1. The Geant3 tale ➢ Motivation... ➢ Why AliROOT+G3 are not reproducing any more the measurements ? ➢ Why stand alone code do reproduce measurements ? talk source: http://www-linux.gsi.de/~abercuci/Contributions/Talks/Software

2. Software Meeting GSI May 23, 20062 G3 tracking Initialise material constants for all the physics mechanisms used by GEANT Initialization of tables for energy loss straggling in thin gas layers Initialization of the values needed in computation of dN^2/dE/dx as in Grishin,Ermilova,Kotelnikov, NIM A307(1991),273 dN^2/dEdx table for a gamma factor Controls tracking of current particle up to end of track for sequential tracking mode, or through current volume for parallel tracking mode. Electron type track. Computes step size and propagates particle through step. Subroutine to decide which method is used to simulate the straggling around the mean energy loss. Simulation of energy loss straggling in thin layers. Sampling is done from the tables which are prepared in GSTINI. Generates Delta rays Charged hadron type track. Computes step size and propagates particle through step.

3. Software Meeting GSI May 23, 20063 Too much memory economy ? ● location G3STXIN ● location G3STTAB EMAX=LOG(DCUTE*1.E9) EMAX=29.9335 Calculate d2N/dEdx Calculate dN/dx ● What are CERN people saying ? “This is not a bug but a feature. [...] If you decide to produce explicitly d-rays above a certain threshold, then you should adjust the BB so that these are not counted twice. The "correction" you propose would lead to a double counting of the energy lost by the particle, so it is definitely wrong.” *) see G3 Manual pg. 264

4. Software Meeting GSI May 23, 20064 Who's afraid of 0 ? ● What are CERN people saying ? AMU = DAN*STEP CALL G3POISS(AMU,N,1) NICOLL = MAX(N,1) NICOLL = N ● location G3STREN The reason for this protection is to have at least one collision, otherwise there is no fluctuation and no straggling and the distribution is too narrow.

5. Software Meeting GSI May 23, 20065 Redundancy ? ● location G3TELEC IF((IPART.EQ.2).OR. ((IPART.EQ.3).AND.(GEKIN.GT.2.*DCUTE))) CALL G3DRAY ● Moeller theory for e- ● Bethe theory for e+ 4 GeV/c electrons G3 11.73 % delta per primary track 124.1 keV mean energy Stand alone code based on PAI 22.2 % delta per primary track 167.8 keV mean energy “PAI should not be used. If it is (used), it should be switched off and all the tests redone with the standard energy fluctuation. PAI is for very specific studies, and not for general use. There is no point in discussing it for a general case. [...]This is not the standard GEANT.”

6. Software Meeting GSI May 23, 20066 What about AliROOT... ● AliTRD::CreateMaterials() ● AliTRDv1::FixStep() wXeCO2[3] = { 0.85, 0.0375, 0.1125}; wXeCO2[3] = { 8.5, 1.5, 3. };..................................................... AliMixture(10,"Gas mixture",aXeCO2, zXeCO2, dgm, 3, wXeCO2); AliMixture(10,"Gas mixture", aXeCO2, zXeCO2, dgm, -3, wXeCO2); if (( gMC->TrackCharge() ) && (!gMC->IsTrackStop() ) && (!gMC->IsTrackDisappeared())) { *) Thanks to Andreas Morsch for pointing it out.

7. Software Meeting GSI May 23, 20067 Results and answers... Why AliROOT + G3 are not reproducing any more the measurements ? Most probable because of: a) DCUTE b) material definition(s) c) FixStepManager() Why stand alone code do reproduce measurements ? Because of luck ! Why using the PAI model for TRD ? a) Because the number of inelastic collisions is small ! 2 GeV/c pi+ = 64.5/15 end is even more reduced by the magnetic field ! b) Because we need the number of inelastic collisions. c) Alternatives... ?! more results can (or will) be found @: http://www-linux.gsi.de/~abercuci/MyWork/ALICE-TRD/dEdx/FixStep/Results/New

8. Software Meeting GSI May 23, 20068 The future... ● Can we use G3 in AliROOT (TRD) ? ● What do we learn from the G3 lesson ? ● Efficiency ? ● What about the G3 community ? Disclaimer: I have used for the current analysis AlirROOT v4-04-02 and ROOT v511-01-alice some results may not be in agreement with AlirROOT v4-04-09 and ROOT v511-02 or the G3 distribution which is accompanying them.