  Two-fluxon state in an annular Josephson junction in the presence of external
magnetic field is studied analytically, numerically and experimentally. We
obtain an analytical expression for the potential of interaction between the
fluxons moving at arbitrary velocities (without the use of the
"nonrelativistic" approximation). Treating the fluxons as quasi-particles, we
then derive equations of motion for them. Direct simulations of the full
extended sine-Gordon equation are in good agreement with results produced by
the analytical model, in a relevant parameter region. Experimental data
qualitatively agree with the numerical results.
