All values of constants, dimensions, defaults, cuts, etc. must be grouped in well-identified places:
The geometry, calibration and alignment constants must be in the geometry/conditions data base;
All numbers relative to a single algorithm must be in a single (header?) file and assigned to variables with appropriate, meaningful names.
Rule 2: Adopt the CLHEP SystemOfUnits header file
The SystemOfUnits.h header, with the unit definitions, must be included in all places where there is any number.
Only two basic units differ from current (Geant3 inherited) practice:
millimeters for length
MeV for energy
These units will also be used for the persistent representation of all ATLAS data.
Rule 3: All numbers come with units
All numbers come with the units in which they are specified.
Examples:
double pixelLength = 400 * micrometer;
double pT_Cut = 1.0 * GeV;
double coolingFluidPressure = 2 * bar;
Rule 4: Interfaces with external packages
Some external packages are beyond our control and use their own systems of units,
there is no way to change that. Wherever this happens, a well-defined interface to the rest of our code must be defined.
Rule 5: Reference frame
The ATLAS reference frames are defined in the
TC Document
Rule 6: Range of φ
For most applications, it is irrelevant to specify the range of φ:
(-π, +π] or [0, 2*π).
In practice, φ is calculated mostly through the arc tangent function,
which returns angles in the range (-π, +π] both in Fortran and
in C/C++ (CLHEP methods in particular return phi() in this range).
We therefore choose the φ range as (-π, +π].
Care has to be taken when filling histograms or calculating Δφ.