1. 20/12/2011Christina Anna Dritsa1 Design of the Micro Vertex Detector of the CBM experiment: Development of a detector response model and feasibility studies of open charm measurement C.Dritsa

2. 20/12/2011Christina Anna Dritsa2 Outline Motivation Motivation Detector response studies Detector response studies Detector response model Detector response model Simulation results Simulation results Conclusion Conclusion

3. 20/12/2011Christina Anna Dritsa3 QCD phase diagram Exploration of the phase diagram of nuclear matter is important for the understanding of QCD Explore fundamental properties of strong interaction and origin of mass for light quark.

4. 20/12/2011Christina Anna Dritsa4 The FAIR accelerator

5. 20/12/2011Christina Anna Dritsa5 The CBM experiment Onset of deconfinement phase transition at high net baryonic densities Chiral symmetry restoration at high net baryonic densities Phase transition between hadronic and quark-gluon matter Search for the QCD critical endpoint Open charm particles are produced during the early stages of the collision => among most important observables. MVD+STS Tracking, vertexing, momentum characterisation in magnetic field Particle ID: RICH, TRD (electrons) ECAL (photons) TOF (hadrons) PSD: Event characterisation

6. 20/12/2011Christina Anna Dritsa6 Driving CBM experimental requirements Particle multiplicity x branching ratio min. bias Au+Au collisions at 25 GeV (from HSD and thermal model) P.Senger SPS Pb+Pb 30 A GeV Need for high beam intensities

7. 20/12/2011 Christina Anna Dritsa 7 D 0 detection strategy Identify decay vertex (SV) from the primary collision vertex (PV) few 100 µm in the lab Avoid multiple scattering: x ~ 0.3 % X o Measure precisely: spatial resolution < 5µm High beam intensities: granular and radiation hard

8. 20/12/2011Christina Anna Dritsa8 Choice of technology Hybrid Pixels ~25ns >10 14 n eq ~ 30µm ~ 1% X 0 Topic s.p. resolution Material budget Time resolution Rad. hardness ~5µm ~100µs* ~10 10 n eq * CCDs ~ 0.1% X 0 1.5µm ~ 1 ms MAPS (2003) ~10 12 n eq ~ 0.1% X 0 [after M.Deveaux, PhD Thesis ] Technology of choice: Monolithic Active Pixel Sensors (IPHC, Strasbourg) CBM (10 5 coll/s) ~ 5µm Few 0.1% X 0 ~10 µs ~10 13 n eq /cm² Not yet optimised

9. 20/12/2011Christina Anna Dritsa9 MAPS ( expected 2015) MAPS performances over time Topic s.p. resolution Material budget Time resolution Rad. hardness CBM (10 5 coll/s) ~ 5µm Few 0.1% X 0 ~10 µs ~10 13 n eq /cm² 1.5µm ~ 1 ms MAPS (2003) ~10 12 n eq ~ 0.1% X 0 3.5µm ~ 100 us MAPS (2011) >10 13 n eq ~ 0.05% X 0 3.5µm ~ 30 µs <10 14 n eq ~ 0.05% X 0 Compromise: Accept pile up of several events in the MVD? How many? What about the increased hit density? Delta electrons?

10. 20/12/2011 Christina Anna Dritsa 10 Is open charm measurement possible? Very high hit density for most upstream detector stations Delta electrons increase the occupancy by one order of magnitude previous studies no studies +

11. 20/12/2011 Christina Anna Dritsa 11 MAPS operation principle - - - - - - - - - - - epitaxial layer substrate micro-circuits Pixel pitch ~20 µm spatial resolution few µm low material budget (~ 0.05% X 0 ) diffusion

12. MIMOSA 26 20/12/2011Christina Anna Dritsa12 MAPS operation principle not sensitive (chip intelligence) SENSITIVE AREA 1152 x 576 Pixels 18.4 µm pitch Column parallel readout: scales with number of pixels in column

13. 20/12/2011 Christina Anna Dritsa 13 Is open charm measurement possible? Very high hit density for most upstream detector stations Delta electrons increase the occupancy by one order of magnitude previous studies no studies +

14. 20/12/2011Christina Anna Dritsa14 IDEAL RESPONSE REAL RESPONSE × REAL MEASURED RESPONSE Need for realistic response MVD station MAPS PIXEL SENSOR (MIMOSA 17 ) ×

15. 20/12/2011Christina Anna Dritsa15 Need for realistic response IDEAL RESPONSE REAL RESPONSE × MVD station × Detailed simulation requires precise understanding of the sensor: Charge distribution among pixels Number of significant pixel in cluster Response to inclined tracks … Perform measurements at CERN-SPS

16. 20/12/2011 Christina Anna Dritsa 16 Experimental setup Sensor: MIMOSA-17 30 µm pixel pitch Analog readout “Standard” 14 µm epitaxial layer AMS 0.35 µm OPTO S2: Scintillator Mi 17 Angles: 0°–75° S1: Scintillator Mi 17 Beam direction

17. 20/12/2011Christina Anna Dritsa17 MPV Important definitions Landau Lorentz

18. 20/12/2011Christina Anna Dritsa18 Important definitions Number of Pixels Accumulated charge (MPV) Q1Q2 Q3 Q1>Q2>Q3… Qi Q1Q2 Q3 Q1Q1+Q2Q1+Q2+Q3 Q1+Q2+…+Qi Qi<0 (i~N) 123 N … The accumulated charge plot describes the charge sharing among the pixels of the cluster: e.g. the seed pixel collects ~30% of the total cluster charge. Accumulated charge plot

19. 20/12/2011Christina Anna Dritsa19 Accumulated charge plot