  Edge states of two-dimensional topological insulators are helical and
single-particle backscattering is prohibited by time-reversal symmetry. In this
work, we show that an isotropic exchange coupling of helical edge states (HES)
to a spin 1/2 impurity subjected to a magnetic field results in characteristic
backscattering current noise (BCN) as a function of bias voltage and tilt angle
between the direction of the magnetic field and the quantization axis of the
HES. In particular, we find transitions from sub-Poissonian (antibunching) to
super-Poissonian (bunching) behavior as a direct consequence of the helicity of
the edge state electrons. We use the method of full counting statistics within
a master equation approach treating the exchange coupling between the spin-1/2
impurity and the HES perturbatively. We express the BCN via coincidence
correlation functions of scattering processes between the HES which gives a
precise interpretation of the Fano factor in terms of bunching and antibunching
behavior of electron jump events. We also investigate the effect of
electron-electron interactions in the HES in terms of the Tomonaga-Luttinger
liquid theory.
