Studies of hadronic structure using electromagnetic probes

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One of the central problems of modern physics research concerns our understanding of the building blocks of the atomic nucleus - the protons, neutrons, and other particles (mesons) that bind them. Notable discoveries have indicated that these particles consist of yet more fundamental constituents, the quarks and gluons. While a very good theoretical framework (called Quantum Chromo-Dynamics, or QCD) is able to accurately describe how quarks and gluons interact at extremely high energies (or, equivalently, when the quarks are very close together), it has been very difficult to apply QCD to lower energy (longer distance) phenomena. One of the obstacles to our improved comprehension has been a lack of quality data, particularly for the Deep Exclusive Reactions, which provide the clearest picture of the inner workings of QCD. Jefferson Lab enables these studies with its continuous, high luminosity electron beams, and detectors with good particle identification and reproducible systematics. 1. p+ and K+ Elastic Form Factors (Fp, FK): The internal structures of the two lightest mesons are parametrized by their form factors, and are critical hadronic structure quantities to understand. The p+ form factor offers our best hope of directly observing QCD's transition from strong-confinement at long-distance scales to asymptotic-freedom at short-distances. The K+, where the d quark is replaced with an s quark, provides a vital second study case.  Objectives: (i) establish the reliability of the extraction of the meson form factors from electroproduction data over a broad kinematic range, (ii) obtain high quality form factor measurements to high momentum transfer. The K+ data were acquired in 2018-19, while the p+ data were partly acquired in 2019, with the bulk to be acquired in 2021, 2023. The data are being analyzed by four Regina PhD students. 2. Unique Access to u-Channel Physics: Backward-angle meson electroproduction is anticipated to offer unique access to the three quark plus sea component of the nucleon wave function. We pioneered a new technique, which demonstrated the feasibility of high quality measurements in this previously unexplored region. Our upcoming experiment will take ?, ? and f data with goals: (i) establish existence of a backward-angle cross section peak for multiple reaction channels; (ii) extract the separated cross sections over a wide kinematic range; and (iii) establish whether the collinear factorization scheme is valid. 3. Study of Generalized Parton Distributions (GPDs):  SoLID (Solenoidal Large Intensity Device) will allow new data to be acquired on GPDs, "universal objects" providing a comprehensive description of the quark and gluon structure of the nucleon. Regina students will participate in SoLID detector construction and testing, planned for 2022-25, and contribute to our experiment probing the poorest known GPD, E-tilde. Funds are requested for Jefferson Lab fieldwork, PhD student and PDF salary support.