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Background simulation issues G. Calderini (LPNHE, Paris) on behalf of the Background Simulation Group 2 nd Super B Detector R&D Workshop SLAC, February 14 -16, 2008
What has been done so far - The initial strategy - The results achieved - The limitations Time for a step forward - The necessity of an improved framework - Some standardization - The integration with the physics simulation The tool development
Phase I - Most of the background simulation for the CDR was done by a restricted group of 2 -3 people - The contribution from the subsystems was limited to qualitative descriptive input and analysis of the produced background files - There was no time and no manpower for a detailed detector description, either new or borrowed from Ba. Bar (but no need, at that stage) ! - A very simplified geometry model was setup. - A totally standalone Geant 4 framework was used
CDR detector IFR EMC DCH (conical endplate) SVT The shapes limited to cylinders/discs and cones
- No detector segmentation (strips, cells, tubes) - Detector geometry hardwired in the code - Just hits with coordinates and some additional information subdetector-dependent - energy loss in the relevant material - timestamp - etc IFR
That was perfect for the initial rate studies SVT Choice of detector technology Example: strips. Layer 0 pixels vs First evaluation of radiation tolerance Pair production EMC Radiative Bhas IFR No news here!
Touschek: some news Particles in the same bunch can undergo Touschek scattering and escape the ring energy acceptance window. Off energy particles hit the storage ring material producing backgrounds. Manuela Boscolo (LNF) developed a tool to simulate Touschek scattering around the ring, which was used already for the CDR simulation. The rate was rather high and this triggered a change of the machine parameters.
Touschek particles hitting the beam pipe (Manuela) E. Paoloni
Brand new set of machine parameters (Pantaleo) and beam scrapers (M. Boscolo) e- e+ LER HER E. Paoloni CDR LER New HER LER HER Vert. emitt. (pmr) 4 7 4 Hor. emitt. (nmr) 2 2. 8 1. 6 particles/bunch 1010 6. 16 3. 52 Touschek lifet. (min) 5. 5 38 5. 52 13. 8 20. 6
The new set of parameters almost zeroed the impact of LER Touschek on the SVT The other subdetectors should also check these new data ! Unfortunately, according to Pantaleo, the new machine parameters might need to be changed !
General disclaimer on background simulations They are always very challenging and sometimes not fully reliable 1) At Ba. Bar, a huge discrepancy is present between 2)(even recent) backgrounds simulations and data, 3)in spite of a very detailed detector and beamline 4)description.
2) Massive detector shielding (2. 7 Tons tungsten) has been included for the CDR studies E. Paoloni Very expensive and probably unrealistic in the final design; but without it the background rates for the present IR & optics would be very different
Phase II: something new At the December computing workshop the basis was laid for a common computing framework A fast simulation is being discussed (something existed only for some subdetector) A full detailed simulation of the detector, necessary for later studies is also being discussed Points of contact with the present simulation - Common issue of detector description - Both fast- and full simulation will need the presence of backgrounds hits
While the present backgrounds collections are good for occupancy and radiation studies… … they are almost useless for a fast (non-G 4) physics simulation In principle one could think of an interface to translate background hits into the fast simulation re-adjusting the unavoidable discrepancies in the position and copying them in the final environment Sounds unpractical
Nevertheless it is also difficult to interface background collections to a detailed G 4 simulation (also in that case, the two geometry descriptions don’t probably match precisely and anyway the detector segmentation would be different) It would be necessary to keep the two geometries exactly compatible. Any change to the detector in one environment would need the corresponding change in the other Once again this seems unpractical
Need to converge towards a single description as flexible as possible The detector sketch: - Detector geometry needs to be taken out from the code GDML / XML (or some other standard) external files to speed-up geometry changes and iterations Present code then needs to be modified to read these files - The same mechanism could be used for the shielding description
- Geometry description needs to be refined -> better level of detail -> detector upgraded with respect to CDR - Background root files contents revisited according to subsystems’ requirements I think anyway that not everything can be written from scratch, something will need to be inherited unless enough volunteers to rewrite the signal processing of each subdetector, the digitization, the reconstruction etc…
The beamline: - Need to complete the automated translation procedure Tool to convert MAD to G 4 still incomplete Some of the material information is not even present in the decks - Since the most relevant backgrounds are radiative Bha and Touschek, the beamline description will be crucial - Many accelerator/detector iterations needed during the machine design and the shielding studies -> flexibility
Still missing studies: 1) Full simulation of pairs production Not fully propagated in Geant 4 for the CDR 2) Lost particles Mainly Coulomb/Bremsstrahlung interactions with residual gas molecules in the beampipe For the CDR effort, the estimate was based on PEP-II rates and rescaled for beam currents. Found to be negligible with respect to other sources (1 -2 MHz/cm 2 in Layer_0, decreasing with R). Probably a pessimistic extrapolation, given the absence of permanent dipoles close to the IP in Super. B final focusing. Further effort necessary A Turtle description or something equivalent should be setup
3) Synchrotron radiation The interaction region has been designed to reduce the bending of incoming beam trajectories, but some photons can still hit the beampipe. With the present final focusing, the expected rate on the IR mask tip is about 800 g/BX for energies above 20 Ke. V, reducing to a few g/BX on the Be pipe, a rate similar to Ba. Bar. Preliminary design of adequate mask system already available. Update necessary to the next final focus design, and additional studies for the residual backgrounds in the detector.
Manpower issues The number of people presently working in the core group (and the fraction of their time on this) is clearly not enough for the planned amount of tasks At least one more person would be needed to work with subdetectors on the xml construction It would be advisable to have one or two more for working on the beamline update and on the simulation of the sources of backgrounds which are still missing
Short-term (urgent) goals: Elba challenging but feasible with the present manpower, well-reasonable if additional help available Interact with the subdetectors to: - define the geometry standard - review the rootfile content Clarify the status of the tools wrt the beamline Integrate the pair-production in Geant 4
Conclusions A very big effort has been made on background simulations by a small group of people for the CDR publication. In my opinion, the work has been much more detailed and elaborate with respect to what expected for a CDR standard, but it is definitely small with respect to what is needed next. A participation from the subdetectors is getting mandatory, and fortunately there are signs this is actually beginning. The long list of items to be done requires that a minimum amount of 2 -3 new people join the background simulation group to maintain the project going