  The curvature in blazar spectrum has the potential to understand the particle
dynamics in jets. We performed a detailed analysis of simultaneous Swift-XRT
(0.3-10 keV) and NuSTAR (3-79 keV)} observations of Mkn 421. Our analysis of
NuSTAR observations alone reveals that, during periods of low flux, the hard
X-ray spectra are best represented by a steep power-law with photon index
reaching $\sim$ 3. However, the spectrum exhibits significant curvature during
its high flux states. To investigate this, we explore plausible diffusion
processes facilitating shock acceleration in the emission region that can
contribute to the observed spectral curvature. Particularly, such processes can
cause gradual fall of the photon spectrum at high energies which can be
represented by a sub-exponential function. The parameter that decides this
spectral change can be used to characterise the energy dependence of the
diffusive process. Our results suggest that the X-ray spectra of Mkn 421 are
consistent with a scenario where particle acceleration is mediated through
Bohm-type diffusion and the spectra beyond the synchrotron peak is modulated by
the radiative loss process.
