Session KL: Mini-Symposium: Nucleon Structure in the Valence Regime I

Overview of Physics in the Valence Regime

A general overview of the theoretical and experimental \newline status of physics in the valence regime will be presented, with a \newline particular emphasis on new data from Jefferson Lab. A special focus will \newline be put on the recent results of the ratios of proton to deuteron structure \newline functions at large x from Hall C, and their impact on parton distribution \newline function studies.   

Results from the JLab MARATHON Tritium DIS Experiment

Results from the Jefferson Lab (JLab) MARATHON experiment on electron deep inelastic scattering (DIS) from light nuclei will be presented. The experiment measured, using an 11 GeV electron beam and the JLab Hall A spectrometer facility, DIS cross section ratios for 2H, 3H and 3He in order to determine, using a novel method, free of the theoretical uncertainties present in previous SLAC measurements, the ratio of the F2n/F2p structure functions of the neutron and proton. The results from the experiment are expected to test predictions of the quark model of the nucleon and of perturbative quantum chromodynamics, and to constrain the nucleon's parton distribution function parametrizations needed for the interpretation of high energy collider data. Another goal of the experiment was a precise determination of the EMC effect of the two A$=$3 mirror nuclei, which will be also presented. The results are considered essential for the explanation of the EMC effect, which represents the modification of the nucleon structure functions in the nuclear medium.   

F2d/F2p structure function ratio at large-x

Nucleon structure functions are essential tools in studying the partonic dynamics within the nucleon. Jefferson Lab experiment E12-10-002 measured data covering the large-x kinematic region where parton distributions are hard to extract both experimentally and theoretically (higher twist and target mass effects and, often for the neutron, deuteron nuclear corrections). These high precision data, facilitated by the new Jefferson Lab 12 GeV electron beam energy and Hall C equipment, will provide constraints for theoretical models and should help with global fitting efforts. As it avoids or at least minimizes the influence of several experimental and theoretical effects the F2d/F2p ratio was extracted from the E12-10-002 data and will be presented.   

The BONuS Experiment with CLAS12

The BONuS experiment aims to measure structure functions of the neutron by tagging electrons scattered off the neutron inside deuterium with slow, backward-moving protons detected in coincidence. Using the energy-upgraded Jefferson Lab electron beam and the CLAS12 spectrometer, the experiment can access the neutron structure function F$_{\mathrm{2}}$ up to Bjorken-$x \quad =$ 0.8, thereby testing definitively various theoretical predictions for the behavior of valence up and down quark distributions as $x$ goes to 1. In addition, we will measure, for the first time, fully exclusive Deep Virtual Compton Scattering (DVCS) on the neutron. A new Radial Time Projection Chamber (RTPC) has been constructed to match the high event rate and geometrical constraints of CLAS12, and has been successfully operated during the data taking runs of BONuS in Winter and Summer of 2020. We will discuss the construction, calibration and operation of the RTPC, and give a first glimpse of the data set collected.   

A Novel Feature of Valence Quark Distributions in Hadrons

We report an observation of a strong correlation between the height of the maximum of valence quark structure function, xq(x), and its Bjorken-x position as a function Q$^{\mathrm{2}}$. The observed correlation is used to derive a new model independent relation which connects the partial derivative of the valence parton distribution functions (PDFs) in x to the QCD evolution equation through the x-derivative of the logarithm of the correlation function. The numerical analysis of this relation using empirical PDFs results in a constant factor for the Q$^{\mathrm{2}}$- range covering four-orders of magnitude. The obtained constant factor allows us to express the ``height- position" correlation function in a simple exponential form which is valid for the all Q$^{\mathrm{2}}$ range of valence PDFs being considered. A similar relation is observed also for pions, indicating that the obtained relation may be universal. The observed ``height - position" correlation is used to prove the ``mean field theorem" according to which no fixed number constituent exchanges can be responsible for the valence quark distributions that produce a peak in the xq(x) distribution, thereby in the hadron structure function, F$_{\mathrm{2}}$(x).   

Modeling the proton structure functions: from deep inelastic scattering to the resonance region

We develop a phenomenological model of the proton inelastic structure functions applicable for both the deep inelastic scattering (DIS) and the nucleon resonance region. In the DIS region we compute the structure functions in terms of the parton distributions with higher-twist corrections which are determined from a global QCD fit. The resonance region is addressed in terms of Breit-Wiegner contributions from the $\Delta(1232)$ resonance, $N(1440)$ Roper resonance and three effective resonances for the second and the third resonance regions. The nonresonance background is described using an extrapolation of the DIS structure functions into the region of low invariant masses of produced states $W$ as well as low invariant momentum transfer $Q$. We independently treat the transverse, $F_T$, and the longitudinal, $F_L$, structure function and determine the model parameters from a fit to the proton differential cross section data. We demonstrate a very good performance of the model by comparing our predictions with a number of measurements of the structure functions $F_2$ and $R=F_L/F_T$ in the resonance and the DIS region.   

Non-Perturbative Model of Valence Quark Distributions in the Nucleon

We develop a non-perturbative model for valence parton distribution functions (PDFs) in the nucleon to explain quark interactions in the mean field of the nucleonic interior. The main motivation for the model is to obtain a mean field description of the valence quarks as a baseline upon which to study the short-range quark-quark interactions that generate the high x tail of valence PDFs. Our non-perturbative model is based on assumption of a factorization of the short-range valence quark and long-range dynamics in the residual system. The nucleon structure functions are calculated using light-front dynamics and the effective Feynman diagrammatic approach is used to introduce valence quark and residual wave functions. The parameters of these wave functions are fixed by the position of the peak of $xf(x)$ fixed at an initial $Q^2$ equal to the mass of the charm quark. With a few parameters we achieved a very reasonable description of the up and down valence quark distributions in the moderate x region ($x<0.5$), where one expects the mean field dynamics to dominate. The model, however, systematically underestimates the high x region where we expect enhanced contribution from partonic correlations. A review of PDFs is also presented, with attention given to the evolution of the