restricted to 
a maximal total deviation from the nominal values <i>(p_x0, p_y0, p_z0)</i>, 
for the second incoming particle, like
<br/><i>
(p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 +
(p_z - p_z0)^2/sigma_pz^2 < maxDev^2
</i><br/>
(Note the absence of a factor 2 in the denominator, unlike the Gaussians 
used to pick <i>(p_x, p_y, p_z)</i>.) 
  

<h3>Beam interaction vertex</h3>

On this page you can set the spread of the interaction vertex according to 
a simple Gaussian distribution. If you in