Measurement of spin-density matrix elements in Δ++(1232) photoproduction

• DOI: 10.1016/j.physletb.2025.139368
• Publication Date: 2025-02-28T17:12:23Z
• Authors: F. Afzal; C.S. Akondi; M. Albrecht; M. Amaryan; S. Arrigo; V. Arroyave; A. Asaturyan; A. Austregesilo; Z. Baldwin; F. Barbosa; J. Barlow; E. Barriga; R. Barsotti; D. Barton; V. Baturin; V.V. Berdnikov; T. Black; W. Boeglin; M. Boer; W.J. Briscoe; T. Britton; S. Cao; E. Chudakov; G. Chung; P.L. Cole; O. Cortes; V. Crede; M.M. Dalton; D. Darulis; A. Deur; S. Dobbs; A. Dolgolenko; M. Dugger; R. Dzhygadlo; D. Ebersole; M. Edo; H. Egiyan; T. Erbora; P. Eugenio; A. Fabrizi; C. Fanelli; S. Fang; J. Fitches; A.M. Foda; S. Furletov; L. Gan; H. Gao; A. Gardner; A. Gasparian; D.I. Glazier; C. Gleason; V.S. Goryachev; B. Grube; J. Guo; L. Guo; J. Hernandez; K. Hernandez; N.D. Hoffman; D. Hornidge; G. Hou; P. Hurck; A. Hurley; W. Imoehl; D.G. Ireland; M.M. Ito; I. Jaegle; N.S. Jarvis; T. Jeske; M. Jing; R.T. Jones; V. Kakoyan; G. Kalicy; V. Khachatryan; C. Kourkoumelis; A. LaDuke; I. Larin; D. Lawrence; D.I. Lersch; H. Li; B. Liu; K. Livingston; G.J. Lolos; L. Lorenti; V. Lyubovitskij; R. Ma; D. Mack; A. Mahmood; H. Marukyan; V. Matveev; M. McCaughan; M. McCracken; C.A. Meyer; R. Miskimen; R.E. Mitchell; K. Mizutani; V. Neelamana; L. Ng; E. Nissen; S. Orešić; A.I. Ostrovidov; Z. Papandreou; C. Paudel; R. Pedroni; L. Pentchev; K.J. Peters; E. Prather; S. Rakshit; J. Reinhold; A. Remington; B.G. Ritchie; J. Ritman; G. Rodriguez; D. Romanov; K. Saldana; C. Salgado; S. Schadmand; A.M. Schertz; K. Scheuer; A. Schick; A. Schmidt; R.A. Schumacher; J. Schwiening; N. Septian; P. Sharp; X. Shen; M.R. Shepherd; J. Sikes; A. Smith; E.S. Smith; D.I. Sober; A. Somov; S. Somov; J.R. Stevens; I.I. Strakovsky; B. Sumner; K. Suresh; V.V. Tarasov; S. Taylor; A. Teymurazyan; A. Thiel; T. Viducic; T. Whitlatch; N. Wickramaarachchi; Y. Wunderlich; B. Yu; J. Zarling; Z. Zhang; X. Zhou; B. Zihlmann
• Keywords: Spin density; Matrix (chemical analysis); Density matrix

We measure the spin-density matrix elements (SDMEs) of the $Δ^{++}(1232)$ in the photoproduction reaction $γp \to π^-Δ^{++}(1232)$ with the GlueX experiment in Hall D at Jefferson Lab. The measurement uses a linearly--polarized photon beam with energies from $8.2$ to $8.8$~GeV and the statistical precision of the SDMEs exceeds the previous measurement by three orders of magnitude for the momentum transfer squared region below $1.4$ GeV$^2$. The data are sensitive to the previously undetermined relative sign between couplings in existing Regge-exchange models. Linear combinations of the extracted SDMEs allow for a decomposition into natural and unnatural--exchange amplitudes. We find that the unnatural exchange plays an important role in the low momentum transfer region.