  We analyze the time evolution of spin-polarized electron wave packets
injected into the edge states of a two-dimensional topological insulator. In
the presence of electron interactions, the system is described as a helical
Luttinger liquid and injected electrons fractionalize. However, because of the
presence of metallic detectors, no evidences of fractionalization are encoded
in dc measurements, and in this regime the system do not show deviations from
its non-interacting behavior. Nevertheless, we show that the helical Luttinger
liquid nature emerges in the transient dynamics, where signatures of
charge/spin fractionalization can be clearly identified.
