title: X-wave bullets with negative group velocity in vacuum.
abstract: Propagation-invariant, X waves with negative group velocity are reported. Beam aperturing allows for a comprehensive analysis concerning the causality of the optical signal and forerunner formation.
authors.affiliation: Departamento de Optica, Universidad de Valencia, Burjassot, Spain. carlos.zapata@uv.es
authors.forename: Carlos J
authors.initials: CJ
authors.lastname: Zapata-Rodríguez
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Porras