A new attempt towards the unification of space-time and internal gauge   symmetries
  The neat formulation that describes the gauge interactions associated with internal symmetries is extended to the case of a simple, yet non-trivial, symmetry group structure which mixes gravity and electromagnetism by associating a gauge symmetry with a central extension of the Poincar\'e group. 
V. Aldaya and E. S\'anchez-Sastre