Session JI: Mini-Symposium: Nucleon Structure and Interactions I

Don't stand so close to me: a story of short-range nuclear repulsion

 

Much of what we know about high-energy components of nuclear structure comes from recent measurement campaigns at Jefferson Lab.  Experiments from the 6 GeV era have provided precise results about short-range nucleon-nucleon correlations and their nuclear dependence. Additionally, an intriguing correlation was observed to measurements of modifications of nuclear quark distributions (EMC effect).   I will highlight key insights gained from previous measurements (including recent ones) and present future experiments aimed at further illuminating these exotic components of nuclear structure.   

Learning about the 3D nucleon structure with novel hard exclusive processes

The so-called Generalized Parton Distributions have been introduced more than 30 years ago now, and their interpretation allows for a better understanding of the multidimensionnal "position" versus "momentum" partonic distributions in the nucleon. Yet, very few measurement exist beyond DVCS and light vector mesons in worldwide experiments and data. Recent theory progresses however emphasize the importance of new measurements to constrain GPD models and allow for interpretations.

In this talk, I will discuss why it is fundamental to complement the existing measurements with novel reactions that have not been measured yet, or are poorly studied. I will present a brief summary of the outcome of a dedicated workshop we are hosting in July 2022 about this topic, with theorist and experimental specialists working at different facilities. I will also provide projections and physics interest for some reactions that can be measured in a short time-scale at Jefferson Lab, including (non-DVCS) Compton-like processes and vector mesons.   

Measurement of Direct Photon Cross Section and Double Helicity Asymmetry at √s = 510 GeV in pp Collisions at PHENIX

Understanding the gluon spin contribution to the proton spin is important to solving the proton spin puzzle. Direct photon production in the longitudinally polarized proton (pp) collisions provides a "clean" probe to the gluon spin at leading order. Compared with hadron production, there is little fragmentation involved in direct photon production. The Relativistic Heavy Ion Collider (RHIC) is the only collider that is capable of producing two longitudinal polarized proton beams. The RHIC run of the year 2013 provides the largest integrated luminosity (155 pb^-1) in pp collisions. The Electromagnetic Calorimeter at PHENIX has fine granularity to identify the π⁰ background up to transverse momentum p_T of 30 GeV/c. In this talk, I will present the direct photon cross section and double helicity asymmetry (A_LL) for photon p_T of 6--30 GeV/c and 6--20 GeV/c, respectively. When included in future global analyses, our results will provide an independent constraint on the gluon spin contribution to the proton spin.   

Factorization of Lepton Radiation in SIDIS

The factorization theorem plays an important role in the analysis of high energy QCD processes, separating the nonperturbative hadronic interaction into the universal parton distribution functions (PDFs) and fragmentation functions (FFs). However, the lepton-hadron collision induced QED radiation can change the momentum of the exchanged unobserved virtual photon, making the photon-hadron frame, where the factorization formalism for lepton-hadron semi-inclusive deep inelastic scattering (SIDIS) was derived, ill defined. An new analogous factorization approach has been introduced to separate the leading power process independent QED radiative contributions to the single photon exchange by introducing lepton distribution functions (LDFs) and fragmentation functions (LFFs), while process dependent effects are calculated with large logarithms removed [J. High Energ. Phys. 2021, 157 (2021)]. These LDFs and LFFs are considered global, as they appear in many different interactions, such as e⁺ e^- and SIDIS, so data from experiments can be used to describe these functions across a wide range of lepton scattering. In this talk, I will apply this hybrid approach to lepton-hadron SIDIS to study the transition between the two-scale region, where the transverse momentum dependent (TMD) factorization is relevant, and the single scale region, where collinear factorization approach is more relevant, treating both QCD and QED factorization equally. I also derive the NLO perturbative coefficients for SIDIS in both regions, and investigate the matching between these two regions.   

Double-pion electroproduction off protons in deuterium: measurements of quasi-free cross sections and study of final state interactions

Exclusive reactions of meson photo- and electroproduction off protons are a very powerful tool to investigate the nucleon structure and the principles of strong interaction. Among them, the production of pion pairs plays a particularly important role: the channel contributes significantly to the total inclusive cross section for W ≤ 1.6 GeV and starts to dominate all other exclusive channels for W ≥ 1.6 GeV.

By now exclusive reactions off free protons have been studied in considerable detail, while experimental information on reactions occurring off nuclei is scarce and mostly limited to inclusive measurements. This situation causes a strong demand for exclusive measurements off bound nucleons, and the deuteron, being the lightest and weakly-bound nucleus, is the best target for initiating these efforts.

This talk will present the quasi-free cross sections for the process of charged double-pion electroproduction off protons bound in deuterium extracted in the second resonance region with the CLAS detector. In the talk, I will introduce the details of the cross section measurements and also focus on the analysis specificities caused by the Fermi motion of the initial proton and complex effects of the final state interactions between the reaction final hadrons and spectator neutrons.   

Probing Flavor Dependence of the EMC Effect

The correlation between the EMC effect and the short-range correlations (SRCs) suggests that there is potentially a closer connection between these two effects. This raises the possibility of an isospin dependent EMC effect in non-isoscalar nuclei due to the np dominance of the SRCs. However, there has been no conclusive experimental evidence for the isospin dependence or lack thereof for the EMC effect. Recent analyses have investigated the EMC-SRC correlation under two separate assumptions: the dominance of the np pairs contributing to the EMC effect (high virtuality) and the isospin-independent EMC effect (local density), and observed no isospin dependence of the EMC effect. We will present the new results on a global analysis that has been performed using the same approaches followed in these analyses with all the existing experimental data.