  The spatio-temporal evolution and breaking of relativistically intense wave
packets in a cold homogeneous unmagnetized plasma has been studied analytically
and numerically. A general expression for phase mixing time scale as a function
of amplitude of the wave packet and width of the spectrum has been derived.
Results have been compared with the existing formulae in literature. It is
shown that phase mixing time scale crucially depends on the relative magnitude
of the amplitude and dimensionless spectral width of the wave packet. Finally
we have verified our results with respect to both the amplitude of the wave
packet and width of the spectral using numerical simulations based on Dawson
Sheet Model.
