For Weyl semimetal, the effective Hamiltonian reads:
$$H=E_0 \mathbb{1} + v_0 \cdot \mathrm{q} \mathbb{1}+\sum_{i=1}^{3} \mathrm{v}_i \cdot \mathrm{q} \sigma_i$$
Why is the chirality given by $${\rm sgn}(\mathrm{v}_1 \cdot \mathrm{v}_2 \times \mathrm{v}_3)~?$$
