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BaBar Silicon Tracker Perspective at High Luminosity

G. Calderini

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Which will be the performance of theBaBar SVT when the lumi increases?

Performance of the present SVT withminor modifications:(Short term perspective: n x 1034)

Strategy to cope with future physicsprograms: (Long term perspective: n x 1035 to 1036)

Main issues: radiation damage, occupancy

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Present detector: Short Term extrapolation

Extrapolation for the short term is based on expectations for currents

Step 1

Currents are used to calculate instantaneous dose rates, by using background studies

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Instantaneous dose

Integrated dose(radiation damage)

Step 2

Occupancy(performance)

SVT L1-Signal/Noise vs dose

0

1000

2000

3000

4000

5000

6000

0 5 10 15 20 25

Dose (Mrad)

No

ise

(ele

ctro

ns)

0

5

10

15

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25

Sig

nal

/No

ise

noise_Ileakage

noise_Cload

noise_total

S/NG.Rizzo, G.C.

B.Petersen

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Occupancy (performance)

Step 3

extrapolated hit efficiency

extrapolated hit resolution

M.Mazur

M.Mazur

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Extrapolations suggest that a detector likeit is now, can work well up to a 2-4 x 1034

Which are the limits imposed to the trackerby more aggressive physics programs?

The 1035 scenario

The 1036 scenario

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The (1-2) x 1035 and 1036 scenarios present similar concerns:

Machine-related background(continuous injection!)

Radiation damage

Rate

Physics backgrounds

But they are completely different worlds

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In a 1035 world, a BaBar-like trackerwith SVT-DCH is somehow possible

More phase-space for solutions!

In a 1036 world, life is different, moreeffort necessary in the design

DCH

7MRad/y

SVT

100% Occupancy

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Vertexing and Tracking at high luminosity

The inner tracker

The beam-pipeThe beam-pipe radius is a big issue, the choice may depend strongly on the machine design

KEK-B plans on 1cm more performingPEP-II plans on 1.5-2 cm safer

One or two layers of pixels very close to beam-pipemainly required for background suppression, integratedby a few additional layers of silicon strip detectors(vertexing, impact parameter resolution, low-P tracking)

The central tracker: two optionsa) More silicon layersb) Small cell/fast gas drift chamber, combined with normal drift chamber

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Some keypoints

Radiation hardness: possible using LHC technology

Material budget: current hybrid pixel layers are thick; the all-silicon solution can get pretty heavy

Rate capability: effects on silicon segmentation and drift chamber cell size

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Babar possible approach to tracking

• All silicon tracker, with lampshade shaped modules to reduce material

• Start to explore different options• Main issue is material

– Need R&D on thinDSSD and pixels

Pixel (2 layers)

Intermediate DSSD(3 layers)

Central Silicon Tracker(4 layers)R(outer) = 60 cm

B abar ™ and © L. de B runhoff

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Pixels (I)

Front-End Chip

Sensor

A) Hybrid pixels

In hybrid pixel systems the readout chip is connected to the sensor through solder or Indium bumps

Separate development of readout electronics and sensors

Use best available technology for each component

Complexity and reliabilityissues in assembly

Material budget is high dueto overlap of sensor andreadout chip.

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Example: Pixels at LHC

– Radiation hardness and rate capability are highThey should be OK for a Super B-factory as well.

– Material budget is serious:At least 1-2% X0 per layer (current Babar Si is around 0.4% X0)

Atlas pixel modules

Overlap of:• Sensor• Front-end chip• Flex hybrid with

control chip, caps• Mechanical

structure and cooling

LHC experiments use hybrid pixels

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What dominates resolution?

cp

XXR

sin

/014.0)scatt. mult.(

2/50

Here material budget is critical !

Impact parameter resolution is dominated by resolution on first hit

(point)2 = (mult.scatt.)2 + (detector)2

Typical SVT detector resolution at BaBar is

= 12 - 14 m at 90°

For p=1 GeV/c, for R=3 cm,X(beampipe+1st layer) = 1.4% X0

(mult. scatt.) = 50 m at 90°

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Model for resolutionWe can model the SVT performance using a resolution-weighted average of the detector radii.

Model for current SVT sigma d0

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

0 1 2 3P(GeV)

sigm

a d0

sigmad0MS

sigmaD0total Data

1 GeV58m3 GeV25m

1 GeV55m3 GeV23m

F.Forti

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Where can we gain ?

Model for Upgraded SVT sigma d010mm, 0.5% X0, 5um

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

0 1 2 3P(GeV)

sig

ma

d0

sigmad0MS

sigmaD0total

1 GeV12m3 GeV7 m

We could gain a lot by reducing the beam pipe radius and the detector + beam pipe thickness.The point resolution can be improved

F.Forti

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Proven by the success of CMOS video cameras, replacing CCDs.

B) Monolithic Active Pixels (MAPS)

Pixels (II)

Sensor and electronics on the same substrate.

Possible approaches:

Integrate electronics on the high resistivity substrate usually employed for sensors

The fabrication process is highly non-standard with large feature size (>1-2 m)

Signal is high quality, and large

Active components are not of the best quality

Use the low resistivity substrate of standard CMOS process as sensor

Small signal due to the collection mechanism

Standard sub-micron process with state-of-the-art electronics

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CMOS MAPSUse epitaxial layer of CMOS low-resistivity substrate to collect charge (thermal diffusion)

Potential for low cost and very small thickness (reduced substrate).

Radiation hard if using sub-micron CMOS process

Until now: miniscule pixel size (a few um) prevents usage in large system

Low power-consumption (circuitry active only during read-out)

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Pixel: ongoing R&DConventional hybrid pixels

Reduce thickness

It doesn’t seem possible to reduce too much preserving also the mechanical stability

MAPSDevelop large-area detectors (already some results)

Development on-going in several places:LEPSI, LBNL, Japan, Perugia

Project launched by Pisa-Pavia-Bergamo-Trento-Trieste-Modena to the Italian Ministry for Education and Scientific Research

- Main goal is to develop a submicron CMOS MAPS that can be used on large area systems

- Time frame is 2-3 years

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StripsIn the central silicon tracker momentum resolution dominated by material budget

1) Reduce the thickness of the active silicon

Signal reduction (1 MIP ˜ 8000 e/100um Si)

2) Reduce the amount of inactive materialBring the signal out of the active tracking volume

Mechanical issues: silicon greatly contributes to module stiffness

Done already for the present SVT

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Reduce thickness of readout electronics

For the chip themselves is mainly a mechanical problem, could be solved.

It is harder to do for the hybrids (capacitors, traces, etc.,)

Reduce power dissipation (ie cooling)

Very hard if one has to improve the S/N ratio to be able to readout smaller signals

One more reason to go for sub-micron process

Need some local signal amplification

The same technique cannot be used in the larger volume Central Silicon Tracker

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All-silicon performance• Momentum resolution at low-

p is dominated by multiple scattering in silicon material

• To keep a reasonable performance we need 100um thick silicon, which isn’t quite ready yet:

– R&D on thin silicon modules• On this large area it will be

impossible to keep all the electronics outside the active volume

– R&D on thin, low power electronics

• How much the requirement on momentum resolution at low momentum can berelaxed ?

– More physics studies

Current DCH

Double-sidedstrip @ 100um

(1/pt)(GeV-1)

B abar ™ and © L. de B runhoff

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Summary and conclusionsPixel layers

Current LHC pixel would work, but too much material.

Develop monolithic pixelsLarge structures, thickness (back-thinning), radiation damage

Central tracker

Most likely it won’t be possible to achieve the same performance of current systems at low momentum

It would require a <100um equivalent thickness for a9 layers (total) all Si tracker.

Explore the possibility of gaseous detectors, such as a small-cell DCH.

Some R&D has started, but we need to proceed fast if we want to design a realistic system in 2-3 years.