To cite Synth in publications use:

Alberto Abadie, Alexis Diamond, Jens Hainmueller (2011). Synth: An R Package for Synthetic Control Methods in Comparative Case Studies. Journal of Statistical Software, 42(13), 1-17. URL http://www.jstatsoft.org/v42/i13/.

Corresponding BibTeX entry:

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