoproduction gamma d--\textgreater pi-pp near threshold,~ have been used to determine the E0$+$ multipole and total cross sections of the reaction gamma n--\textgreater pi-p, also near threshold. These are the first measurements of the reaction gamma d--\textgreater pi-pp in the threshold region. The value of E0$+$ is extracted through a fit to the deuteron data in a photoproduction model accounting for final-state interactions. The model takes an S-wave approximation for the elementary reaction gamma n--\textgreater pi-p with E0$+ \quad =$ const in the threshold region.~ The obtained value E0$+ \quad =$ (-31.86$+$-0.8) x 10\textasciicircum -3/m\textunderscore pi$+$ is in agreement with other existing results. Model predictions for the total cross section are also given.