Year: 2026
Type: preprint
Abstract: We propose to measure hard exclusive $α$-knockout reactions from the light $α$-conjugate nuclei $^{12}$C, $^{16}$O, and $^{20}$Ne using electron scattering. Three reaction channels will be measured for the first time on the same target set: (i) the quasi-elastic knockout $(e, e' α)$ knockout, which establishes a baseline against $α$-clustering model calculations and tests final-state-interaction (FSI) models; (ii) coherent deeply virtual Compton scattering (DVCS) on a bound $α$, which provides a quark-level probe of the bound system through the beam-spin asymmetry; and (iii) coherent vector-meson production ($ϕ$ and J/{$ψ$}) on a bound $α$, which probes the gluon distribution of the bound cluster through two-gluon exchange. The central observables are ratios of cross sections and beam-spin asymmetries between the bound $α$ in $^{12}$C, $^{16}$O, and $^{20}$ Ne and the free $^4$He ground state. Together these measurements directly address the question of the role of QCD in the structure of light nuclei : do the quark and gluon distributions of an $α$ embedded in a heavier nucleus differ from those of a free 4He, and do those modifications track the increasing $α$-clustering strength along the $α$-conjugate chain? The measurement will use the 12 GeV CEBAF accelerator in Hall B with the CLAS12 spectrometer instrumented with the ALERT low-energy recoil detector and high-pressure gas straw targets. ALERT, which replaced the CLAS12 silicon vertex tracker, was successfully commissioned and operated during Run Group L (RG-L) from April through August 2025. It uniquely provides identification of low-momentum recoiling light ions (p, d, t, $^3$He, and $^4$He) close to the beamline at the high luminosities required by these exclusive channels.
Source: arXiv (Cornell University)
Authors: Armstrong, Whitney R., Cloët, Ian, Flor, Fernando A., Joosten, Sylvester, Kim, Bobae, Kim, Minho, Klest, Henry T., Meziani, Zein-Eddine, Peng, Chao, Klimenko, Valerii, Pilleux, Noémie, Żurek, Maria, Dulya, Jacob C., Xu, Zhiwan, Fassi, Lamiaa El, Smith, Penn, Ifat, Nazmus S., Lee, Sangbaek, Shrestha, Suman, Sparveris, Nikos, Benmokhtar, Fatiha, Ploen, Christine, Dupré, Raphaël, West, Jennifer Rittenhouse, Brodsky, Stanley J.
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7221771872