Relativistic quantum mechanics in the einbein field formalism
  The system of two relativistic particles with einbein fields is quantized as a constrained system.A method of the introduction of the Newton--Wigner collective coordinate is discussed in presence of different gauge fixing conditions. Some arguments are involved in the favour of Lorents covariant gauge fixing conditions. 
Yu. S. Kalashnikova and A. V. Nefediev (ITEP, Moscow)