Re: Origin in simulations

[Richard Jones <Richard.T.Jones@uconn.edu>]

Hi James,

The answer to your question is found in the following comment which I cut and paste from the head of the hdgeant source file hddmInput.c.

 * Usage Notes:
 * 1)   Most Monte Carlo generators do not care where the vertex is placed
 *      inside the target, and specify only the final-state particles'
 *      momenta.  In this case the vertex position has to be randomized by
 *      the simulation within the beam/target overlap volume.  If the vertex
 *      position from the generator is (0,0,0) then the simulation vertex is
 *      generated uniformly inside the cylinder specified by TARGET_LENGTH,
 *      BEAM_DIAMETER, and TARGET_CENTER defined below.
 */

#define TARGET_LENGTH 30
#define BEAM_DIAMETER 0.5
#define TARGET_CENTER 65


All hits reported in the hitView tag are in the master reference system, as thy should be.  The only thing I had forgotten is how the physical vertex is actually generated by geant, if it was left at 0,0,0 by the generator.  Do you need a new "truth"
tag in the hitView group that reports the actual vertex?  If you think that would be useful then I can add it, although I would warn you against relying on it since for real data that information will be missing.

Richard Jones