To cite Synth in publications use:

  Jens Hainmueller aut cre, Alexis Diamond aut, Alberto Abadie aut
  (2011). “Synth: An R Package for Synthetic Control Methods in
  Comparative Case Studies.” _Journal of Statistical Software_,
  *42*(13), 1-17. <https://www.jstatsoft.org/v42/i13/>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Synth: An R Package for Synthetic Control Methods in Comparative Case Studies},
    author = {{Jens Hainmueller aut cre} and {Alexis Diamond aut} and {Alberto Abadie aut}},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {42},
    number = {13},
    pages = {1--17},
    url = {https://www.jstatsoft.org/v42/i13/},
  }