1 Erratum to: Eur. Phys. J. C https://doi.org/10.1140/epjc/s10052-019-7506-9

Equation (38) of the article is incorrect. Indeed, adopting the same notations, Eq. (33) reads as

$$\begin{aligned} \rho '= & {} {{{\mathcal {D}}}}^{1/2}(\Omega )^{\dagger }\rho ^{f_1}{{\mathcal {D}}}^{1/2}(\Omega )\\= & {} \frac{1}{2} [I + {{\mathcal {D}}}^{1/2}(\Omega )^{\dagger }{\vec {\sigma }} \cdot \vec {P^{f_1}} {{\mathcal {D}}}^{1/2}(\Omega )]. \end{aligned}$$

This implies, instead of Eq. (38),

$$\begin{aligned} \rho ' = \frac{1}{2}(I + \sigma _1 P_T + \sigma _2 P_N + \sigma _3 P_L), \end{aligned}$$

with

$$\begin{aligned} P_i = \vec {P^{f_1}}\cdot {\hat{e}}_i, \quad i = T, N, L, \end{aligned}$$

and

$$\begin{aligned} {\hat{e}}_L = \frac{\vec {p}}{p},\quad {\hat{e}}_N = \frac{{\hat{e}}_L \times {\hat{k}}}{|{\hat{e}}_L\times {\hat{k}}|},\quad {\hat{e}}_T = {\hat{e}}_L \times {\hat{e}}_N. \end{aligned}$$

These unit vectors differ from those of Eq. (36), which define the helicity frame. The corrections do not affect the successive equations; however, the matrix R, which appears in Eq. (66), describes a rotation around \(-\vec {p}\) and not around \(\vec {p}\), as claimed in the text.