We have conducted a test aimed at validating the TGeo
propagation engine, as compared to the standard Geant 3.21.
In the course of this test, an identical set of kinematic
tracks was injected into respective models of the STAR
detector, produced from the same source (based on the
standard STAR description within Geant 3.21; the TGeo
model was obtained by using a converter now incorporated
in the St_geant_Maker, and/or by running a standalone
utility g2root (such results are concidered separately).
Consider a particular volume. Tracks entering this volume
will travel a certain distance inside it before exiting.
The following metrics were considered in this test:
The rationale behind it was of course that on the one hand
it allows to cover a large number of volumes without specific
coding, and that on the other hand close numerical results
in these metrics would signal the consistency of track
propagation across both engines.
- The average length of tracks having traversed the volume
- The width of the distribution of the above
Let's enumerate the two cases as 1 and 2.
The following convention was used:
The following "goodness" vlaues were then defined:
- Nk1,2 -- numbers of entries into the volume number "k"
- Ak1,2 -- average track lengths in the volume number "k"
- Dk1,2 -- widths of the above, in the volume number "k"
dN = (N1-N2)/sqrt(N1+N2)
dA = (A1-A2)/sqrt(D1*D1/N1 + D2*D2/N2)
Test cases included a "TPC ONLY"
geometry, as well as the full STAR with
the exception of the SVT, which due its
complexity merits its own study. We
also used different conversion schemes
as mentioned above. In the end, one
has the following sets of data:
- tpc_only + GSTAR
- tpc_only + STAR conversion to TGeo
- tpc_only + G2ROOT conversion to TGeo
- full-svt + GSTAR
- full-svt + STAR conversion to TGeo
- full-svt + G2ROOT conversion to TGeo
Below is a condensed summary of the tests that have
- 1 vs 2: discrepancies were found which can be explained by
(a) handling of the divisions in Geant 3 vs emulation of the divisions
with volume copies in TGeo, which may lead to tiny cracks between
volumes due to finite numerical accuracy (b) Imprefection in the
STAR conversion scheme for volumes of zero size, whereby there
is a duplicate name rejected by TGeo
- 1 vs 3: match
- 4 vs 5: discrepancies of the kind described above
- 4 vs 6: g2root failure