# Presentation: 551118

## A high-performance Silicon Tracker for the CBM experiment at FAIR

- J.M. Heuser, W. Müller, P. Senger  (GSI Darmstadt)C. Müntz, J. Stroth  (University of Frankfurt)
- for the CBM Collaboration
- PANIC05  – Santa Fe, New Mexico, October 2005

- _Overview:_
- The future accelerator facility FAIR in Darmstadt
- The **C**ompressed **B**aryonic **M**atter experiment
- The CBM Silicon Tracker
- Performance requirements
- Detector concept
- R&D directions

## Facility for Antiproton and Ion Research

- FAIR: Future international accelerator complex at GSI, Darmstadt, Germany
- _Research program includes physics with:_
-  Radioactive ion beams:    Structure of nuclei far from stability
-  Anti-proton beams:    Hadron spectroscopy, anti hydrogen
-  Ion and laser induced plasmas:    High energy density in matter
-  High-energy nuclear collisions:      Strongly interacting matter at     high baryon densities

**C**ompressed **B**aryonic **M**atter Experiment

- _Project Management: _
- Start of construction:   2007/2008
- First beams:                2011
- Full operation, **CBM**:   2015

**SIS 100**
**SIS 300 **

- UNILAC

**max.  ****U: 35 GeV/n **
**p: 90  GeV **

**Heavy-ion synchrotrons**

-  see talk of L. Schmitt

## CBM - Physics Motivation

- confinement, broken chiral symmetry, hadron masses.
- _CERN-SPS and RHIC:_
-   indications for a new state of      matter: „Quark Gluon Plasma“.   Produced at high T and low B.   LHC: even higher T, lower B.
- _QCD phase diagram: _
-   poorly known at low T, high B:  new measurements at **FAIR**:       with highest baryon densities,         and with new probes!

****** ****CBM Experiment **
**      **

**Notes:**
intermediate range: limited number of observables!
region to be expolred by CBM
several aspects which make this region very interesting: dense baryonic medium -> in medium properties of hadrons
									critical endpoint, deconfinement?

## Physics and Observables

_**Tracking challenge:**_
-   up to 107 Au+Au reactions/sec @ 25 GeV/nucleon
-   ~ 1000 charged particles/event, up to ~100 tracks/cm2/event
-   momentum measurement with resolution < 1%
-   secondary vertex reconstruction ( 30 m)
-   high speed data acquisition and trigger system

| Physics | Observables |
| --- | --- |
| In-medium modifications of hadrons:
Onset of chiral symmetry restoration  | , ,     e+e-   (μ+ μ-)open charm: D0, D± |
| Indications for deconfinement:
Anomalous charmonium suppression ? | D0, D±, J/    e+e-   (μ+ μ-) |
| Strangeness in matter:
Enhanced strangeness production | K, , , ,  |
| Critical point:
Event-by-event fluctuations  | , K |

**Open charm measurement: **
- One of the prime interests of CBM, one of the most difficult tasks!

- URQMDAu+AU 25 GeV/nucleon

## The CBM Experiment- Conceptional Design -

- The CBM Experiment- Conceptional Design -

-   Tracking, momentum measurement, vertex reconstruction:      Exclusively with a Silicon Tracking System (STS)
-   Electron ID: RICH & TRD (& ECAL)
-   Hadron ID: TOF (& RICH)
-   Photons, 0, : ECAL
-   High interaction rates
-   No central trigger
-   Data-push r/o       architecture

- beam

- target

- STS
- (5, 10, 20, 40, 60, 80, 100 cm)

- TRDs
- (4,6, 8 m)

- TOF
- (10 m)

- ECAL
- (12 m)

- RICH

- magnet

- Further specific detector configurations under study.

## The Silicon Tracking System

- - Conceptional Geometry -

- Assume 7 planes
- 2 or 3 _thin_ pixel stations:
-        secondary vertex detection
-           (benchmark: open charm)
- 4 or 5 _thin_ strip stations:
-        tracking
- Acceptance: 50 to 500 mrad
- First plane:
-        z=5cm ;   size 25 cm2
- Last plane:
-       z=100cm; size  ~1 m2
- Magnetic dipole field:
-       ~ 1Tm, p/p <1%  @  p=1 GeV

## Challenge: Open Charm Reconstruction

- _Some hadronic decay modes:_
- D (c = 317 m):
- D+  K-++  (9  0.6%)
- D0 (c = 124.4 m):
- D0  K-+  (3.9  0.09%)

-  High-granularity sensors. Thin tracking stations.

- Rare probe:   High level charm trigger.

## Pixel Detectors for Vertexing

- I. Vassiliev et al., GSI

- What kind of **pixel detectors** can do the job?  Study of different detector types, characterized by their material budgets and pixel sizes:

- D0 impact parameter measurement

-  = 38 μm

- MAPS 25x25 μm2 100 μm thick
****** ****= 38 ****μ****m**

-  small pixels ≤ 25 x 25 μm2 ( thin sensors ~ 100 μm )

## D0→ K-π+ reconstruction using MAPS

- study by  I. Vassiliev, GSI

**~10 % efficiency**

- Signal

- Cuts

## Pixel Detector Requirements

-  Small pixels – less than 25 x 25 μm2
-  Thin – less than ~100 μm silicon
-  Radiation hard > 1014 nequiv/cm2
-  Fast readout – interaction rate up to 107/s

- Such a detector does not exist !
- Two possible R&D directions:

- _Monolithic Active Pixel Sensors (MAPS):_
- - small pixels:    25 x 25 μm2
- - thin:        standard 120 μm; study: 50 μm
- - spatial resolution: ~3 μm
- - too slow for CBM: ~ms/Mpixel full frame
- - limited rad. hardness  (bulk damage)
-  Improve r/o time, radiation tolerance.

- _Hybrid Pixel Detectors (LHC type):_
- - fast readout
- - radiation hard
- - too large pixels:       50 x ~400 μm2 - spatial resolution:  ~15 (115) μm
- - thick: standard       > 350 μm
-  Reduce pixel size and thickness.

- We started persuing the MAPS option, together with IReS  Strasbourg.Alternative to consider:  DEPFET sensors (MPI Munich).

## R&D goals with MAPS: Radiation tolerance & readout speed

- _R&D goals with MAPS:_
- radiation tolerance:  ~1012   1013 1 MeV nequiv.
- readout time:           10 sec, column parallel r/o,                            in reach in next years
- _Expected situation in CBM: _
- Fluence at 1st MAPS station:
-      ~10  1-MeV nequiv. per event
-        detector partly destroyed after 1012 reactions corresponds to 105 D mesons detected                                (decent measurement!)
- _Possible running conditions:_
-      a)   1 day detector lifetime at 107 reactions/s,      100 events piled up,       or
-      b)    4 month detector lifetime at 105 reactions/s,          no pile-up events.

**URQMD, Au+Au 25 GeV/nucleon**
**Fluence of ****1 MeV n****equiv.****/cm****2**** ****in 1****st**** MAPS station at z = 5cm**

- _Consider also future developments: _Hybrid pixels ~ 50x50 μm2, few hundred μm thick, with higher radiation tolerance and faster readout.

- Kpn

## Tracking in Silicon Strip Stations

| MAPS pile-up (events) | 
0 | 
5 | 
10 | 
20 | 
50 | 
100 |
| --- | --- | --- | --- | --- | --- | --- |
| Track category | Efficiency (%) |  |  |  |  |  |
| Reference primary | 96.37 | 96.08 | 95.84 | 95.15 | 93.79 | 91.47 |
| Ref.  set | 92.87 | 92.55 | 92.30 | 91.58 | 90.06 | 87.94 |
| All set | 86.17 | 85.52 | 84.97 | 83.69 | 80.97 | 78.47 |
| Extra set | 63.33 | 61.57 | 59.98 | 56.79 | 51.60 | 47.88 |
| Clone | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |   0.00 |
| Ghost | 2.47 | 3.59 | 4.55 | 6.53 | 9.85 |  13.33 |

- _First attempts:_ Problem - High occupancy with many combinatorial hit points in silicon strip stations.
- _Recent approach:_  Cellular automaton technique: Works!
- _Example:_4 strip stations + 3 MAPS stations

- MAPS pile-up: 10 events

- I. Kisel, Heidelberg, and S. Gorbunov, DESY

## Tracking Requirements

-  Tracking with microstrip and pixel stations:
-     Works despite of combinatorial hits and pile-up!
-      But:  Noise, misalignment,
-              detector inefficiencies
-              etc. not taken into account!
-  Consider more tracking redundancy!
-    Comprehensive study on the way to    optimize the Silicon Tracker's layout,    including:
-     - more tracking stations,
-     - several strip geometries,
-     - additional (hybrid?) pixel detectors        supporting the tracking, and
-     - detailed modeling of the detectors.

## Pixel Detector – Module Concept

- _Detector module_: BTeV inspired design ladders mounted on either side of a substrate providing (active?) cooling.

- _Active cooling support:_
-  a carbon fiber structure with micro     pipes?      ~ 0.3% X0
-  glass or silicon wafers with buried     micro channels?     ~ 0.1-0.3% X0

- _CMOS MAPS chips for CBM:_
- - size: ~0.5 x 1 cm2 - ~50% sensor, ~50% r/o.- column readout in ~10 μs
- CBM MAPS ladders with 4 or 5 "chips".

## Strip Detector – Modules & Stations

- _Basic sensor elements:_
- 200 m thick silicon wafers.
- double-sided, rad-tolerant. 50 m (25 m?) strip pitch.
- Inner  : 	6x4 cm
- Middle :   	6x12 cm
- Outer  :	6X20 cm

- _Study of:_
- strip length, pitch, stereo angle(to reduce fake hits)
- single-sided sensor option
- location of read-out chips(on sensor / outside acceptance)

- _Four detector stations:_
- built from a few wafer types.

## Summary

**CBM**   – High-rate fixed-target heavy-ion experiment planned at FAIR/SIS300.
-            – Strong-interaction physics, high baryon densities: Au+Au up to 35 GeV/nucl.
-            – Challenge: Rare probes - Open charm, low-mass vector mesons  di-leptons.
**Experimental concept, new to heavy-ion physics:**
-            – Tracking exclusively with a high-performance Silicon Tracker.            – **Very important detector system,  key to the physics of CBM. **
**Silicon Tracker performance requirements:**
-            – Efficient tracking, high momentum resolution.
-            – High-resolution vertexing.  Benchmark: Open charm.                Small pixels, thin, radiation tolerant, fast r/o. **Beyond state-of-the-art!**
**Detector R&D started:    **
-                       – Thin, fine-pitch double-sided microstrip sensors (tracking).
-            – **MAPS with improved radiation hardness, readout speed (vertexing). **
-            – Readout electronics.
**Open for new ideas!**         http://www.gsi.de/fair/experiments/CBM/index_e.html

## Discussion

## Low Mass Dilepton Spectroscopy

- If missed ...

- ... fake open pair is formed.

_**Signal:**_  vector meson decays
-                 , ,     e+e-

_**Background:**__   _
- 0 decay (365/event)
- 0  e+e-  (1.2%)	 0   98.8%)
- conversion    e+e-

_**Detector requirements:**_
-  first stations with large     acceptance
-  tracking efficiency     down to p = 0.1 GeV/c     to suppress background
-  detect conversion pairs:     → small pixels

## Delta Electrons

- study by  I. Vassiliev, GSI

- _Beam ions on target:_
-  produce delta-rays  dominate occupancy      when integrated over      many events.
-  high local radiation damage,      comparable to bulk damage. hits spoil track finding
-  limits rate capability
- _Only way out: _Fast detector readout to avoid electron hit pile-up.

-  hits in 1st MAPS station: 1000 min. bias URQMD events, Au+Au 25 AGeV.

## Data-Push Architecture, Data Flow

- Each detector channel detects **autonomously **all hits FEE design.
- An **absolute time stamp**, precise to a fraction of the sampling period, is associated with each hit.
**All hits are shipped **to the next layer (usually data concentrators).
- Association of hits with events done later using time correlation.
- _Typical parameters:_
- (few % occupancy, 107 interaction rate)
- some 100 kHz hit rate per channel
- few MByte/sec per channel
- whole CBM detector: ~ 1 Tbyte/sec