Function lab_angles 

pub fn lab_angles(theta: f64, mu: f64) -> (f64, f64)

Laboratory deflection of projectile and recoil for a centre-of-mass scattering angle theta and mass ratio mu = M1 / M2.

Classical elastic two-body kinematics (e.g. H. Goldstein, Classical Mechanics, 2nd ed. (1980), Sec. 3.11; M. T. Robinson and I. M. Torrens, Phys. Rev. B 9 (1974) 5008):

• projectile: tan(psi) = sin(theta) / (cos(theta) + mu), psi in [0, pi],
• recoil: phi = (pi - theta) / 2, on the opposite azimuth,
• energy transfer: T = gamma E sin^2(theta/2), gamma = 4 M1 M2 / (M1 + M2)^2.

Returns (psi, phi).