I am going to make a solenoid magnet simulation with a beam source. So, in order to define the beam direction, there are different parameters such as $x$, $x'$ (or $a$ or $P_x/P_0$), $y$, $y'$ (or $b$ or $P_x/P_0$) also known as 4D phase space.
$a$ parameter defines the angle between $Z$ (beam) axis and $X$-axis, $b$ defines the angle between $Z$ (beam) axis and $Y$-axis
I started with easy way and I defined one constant $b$ value (in rad). As a further step, I wanted to emit the particles as a cone shape by keeping this angle ($\varphi$ angle in the image below).
I do not know how can describe this angle by using only $a$ and $b$ angles.
