DOC: cite Jacob & Wick

docs/bibliography.bib

@@ -113,6 +113,20 @@ @techreport{chungSpinFormalismsUpdated2014
   url = {https://suchung.web.cern.ch/spinfm1.pdf}
 }
 
+@article{Jacob:1959at,
+  title = {On the General Theory of Collisions for Particles with Spin},
+  author = {Jacob, M. and Wick, G.C.},
+  year = {1959},
+  month = aug,
+  journal = {Annals of Physics},
+  volume = {7},
+  number = {4},
+  pages = {404--428},
+  issn = {00034916},
+  doi = {10.1016/0003-4916(59)90051-X},
+  url = {https://linkinghub.elsevier.com/retrieve/pii/000349165990051X}
+}
+
 @article{JPAC:2019ufm,
   title = {Dalitz-Plot Decomposition for Three-Body Decays},
   author = {Mikhasenko, M. and Albaladejo, M. and Bibrzycki, Ł. and {Fernandez-Ramirez}, C. and Mathieu, V. and Mitchell, S. and Pappagallo, M. and Pilloni, A. and Winney, D. and Skwarnicki, T. and Szczepaniak, A. P.},

docs/formalisms/helicity.md

@@ -64,6 +64,8 @@ what part in the equation. So we would refer in the Doxygen docs to
 [IntensityBuilderXML](https://compwa.github.io/ComPWA/classComPWA_1_1Physics_1_1IntensityBuilderXML.html)
 -->
 
+{cite}`Jacob:1959at`
+
 ## Canonical formulation
 
 <!-- The canonical formalism gives access to the orbital angular momentum $L$ and