A revision of the concept of the CBM

1
A revision of the concept of the CBM MVDOr Do
we need an intermediate pixel detector?
M. Deveaux, Goethe University Frankfurt/Main

• Why a revision of the concept?
• Strategies to improve detector resolution
• Occupancy and consequences
• Summary and conclusion

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Why a revision?
Sufficient S/B
Harder impact parameter-cut
Conclusion To measure ?c CBM needs thin (less
200?m !) MAPS detectors.
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Why a revision
Optimistic estimate of the material budget of the
first MVD-station
M. Deveaux et al. RD activities for the CBM
Micro Vertex Detector (MVD) CBM collaboration
meeting, 25. 28. Feb 2008, GSI, Darmstadt
There is an obvious misfit between required and
possible material budget Revise global MVD concept
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Standard detector layout (reminder)
Target
MVD 1 z10cm
MVD 2 z20cm
Strip 1 z30cm
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Detector resolution?

• A good detector resolution.
• Detector needs to be better than a standard MVD
  with a first station at 10 cm and 200 µm silicon.
• What does this means in terms of resolution?

Primary Vertex
Secondary Vertex
Lets calculate the impact parameter resolution
of the MVD
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What do we need?
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Impact parameter resolution
We are multiple scattering dominated. We have to
reach an IP-resolution of 45 µm (Easy if first
station at z5 cm). Open Can one put the first
station to 5 cm?
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A vertex detector at z 5cm?
Detector lifetime? _at_10 cm gt 12.0 x 1011 min.
bias collisions _at_ 5 cm gt 4.4 x 1011 min. bias
collisions (46 days at 105 coll/s)
Open issue
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Cluster merging?
Detector
Cluster
Assume We want a lt 1 probability for cluster
merging. How to estimate max. occupancy?
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Cluster merging?
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Track matching probability
Target
MVD 1 z 5 cm
MVD 2 z ???
Strip 1 z30cm
To avoid this scenario, pointing resolution of
station 2 to station 1 must be sufficiently
good. Assume Search cone cluster size ( 20
µm) gt PAmb lt 1
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What about track matching?
Material budget X0
Detector Position cm
Station 3 has to be placed at z 11.5 cm, mat.
budget 700 µm Si equivalent Hit density 1.2 /
mm² / coll.
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MVD STS Track matching
z 11.5 cm
z 7.5 cm
500 µm Si
z 5 cm
STS 1, z 30 cm
Target
MVD
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Summary and conclusion

• Higher, realistic material budget reduces the
  selectivity of the MVD
• To remain sensitive for open charm, the MVD must
  be placed closer to the target
• Close distance to target delta electrons
  generate very high occupancy
• Hit finding and track matching become crucial
  already at 105 coll. /s
• A compact MVD design is seems required for
  reasonable track matching in MVD
• Intermediate pixel detectors might be needed for
  STSgt MVD track matching.
• Assumptions made are conservative
• Hot spot occupancy is assumed
• Option to detect/reject bad clusters or
  ambiguous tracks is ignored
• Might clever algorithms allow for gt 105 col/s
  operation? Needs to be simulated.
• Neglect the occupancy from delta electrons in
  simulation is not justified.