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

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

2. 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

3. 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

4. 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

5. 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

6. Primary Vertex Impact-parameter z1 z2 What do we need? M. Deveaux, CBM collaboration meeting, 14.-17. Oct. 2008, Dubna, Russia

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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