at intermediate rigidities. Due to the CR-wave coupling, any phenomenon with a low-rigidity characteristic scale, affecting either propagation or injection, may thus be at the origin of correlated changes in the CR spectra and the diffusion coefficient. For propagation, such a scale might arise due to the decrease of the CR pressure as CRs get closer and closer to the nonrelativisitic regime, and/or be related to some dissipation of the turbulence power spectrum [@YanEtAl2004; @PtuskinEtAl2005; @PtuskinEtAl2006a; @ShalchiEtAl2010; @EvoliEtAl2014; @XuEtAl2016]. In the following, while remaining agnostic on these specifics, and in contrast to previous B/C studies performed in the context of semi-analytical models ( [@MaurinEtAl2001; @MaurinEtAl2002a; @PutzeEtAl2010; @PutzeEtAl2011]), we want to capture the possibility that the diffusion coefficient departs from a single power law. This is justified by both theoretical arguments and observational evidence, as recalled above.

Starting from general considerations arising in the quasi-linear theory ( [@Jokipii1966; @BerezinskiiEtAl1990; @Schlickeiser2002; @Shalchi2009]), the diffusion coefficient is expected to be linked to the magnetic turbulence spectrum $|\delta B/B|$ through \[eq:k\_linear\_th\] K(E) = , where $v$ is the CR speed, $l_{\rm mfp}$ is the mean free path length, $r_{\rm L}\propto R/B$ is the Larmor radius defined from the rigidity $R=p/Z$, and $k_{\rm L}\propto 1/r_{\rm 