J.M. Heuser et al. CBM Silicon Tracker 1

Requirements for theSilicon Tracking System of CBM

Johann M. Heuser, M. Deveaux (GSI)C. Müntz, J. Stroth (University of Frankfurt)

for the CBM Collaboration

10th Semiconductor Detector Symposium, Wildbad Kreuth, June 2005

Overview:

The future accelerator facility FAIR in Darmstadt

The CBM experiment:

Physics Motivation

Detector Concept

Silicon Tracker – Physics Requirements and Impact on the Design

J.M. Heuser et al. CBM Silicon Tracker 2

Facility for Antiproton and Ion Research

Future accelerator complex FAIR at GSI, Darmstadt:

Research program includes:

• 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

SIS 100 Tm

SIS 300 Tm

U: 35 AGeV

p: 90 GeV

Compressed Baryonic Matter Experiment

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Facility for Antiproton and Ion Research

Photomontage of the existing and the planned research facility at GSI/FAIR.

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Strong-interaction physics:

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!

CBM Physics Motivation

SIS100/300

CBM Experiment

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Physics and ObservablesTransition of normal nuclear matter to a deconfined quark gluon plasma.

Physics Observables

In-medium modifications of hadrons:

onset of chiral symmetry restoration

, , e+e- (μ+ μ-)open charm: D0, D±

Strangeness in matter:

enhanced strangeness production K, , , ,

Indications for deconfinement:

anomalous charmonium suppression ? D0, D±, J/ e+e- (μ+ μ-)

Critical point:

event-by-event fluctuations π, K

Open charm measurement: one of the prime interests of CBM, and one of the most difficult tasks.

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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

beam

target

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

TRDs(4,6, 8 m)

TOF(10 m)

ECAL(12 m)

RICH

magnet

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Tracking Nuclear CollisionsCentral Au+Au collision, 25 AGeV:

URQMD + GEANT 160 protons 170 neutrons360 - 330 + 360 0

41 K+ 13 K- 42 K0

~500 charged primary particles in acceptance 50-500 mrad

Tracking challenge:

~ 1000 charged particles/event

momentum measurement with good resolution secondary vertex detection

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Open Charm Reconstruction

GeV3%18.0

mm50 μm;10

MeV14

0

21

0

21

pX

x

dx

X

x

pdx D0→K

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. High level charm trigger.

target

primary tracks

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The Silicon Tracking System

Assume 7 planes

- 3 or 2 thin pixel planes

- 4 or 5 thin strip planes

Acceptance: 50 to 500 mrad

First plane: z=5cm ; size 25 cm2

covers 100 to 500 mrad

Last plane: z=100cm; size ~ 1 m2

Magnetic dipole field: ~ 1Tm, p/p <1% @ p=1 GeV

vacuum

1

7

3

4vertexing

tracking

2

5

6

pixel detectors strip detectors

z = 5,10,(20) cm

z = (20),40,60,80,100 cm

Conceptional geometry: – mostly guided by open charm needs. – focus on the pixel planes for

secondary vertex detection – tracking

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D0 vertex resolution

primary vertex resolution

MC Studies on Vertex Finding

I. Vassiliev et al., GSI

What kind of pixel detectors can do the job? Material budgets, pixel sizes:

10%

D impact parameter detection

100 µm thick750 µm thick

750 µm thick

small pixels < 25 x 25 µm2

( thin sensors ~ 100 µm )

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Studies on D0→ K-π+ reconstruction

study by I. Vassiliev, GSI

D0 K-,+ signal Background

Rec

onst

ruct

ed e

vent

s

Z-vertex (cm)

~10 % efficiency

signal

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Silicon Detectors Requirements Small pixels – less than 25 x 25 µm2

Thin – less than ~100 µm silicon

Radiation hard – > 1013 n_equiv/cm2

Fast readout – Interaction rate up to 107/s

Such a detector does not exist !R&D on existing detector types. Two possibilities:

Monolithic Active Pixel Sensors:- small pixels: 25 x 25 µm2

- thin: standard 120 µm, study: 50 µm - achieve spatial resolution: ~3 µm

- TOO SLOW for CBM: ~ ms/Mpixel- rad. hard. limited by bulk damage

Improve r/o time, radiation hardness.

Hybrid Pixel Detectors:- fast readout, - radiation hard- larger pixels: 50 x ~400 µm2 - spatial resol.: ~15 (115) µm- thick: standard > 350 µm

Reduce pixel size and thickness.

We started persuing the MAPS option.

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Pixel Detector Stations

Detector module: BTeV inspired design

ladders mounted on either side of a substrate providing (active?) cooling.

Active cooling support:

a carbon fibre 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 5-10 µs

CBM MAPS ladders with 4 or 5 chips.

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Silicon Strip Detector Stations

Basic sensor elements: 200 m thick silicon wafers.double-sided, rad-tolerant. 25 m strip pitch.

Inner : 6x4 cm Middle : 6x12 cmOuter : 6X20 cm

Open questions: strip length, stereo angle

(to reduce fake hits) location of read-out

(on sensor, all at edge ?)

Four detector stations:built from few wafer types

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Tracking in the Silicon Strip StationsCellular automaton/Kalman filter method.

First attempts: Problem - High occupancy with many combinatorial hit points.

Recent approach: - start with long “stiff” tracks- 4 strip stations + 1 pixel station (“no pile-up”)- attached strips are removed - this cleans up the hit pool, reduces the fake strip combinations.

Work in progress.

Ideal conditions: full sensor efficiency, no pile-up:

~ 1% “ghost tracks” remain.

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Consider more Tracking Redundancy

vacuum

1

11

3

vertexing

track finding

track seeding

4

8

9larger area ?larger acceptance, faster r/o?

Under real conditions: - event pile-up in pixels - fake hits in strips

At present: - forward tracking hampered by pile-up - backward tracking hampered by fakes - need for clean track seeds

Consider: - more tracking stations - shorter strips – lower occupancy - fast pixels with clean event association

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Low Mass Dilepton Spectroscopy

If missed ...

ee 0

ee

... 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

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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 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.

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Particle Fluence – Detector LifetimeImpact on D Meson Measurements

Fluence at 1st MAPS station, z = 5cm, r = 5mm: ~ 10 1 MeV nequiv per event.

MAPS rad. tolerance: ~1012 1 MeV nequiv. ; development goal: 1013 1 MeV nequiv.

MAPS readout in potentially 10 s. No pileup at event rate <105/s.

The MAPS lifetime is then about 1 1012/105 s = 1 107 s = 16 weeks.

D0→Kπ: 4 10-5 0.038 0.05 = 7.6 10-8 per event. In MAPS lifetime: 7.6104 D0.

Fluence of 1 MeV nequiv./cm2 in 1st MAPS station at z = 5cm

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CBM Silicon Tracker Requirements- Summary -

Tracking exclusively performed with the Silicon Tracker:Very important detector, key to the physics of CBM.

The Silicon Tracker must deliver high performance: Open charm detection is the bench mark. Pixel planes near the target are of special importance: Thin, small pixels, fast readout, radiation hard.

Such detector does not exist . What comes closest ? What R&D? We chose to start investigating MAPS. Alternative ?

Discussions on latest detector technologies are very welcome!