The LHCb VELO and its use in the trigger

VERTEX 2001, 23-28 September Thomas Ruf CERN

The LHCb VELO and its The LHCb VELO and its use in the triggeruse in the trigger

Vertex 2001 23-28 September

2001Introduction

Silicon R&D

Second Level trigger

Pile up VETO trigger

Thomas Thomas RufRuf


LHCb: LHCb: Systematic studies of CP in the beauty sector by measuring particle - antiparticle time dependent decay rate asymmetries.

IntroductionIntroduction

LHCb requires:LHCb requires:

Reconstruction of pp-interaction pointReconstruction of pp-interaction point Reconstruction of decay vertex of beauty and charm hadronsReconstruction of decay vertex of beauty and charm hadrons Standalone and fast track reconstruction in second Level trigger (L1)Standalone and fast track reconstruction in second Level trigger (L1)

Reconstruction of pp-interaction pointReconstruction of pp-interaction point Reconstruction of decay vertex of beauty and charm hadronsReconstruction of decay vertex of beauty and charm hadrons Standalone and fast track reconstruction in second Level trigger (L1)Standalone and fast track reconstruction in second Level trigger (L1)

VEVErtexrtex LO LOcator of the cator of the LHCb experimentLHCb experiment


VELO OverviewVELO OverviewIntroduction

Number of silicon sensors: 104 Area of silicon: 0.32m2

Number of channels: 204,800

Number of silicon sensors: 104 Area of silicon: 0.32m2

Number of channels: 204,800

~1 m

Precise vertexing requires, to be: as close as possible to the decay vertices with a minimum amount of material between

the first measured point and the vertex

Precise vertexing requires, to be: as close as possible to the decay vertices with a minimum amount of material between

the first measured point and the vertex

silicon sensors are placed in vacuum

silicon sensors are placed in vacuum

Vacuum Vessel


VELO SetupVELO SetupIntroduction

Positioning and number of stations is definedby the LHCb forward angular coverage of

15mrad < <390mradtogether with the inner/outer sensor dimensions

Positioning and number of stations is definedby the LHCb forward angular coverage of

15mrad < <390mradtogether with the inner/outer sensor dimensions

Also need to account for spread of interaction region:

=5.3 cm partial backward coverage

for improved primary vertex measurement

Also need to account for spread of interaction region:

=5.3 cm partial backward coverage

for improved primary vertex measurement

Interaction regionp

p

forwardFinal configuration: 25 stations 1 station = 2 modules (left and right) 1 module = 2 sensors

Final configuration: 25 stations 1 station = 2 modules (left and right) 1 module = 2 sensors

One detector half:


RadiationRadiationIntroduction

Sensors have to work in a harsh radiation environment:

max. fluences: 0.5 x 1014 - 1.3 x 1014 neq / cm2 / year

Nr= 1.6 2.1

neq = damage in silicon equivalent to neutrons of 1 MeV kinetic energy


Sensor DesignSensor Design

Azimuthal symmetry of the events suggests sensors which measure and R coordinates.

Inner radius is defined by the closest possible approach of any material to the beam:8 mm (sensitive area) LHC machine

Outer radius is constrained by the practical wafer size: 42 mm

Advantages of Rgeometry: Resolution: Smallest strip pitch where it is

needed, optimizing costs/resolution. Radiation: Short strips, low noise,

(strixels, 40m x 6283m) close to beam L1: Segmented R-sensor (45o) information is

enough for primary vertex reconstruction and impact parameter measurement.

Introduction


Sensor DesignSensor Design

Strips are readout by using a double metal layer Analog readout for better hit resolution and monitoring

Introduction

2048 strips / sensor 16 FE chips


Silicon R&DSilicon R&D

VELO Design ChallengesVELO Design Challenges Varying strip lengths Double metal layer Regions of fine pitch Large and non-uniform

irradiation

VELO Technology ChoicesVELO Technology Choices Thickness Oxygenation Cryogenic Operation Segmentation p or n strips ?

Tested Prototypes Double sided DELPHI sensor

thickness: 310 m



Tested Prototypes Hamamatsu n-on-n

VELO Technology ChoicesVELO Technology Choices Thickness Oxygenation Cryogenic Operation Segmentation p or n strips ?


irradiation

thickness: 300 m



Tested Prototypes MICRON p-on-n

VELO Technology OptionsVELO Technology Options Thickness Oxygenation Cryogenic Operation Segmentation p or n strips ?


irradiation

thickness: 200/300 m


R&D ResultsR&D Results~5% of VELO sensors

tested with beam

Silicon R&D

Trigger performance

40MHz readout chip SCT128A

Res

olut

ion

vs a

ngle

an

d pi

tch

simulation

Best resolution: 3.6 m

Results about irradiated sensorsResults about irradiated sensorsResults about irradiated sensorsResults about irradiated sensors

First confrontation with alignment issues

See talk of M. CharlesSee talk of M. Charles

http://lhcb-vd.web.cern.ch/lhcb-vd/TDR/TDR_link.htm

May 2001


nn-on--on-nn Prototypes Prototypes

3-chip hybrids

Temperature probes

Repeater card

Sensor readout with 25ns electronics (SCT128A)

Silicon R&D

Variable irradiation with 24 GeV protons at the CERN-PS

Most irradiated region corresponds to 2 years of LHCb operation for the innermost sensor part.

S/N = 21.5


pp-on--on-nn Prototypes Prototypes

After irradiation, silicon needs to be fully depleted, otherwise:

Silicon R&D

Resolution degrades

Charge Collection Efficiency

Res

olut

ion

[m

]


pp-on--on-nn Prototypes Prototypes

After irradiation, silicon needs to be fully depleted, otherwise:

Silicon R&D

Charge is lost to double metal layer

Size of boxes proportional to CCE

2nd metal layer


VELO Technology ChoicesVELO Technology Choices

n-strips, safest solution, sensors can be operated underdepleted Thickness = 300 m, OK for radiation hardness and material budget Oxygenation: nice to have, but not mandatory, less of interest for p-on-n

Silicon R&D

Operation model:

100 days constant fluence, T=-50C

14 days at +22oC

Operation model:

100 days constant fluence, T=-50C

14 days at +22oC

Fully depleted for >2 years, Ubias < 400 VPrototype n-on-n sensors even for 4 years

With n-on-n, can accept 40% under-depletion extending the lifetime even further

Fully depleted for >2 years, Ubias < 400 VPrototype n-on-n sensors even for 4 years

With n-on-n, can accept 40% under-depletion extending the lifetime even further

Prototype n-on-n sensors

behaved much better than

expected for “standard” silic

on

Prototype n-on-n sensors

behaved much better than

expected for “standard” silic

on


LHCb Trigger SystemLHCb Trigger System

L1: vertex trigger 1 MHz 40 kHz L1 buffer = 1927 events

L0 YES: Data is digitized, but stays in memory of individual detector electronics

Data is kept in analog/digital pipelines

L0: look for high pt particles hadron (HCAL), muon (MUON) or electron (ECAL) pile up VETO

40 MHz 1 MHzFixed latency: 4.0 s

pp

L1 YES: Digitized data is read-out, event building CPU farm

L2: refined vertex trigger 40 kHz 5 kHz

L3: online event selection 5 kHz 200 Hz


L1 (Vertex) TriggerL1 (Vertex) TriggerLHCb Trigger

Input rate: 1 MHzOutput rate: 40 kHzMaximum latency: ~2 ms

Input rate: 1 MHzOutput rate: 40 kHzMaximum latency: ~2 ms

Purpose:Select events with detached secondary verticesNeeds:Standalone tracking and vertex reconstruction

Purpose:Select events with detached secondary verticesNeeds:Standalone tracking and vertex reconstruction

~ 1.5 tracks in the seeding region (innermost 450 sectors of R-sensors)

track reconstruction is straightforward

~ 1.5 tracks in the seeding region (innermost 450 sectors of R-sensors)

track reconstruction is straightforward

Occupancy < 1% per channel


AlgorithmAlgorithmL1 Vertex Trigger

Primary vertex: Histogram of 2-track crossings z -segmentation x,y

Primary vertex: Histogram of 2-track crossings z -segmentation x,y

z,y 70 m x,y 20 m

Impact ParameterImpact ParameterReject events with multiple interactionsReject events with multiple interactions

3d reconstruction for tracks with large impact parameter 3d reconstruction for tracks with large impact parameter Efficiency: 95%

Search for 2 tracks close in spaceSearch for 2 tracks close in space

Calculate probability to be a non b-event based on impact parameter and geometryCalculate probability to be a non b-event based on impact parameter and geometry

r2d track reconstruction using only R-hits2d track reconstruction using only R-hits

Efficiency: 98%

z

triplets

Test beam


ImplementationImplementation

Challenge: 4 Gb/s and small event fragments of ~170bytesChallenge: 4 Gb/s and small event fragments of ~170bytes 2d torus topology

based on SCI shared memory

680 Mb/s, limited by number of senders

Readout Unit

150-250 processorsBasic Idea:

SW

ITC

H

CPU farm

Digitizer Board:zero-suppression,common mode correction, cluster finding

1 MHzx100x24

L1 Vertex Trigger


Execution Time of AlgorithmExecution Time of Algorithm450 MHz Pentium III running Windows NT

Expect CPU’s to be 10 times faster in 2005

B+-B+- Minimum biasMinimum bias

L1 latency

L1 Vertex Trigger


Physics PerformancePhysics Performance

Main limitations: No momentum information

significance of impact parameter No particle ID

Main limitations: No momentum information

significance of impact parameter No particle ID

Recent developments: Recent developments:

With new Event Generator, performance degraded by a factor of 2 !

With new Event Generator, performance degraded by a factor of 2 !

Technical ProposalTechnical Proposal

Link L0 objects: large pt (e, , h) with VELO tracks.

Investigate effect of possible momentum information.

Link L0 objects: large pt (e, , h) with VELO tracks.

Investigate effect of possible momentum information.

Working point40kHz

Min

imu

m b

ias

rete

nsi

on

Signal efficiency

L1 Vertex Trigger


Super Level 1Super Level 1L1 Trigger New ideas

B dl = 4 Tm, pt kick = 1.2 GeV/c

Example: Matching with HCAL clusters

HCAL

MUON

ECAL

matching efficiency

B+- : 78% for one

BJ/Ks: 96% for one

L1 rate

B+-

BJ/Ks

B+-

BJ/Ks

NewNew

TPTP

In general, gain back factor 2, in some channels even more.In general, gain back factor 2, in some channels even more.


Mini Level 1Mini Level 1

momentum resolution:

pt)/pt 20%

B0s Ds

-(K+K--) K+

B0d +-

B0s Ds

-(K+K--) K+

B0d +-

L1 Trigger New ideas

For final answer:

See Trigger TDR in 2002

For final answer:

See Trigger TDR in 2002

Silicon tracking station 30% additional data to be send to L1 farm


LHCb chooses luminosity for maximum number of single interactions

LHCb chooses luminosity for maximum number of single interactions

en ! nP

n

Input rate: 40 MHzOutput rate: 1 MHzLatency: 4.0 s

Input rate: 40 MHzOutput rate: 1 MHzLatency: 4.0 s

L0 Pile Up VETOL0 Pile Up VETOLHCb Trigger

Number of inelastic interactions/bunch crossing as a function of luminosity

At L=2x1032cm-2s-1: P>1/ P1 24%

b-events: P>1/ P1 41%

At L=2x1032cm-2s-1: P>1/ P1 24%

b-events: P>1/ P1 41%

Cross-section

LuminosityBunch crossing frequency 30 MHzwith fL /

Purpose:Remove events with multiple interactions.Why ?Multiple interactions are more difficult to reconstruct (specially for L1) and fill bandwidth of L0 (~ 2x probability to pass L0).

Purpose:Remove events with multiple interactions.Why ?Multiple interactions are more difficult to reconstruct (specially for L1) and fill bandwidth of L0 (~ 2x probability to pass L0).

b-event rate: 25kHz


ConceptConceptL0 Pile Up VETO

ZB ZA

RB

RA

ZPV

2 silicon R-stations upstream of VELOB A

If hits are from the same track:

kZZZZ

RR

PVA

PVB

A

B

R

B [

cm]

RA [cm]RA [cm]

True combinations All combinations

ZPV [cm]

ZPV

mask hits belonging to P1search next highest peak P2 (peak size S2)classify: if S2<Slimit then single,

else multiple

build a ZPV histogramsearch highest peak P1


ImplementationImplementationL0 Pile Up VETO

Performance:

Gain of 30-40% of single bb-events at optimal luminosity

Performance:

Gain of 30-40% of single bb-events at optimal luminosity

2 R-stations upstream of the VELO

OR of 4 channels, binary readout, 80Mbit LVDS, 512 lines

Frontend chip: BEETLE running in comparator mode

Large FPGA: >350k gates and >600 I/O pins. Candidates: Altera EP20K400, XILINX XC4000, …


SummarySummary

Technical Design Report of the LHCb VErtex LOcator is completed:

n-on-n silicon strip sensors are the baseline.

Prototyping with different companies continues.

The VELO plays an important role in the second level trigger:

Standalone track finding,

primary vertex reconstruction,

impact parameter determination

Silicon sensors are also used in the first level trigger for a fast determination of the number of interactions.

Technical Design Report of the LHCb VErtex LOcator is completed:

n-on-n silicon strip sensors are the baseline.

Prototyping with different companies continues.

The VELO plays an important role in the second level trigger:

Standalone track finding,

primary vertex reconstruction,

impact parameter determination

Silicon sensors are also used in the first level trigger for a fast determination of the number of interactions.

Documents

The LHCb VELO and its use in the trigger