Super-Belle Vertexing

Talk at Super B Factory Workshop Jan 20 2004

T. Tsuboyama (KEK)

Super B factory Vertex groupPlease visit http://belle.kek.jp/superb/vtx

Outline of this talk• Coverage

– Angle: 17<<150o

• Full acceptance of Super-Belle detector. – Radii:13<r<150 mm.

• Inner radius close to the 10 mm radius IP chamber. (Tajima and Trabelsi’s talks)

• Outer radius wherever CDC can not be operated due to beam background. (Uno’s talk)

• Tracking capability– Spatial resolution: the better the better– Internal tracking for small pt tracks.

• DAQ/Trigger issues (Higuchi/Ito’s talks)– Occupancy: <5 % even at the inner layer.– Dead time: <10 % at trigger rate of 30 kHz. – Trigger latency

• 1st level Trigger from TRG to VTX: 3 sec• Z-Trigger generated by VTX to TRG 15 sec.

• Radiation hardness: 20 Mrad.

15 cm

Sensor configuration• Belle acceptance:

17<<150o must be covered.• Outer radius is determined

by CDC: 150 mm• Inner radius is determined by

beam pipe:10 mm• 5 layers for self tracking.

• Inclined sensors in Layer 4 and 5– Reduce readout channels– Save material budget– Shorten the ladder length (Without slan

t sensors, length ~75 cm)

• Innermost layer suffers serious beam background.– DSSD (striplet) option Super-layer– Pixel option covered by G. Vernar

Expected resolution• Dotted lines: SVD2(Reference)

– Cyan: 0.2 GeV/c, green: 0.5 GeV/c, blue: 1.5 GeV/c, Yellow: 2 GeV/c.• DSSD (Striplet) case:

– Two layer will be useful for robustness under background.– Thickness same as that of SVD2 / layer.– 20 % improvement because of the detector radius.

• Alice type hybrid-pixel case: (Pixel size=50mx400m)– Pixel shape is flat therefore, two layers are necessary since single layer is not enoug

h to achieve good resolution in X and Z.– Material can not be ignored although sensors and amplifiers are thinned.– Consequently, the improvement of the resolution can not be observed.– Instead immunity to background will be better than DSSD case.

Inner layer=2 layer DSSD

Impact parameter resolution (r) Impact parameter resolution (z)

Inner layer=2 layer DSSD

Inner layerAlice type2-layer pixel

Inner layerAlice type2-layer pixel

Occupancy (%)

1

10

100

1000

0 2 4 6

Radius (cm)

SVD1(1usec)SVD2(500nsec)

DSSD for the innermost layer

• The inner most layer sensor has a size of 50m*72mm/readout.

• With 50 um readout pitch and 1sec shaping time, the background estimation results in 200 % occupancy! – In order to obtain <5%

occupancy, ~40 times occupancy reduction is necessary.

5%

DSSD for the innermost layer• Occupancy is proportional to sensitive

area* shaping time.• Readout chip with 50-100nsec

shaping time is available.• Strips area should be 4 to 5 times

smaller. • Narrower strips are not easy.

– We should shorten the strips.• Strips can not be simply cut into 4-5

pieces– Dead regions between groups appear.– Assembly (wire bonding) becomes

difficult.

Striplet design• By arranging strips in 45 dgrees, strip l

ength is shortened automatically. • Small triangle dead region exists.

– About 7 % in Layer1

• Benefits of this design are– Strip arrangement on p-side and n-side ca

n be symmetric.• The above triangular dead region on one side i

s covered by the strips in the other side.

– Strip signal can be read out at detector edge. Assembly with high-density kapton circuit is easy.

– The scheme is applicable in any width/length ratio.

Prototype design in productionProduction of an R&D sensor is going on.

Readout with kapton flex circuits• Readout pitch is 71 m.• Number of channels per

DSSD (512 signals in 8mm width) is 4-5 times denser than SVD2.

• Multi-layer kapton flex circuit solves the readout density issues.

• R&D items– Alignment between

layers. – Production yield and

quality.– Method of assembly

• Installation issue– Number of front end

chip is 4-5 times than the present SVD. Can hybrids be installed?

Assembly

• All strips can be read out

• The kapton flex circuit is assembled as follows.

• Wire bonds are done at sensor edges.

• P-side and n-side are symmetric

Striplet sensorKapton circuit

Readout Hybrid

Kapton circuit

ReadoutHybrid

Wire bonding

Readout overview• Experience of Belle SVD2: L1 sensor

– 10%occupancy /(50m*77mm*1 sec shaping) • Super 　 KEKB: ~20 times larger background

– Strip area ~ 1/5 of Belle SVD2 (striplet)– Additional 1/10 of shaping time gives a 1/50 occupancy.

• Expected trigger rate ~30 kHz– With pipeline length of 3 sec + readout time of 3 sec,

30 kHz trigger rate causes relatively small dead time (needs evaluation). Above 30 kHz, dead time goes up rapidly.

• Z-trigger from VTX– Z information is useful for reducing beam-background ev

ents. – DAQ people requests the Z trigger from VTX becomes to

be ready in 15 sec from the event timing.

Readout chip: APV25• Designed for the CMS sili

con tracker– 50 m pitch, 128 ch/chip.

– 192 stage analog pipe line.

– Readout: multiplexing all channels.

– Built-in deconvolution circuit will not be used.

– Radiation tolerance30 Mrad

– Noise= (246 + 36/pF) enc.

• Operation at Super Belle @42.3 MHz clock– ~4.0 sec pipeline length– ~3.0 sec analog output sc

an: small dead time upt to 30kHz trigger rate.

– Shaping time is adjustable between 50 and 100 nsec depending on background level.

ShaperInverterpreamp192 stageAnalog Pipeline

Analog output

L0 trigger

128 channel multiplexer

Z vertex trigger from VTX• APV25 needs a trigger (L0) from DAQ within ~3 sec from an event.• The internal delay and analog multiplexing(MPX) takes ~7 sec at max.

– This analog multiplexing is not pipelined. Therefore, the subsequent steps can not be fully pipelined.

• Hit cluster search and trigger decision: ~5 sec.• Summing up, L1 trigger from VTX is ready at 15 sec. • If two triggers come to APV25 almost together, the second SVD trigger

decision is done at least 3 sec later than the first decision because of MPX. – The second trigger is available at ~20 sec. – If trigger decision is given up for the 2nd event, 15 sec latency can be regai

ned.

Event timing

L0 trigger from GDL

Analog Multiplex

Track finding in VTXL1 trigger from VTX

0 3 5 7 10 15

APV25 internal

20

R&D items• Finalize the detector configuration

– Material budget in B-factory is limited.– Minimize material in acceptance. – Design detector support system and ladder structure.

• Continue pixel detector R&D• Backend electronics becoming more complex and

important.– 2-3 time of channels.– 10 times of data

• Radiation hardness of sensors.• Design a dedicated readout chip that improves the

performance of Super Belle.

Summary

• Basic requirements to VTX can be fulfilled with striplet sensors and APV25 readout.

• In order to improve robustness of vertexing against the background at higher luminosities, preparation of pixel sensors is essential.

• In parallel, design of a dedicated front-end chip may be helpful to improve the performance of the VTX system.