Session 2WC: QCD and the EIC II

New Opportunities for Insight into the Emergence of Hadron Mass from Studies of Nucleon Resonance Electroexcitation

Understanding the emergence of hadron mass (EHM) represents one of the most challenging and still open problems in the Standard Model. Results of EHM studies from the nucleon resonance electroexcitation amplitudes (g_vpN* electrocouplings) determined from exclusive meson electroproduction data measured with the CLAS detector will be presented in this talk. These CLAS data have provided the first and only available results on the evolution of the γ_vpN* electrocouplings with photon virtuality Q² up to 5 GeV² for most nucleon resonances in the mass range up to 1.8 GeV. A successful description of the CLAS results on the Q²-evolution of the Δ(1232)3/2⁺, N(1440)1/2⁺, and Δ(1600)3/2⁺ electrocouplings achieved within the CSM approach by employing the same dressed quark mass function inferred from the QCD Lagrangian, which also provided a successful description of the pion elastic electromagnetic form factor and PDF, and nucleon elastic form factor, conclusively demonstrated the capability of gaining insight into EHM. The expected results from the CLAS12 detector on the Q²-evolution of the g_vpN* electrocouplings for all prominent resonances in the mass range up to 2 GeV at the still almost unexplored range of Q²>5 GeV² will allow us to place tight constraints on the dressed quark mass function in the range of quark momenta where the substantial part of hadron mass ~50% is expected to be generated. Consistent results on the momentum dependence of the dressed quark mass obtained from analyses of the γ_vpN* electrocouplings of nucleon resonances of different structure will validate insight into EHM in a nearly model-independent way, allowing us to address key open problems in hadron physics on the emergence of hadron mass and nucleon resonance structure from QCD.

 

 

    

The quest to understand the fundamental structure of nuclear matter – outlook to QCD and the Electron-Ion Collider

Nuclear matter is made of quarks that are bound by gluons that also bind themselves. Unlike with the more familiar atomic and molecular matter, the interactions and structures in nuclear matter are inextricably mixed up, and observed properties of nucleons and nuclei, such as mass and spin, emerge out of this complex system. Ongoing and future QCD research offers the exciting prospect to obtain a multi-dimensional picture of the inner quark-gluon structure of protons and atomic nuclei and to inform us how the properties and structure of nuclear matter have emerged from the dynamics of QCD. This program is initiated at the 12-GeV Upgraded Jefferson Lab, concentrating on imaging the region in nucleon and nuclear structure where quarks prevail. A future Electron-Ion Collider (EIC) is planned at Brookhaven National Lab, in partnership with Jefferson Lab. The EIC will have a versatile range of beam energies, polarizations, and ion species, as well as high luminosity, to precisely image quarks, gluons, and their interactions in protons and complex atomic nuclei. The goal is an understanding of the internal structure of nuclear matter comparable to our knowledge of the electronic structure of atoms. The present status of the EIC will also be presented.

    

The Science and Status of the Electron Ion Collider

Despite decades of experimental and theoretical investigations many important questions in Quantum Chromodynamics (QCD), remain unanswered. We know since 1960s that protons and neutrons (nucleons) -- the building blocks of the visible universe --  are made up of quarks and gluons (partons). However, we do not yet understand exactly how the parton interactions constitute the nucleons and impart them properties such as their spin and mass. Questions also  remain in the nuclear regime. For example, what is the quark-gluon nature of nuclear binding? How does a high energy parton interact with cold nuclear matter? How is a color-neutral object formed starting from the colored parton (hadronization)? Lastly, what happens to parton densities in hadrons and nuclei at high energy? QCD suggests emergence of a novel saturated gluonic state of matter often called the Color Glass Condensate. However, it cannot predict exactly what energy and parton density is needed to create it. These and such questions were summarized in the Electron Ion Collider (EIC) White Paper and a case for the EIC was made. The 2015 US nuclear science long range plan enthusiastically supported this. In 2018 the National Academy of Sciences, Engineering and Medicine (NAS) found these questions compelling and extremely important to understand the last remaining secrets of atomic nuclei. The US DOE has since moved to realize the high-luminosity, high-energy EIC with polarized beams as proposed in the White Paper. The EIC is being built at Brookhaven National Laboratory (BNL) jointly by BNL and Jefferson Laboratory (JLab) with enthusiastic international community participating both in the accelerator and detector design and development. In this talk I will summarize the scientific motivation for the EIC and the status of the project.