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

A revision of the concept of the CBM – MVDOr: Do we need an intermediate pixel detector?

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

M. Deveaux, Goethe University Frankfurt/Main

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

Why a revision?

Conclusion:“To measure c

+ CBM needs thin (less 200m !) MAPS detectors.”

Harder impact parameter-cut

Sufficient S/B

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

Why a revisionM

. De

vea

ux e

t al.: “R

&D

activitie

s for th

e C

BM

Micro

Ve

rtex D

ete

ctor (M

VD

)”C

BM

colla

bo

ratio

n m

ee

ting

, 25

. – 2

8. F

eb

20

08

, GS

I, Da

rmsta

dt

z = 10 cm

Requirement

Our running conditions

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

There is an obvious misfit between required and possible material budgetRevise global MVD concept

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

Standard detector layout (reminder)

MVD 1z=10cm MVD 2

z=20cmStrip 1z=30cm

Target

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

Detector resolution?

1) 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 VertexSecondary Vertex

z(Secondary Vtx)Impa

ct-p

aram

eter

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

Impa

ct-p

aram

eter

z1

z2

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

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?

0.002 0.004 0.006 0.008 0.01

20

40

60

80

100

120

z1 = 5 cm

Material budget [X0]

IP-r

esol

utio

n [µ

m]

Required

Iouri’s “thin detector”

Probable material budget

Iouri’s “thick detector”

z1 = 10 cm

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

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 iss

ue

Occupancy?

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

Hit

s / c

oll.

/ mm

²

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

Cluster merging?

Assume: We want a < 1% probability for cluster merging. How to estimate max. occupancy?

Cluster

Detector

Free detectorsurface

Occupied detectorsurface

!

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

Cluster merging?

Free detectorsurface

Occupied detectorsurface

Cluster, 3x3 pixelpixel pitch = 15µm

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

Station at 5cm => ~ 3.5 tracks / (106 µm²) => Pileup = 2

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

Track matching probability

MVD 1z = 5 cm MVD 2

z= ???Strip 1z=30cm

Target

Wrong charm signature

To avoid this scenario, pointing resolutionof 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?

Mat

eria

l bud

get [

% X

0]

Detector – Position [cm]

6 8 10 12 14 16 18 20

0.5

1

1.5

2

2.5

The minimum material budget of detector stations depends on theirposition.

6 8 10 12 14

20406080

100120140160

Accounting for this, we estimate thepointing precision from Station 2 => Station 1

Station 2 has to be placed atz = ~ 8 cm=> Hit density ~1.5 / mm² / coll

Pos (station 2) [cm]

Poin

ting

res

olut

ion

Station 3 has to be placed at z = ~ 11.5 cm, mat. budget = 700 µm Si equivalentHit density: ~1.2 / mm² / coll.

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

MVD – STS – Track matching

MVDTarget

z =

7.5

cm

z =

5 c

m

z =

11.

5 cm

STS

1, z

= 3

0 cm

500 µm Si

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?)

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

Summary and conclusion

Higher, realistic material budget reduces the selectivity of the MVDTo remain sensitive for open charm, the MVD must be placed closer to the target

Close distance to target + delta electrons generate very high occupancyHit finding and track matching become crucial already at ~ 105 coll. /s

A “compact MVD” design is seems required for reasonable track matching in MVDIntermediate 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.