1. Background simulation issues
G.Calderini (LPNHE, Paris) on behalf of
the Background Simulation Group
2nd Super B Detector R&D Workshop
SLAC, February 14-16, 2008

2. 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

3. 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 BaBar (but no need, at that stage) !
- A very simplified geometry model was setup.
A totally standalone Geant4 framework
was used

4. The shapes limited to cylinders/discs and cones
CDR detector
IFR
EMC
DCH
(conical endplate)
SVT
The shapes limited to cylinders/discs and cones

5. 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

6. That was perfect for the initial rate studies
Example: SVT Layer0
pixels vs strips
Choice of detector technology
First evaluation of radiation tolerance
Pair production
EMC
Radiative BhaBhas
IFR
No news here!

7. 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.

8. Touschek particles hitting
the beam pipe
(Manuela)
E.Paoloni

9. Brand new set of machine parameters (Pantaleo) and beam scrapers (M
Brand new set of machine parameters (Pantaleo) and beam scrapers (M.Boscolo) e- e+
LER
HER
E.Paoloni
CDR
New
LER
HER
Vert. emitt. (pmr) 4 7
Hor. emitt. (nmr) 2
2.8
1.6
particles/bunch 1010
6.16
3.52
5.52
Touschek lifet. (min)
5.5 38
13.8
20.6

10. The other subdetectors should also check these new data !
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 !

11. They are always very challenging and sometimes not fully reliable
General disclaimer on background simulations
They are always very challenging and
sometimes not fully reliable
At BaBar, a huge discrepancy is present between
(even recent) backgrounds simulations and data,
in spite of a very detailed detector and beamline
description.

12. 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

13. 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

14. Sounds unpractical While the present backgrounds collections are
good for occupancy and radiation studies…
… they are almost useless for a fast (non-G4)
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

15. Once again this seems unpractical
Nevertheless it is also difficult to interface
background collections to a detailed G4 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

16. 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

17. 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 etc…

18. The beamline: Need to complete the automated translation procedure
Tool to convert MAD to G4 still incomplete
Some of the material information is not even
present in the decks
Since the most relevant backgrounds are
radiative BhaBha and Touschek, the beamline
description will be crucial
Many accelerator/detector iterations needed
during the machine design and the shielding
studies -> flexibility

19. Still missing studies:
1) Full simulation of pairs production
Not fully propagated in Geant4 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/cm2 in Layer_0, decreasing with R).
Probably a pessimistic extrapolation, given the absence of
permanent dipoles close to the IP in SuperB final focusing.
Further effort necessary
A Turtle description or something equivalent should be setup

20. 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 20KeV, reducing to a few g/BX on the Be pipe, a rate similar to BaBar.
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.

21. 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

22. 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 Geant4

23. Conclusions The long list of items to be done requires that a
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