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A revision of the concept of the CBM – MVD Or: 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. Why a revision?.

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a revision of the concept of the cbm mvd or do we need an intermediate pixel detector

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

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

why a revision
Why a revision?

Sufficient S/B

Harder impact parameter-cut

Conclusion:

“To measure c+ CBM needs thin (less 200m !) MAPS detectors.”

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

why a revision1

z = 10 cm

Our running conditions

Requirement

Why a revision

Optimistic estimate of the material budget of the first MVD-station

M. Deveaux et al.: “R&Dactivities 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

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

standard detector layout reminder
Standard detector layout (reminder)

Target

MVD 1

z=10cm

MVD 2

z=20cm

Strip 1

z=30cm

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

detector resolution

Impact-parameter

z(Secondary Vtx)

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

Let’s calculate the impact parameter resolution of the MVD

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

what do we need

Primary Vertex

Impact-parameter

z1

z2

What do we need?

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

impact parameter resolution

z1 = 10 cm

Iouri’s “thick detector”

IP-resolution [µm]

Probable material budget

z1 = 5 cm

Required

Material budget [X0]

Iouri’s “thin detector”

Impact parameter resolution

We are multiple scattering dominated.

We have to reach an IP-resolution of ~ 45 µm (Easy if first station at z=5 cm).

Open: Can one put the first station to 5 cm?

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

a vertex detector at z 5cm

Occupancy?

C. Trageser, Bachelor Thesis (together with S.Seddiki)

Hits / coll. / mm²

A vertex detector at z = 5cm?

Detector lifetime?

@10 cm => 12.0 x 1011 min. bias collisions

@ 5 cm => 4.4 x 1011 min. bias collisions (46 days at 105 coll/s)

Open issue

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

cluster merging

Occupied detector

surface

Free detector

surface

!

Cluster merging?

Detector

Cluster

Assume: We want a < 1% probability for cluster merging.

How to estimate max. occupancy?

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

cluster merging1

Cluster, 3x3 pixel

pixel pitch = 15µm

Occupied detector

surface

Free detector

surface

Station at 5cm => ~ 3.5 tracks / (106 µm²)

=> Pileup = 2

10 µs time resolution => maximum collision rate ~ 2 x 105/s

Cluster merging?

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

track matching probability

Wrong charm signature

Track matching probability

Target

MVD 1

z = 5 cm

MVD 2

z= ???

Strip 1

z=30cm

To avoid this scenario, pointing resolution

of station 2 to station 1 must be sufficiently good.

Assume: Search cone = cluster size (~ 20 µm) => PAmb < 1%

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

what about track matching

The minimum material budget

of detector stations depends on their

position.

Accounting for this, we estimate the

pointing precision from

Station 2 => Station 1

Pointing resolution

Station 2 has to be placed at

z = ~ 8 cm

=> Hit density ~1.5 / mm² / coll

Pos (station 2) [cm]

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.

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

mvd sts track matching

Old Geometrie (with Deltas):

Old Geometrie (no Deltas):

Track matching from STS to MVD turns into a crucial issue.

Probably: Intermediate detectors are needed (Hybrid pixels?)

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

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

summary and conclusion
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 STS=> 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 > 105 col/s operation? Needs to be simulated.
  • Neglect the occupancy from delta electrons in simulation is not justified.

M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia