Appendix C: SIMC Coordinate System and Units.

SIMC almost always uses the standard Hall C coordinate system, as defined in the Hall C Vade Mecum[2]. This means that in all cases, $\hat{x}$ points downwards and $\hat{y}$ points to the left when looking along $\hat{z}$. $\hat{z}$ is along the central ray of the spectrometer when talking about hms or sos quantities, and is along the beamline when talking about scattering angles in the lab. $\theta$ is the scattering angle (angle between the beamline and the outgoing event), and $\phi$ is the azimuthal angle. $\tan(\phi) = y/x$, which means that $\phi = 0$ is down, $\phi= \pi /2$ is beam left, etc.... Thus, $\phi_{spec}^{SOS} = \pi /2$, and $\phi_{spec}^{HMS} = 3\pi /2$.

There are, of course, a couple of exceptions. When generating the interaction point, we use the Accelerator coordinate system (as we do in the engine for the BPMs and Raster). In this case, y is vertical (+y is up??), x is horizontal (+x is left??), and z is downstream. In addition, some experiment specific quantities are defined (or calculated) using another coordinate system. Where this is the case, the code is commented, and a transformation is made between the standard SIMC coordinates and the desired values. However, any global variables SIMC uses should use the standard coordinate system.

Units are another matter entirely. As a rule, all distances are in cm, and most energies are in MeV. After that, it's anybody's guess. In order to make the code less unfriendly, I have tried to make sure that all input or output has comments that give the units, and have made all of the Ntuple variables follow the Hall C convention for units (GeV/cm/radians) as well as the standard naming conventions for the engine ntuples.