invariant yield

unit derivation:

$$\begin{eqnarray*} \frac{d^3N}{dp^3} & = & \frac{d^3N}{dp_x dp_y dp_z}\\ & = & ... ...}dm_t\\ \frac{d^3N}{dp^3} & = & \frac{d^2N}{2\pi E m_t dm_t dy} \end{eqnarray*}$$