The LHCb VELO and its use in the trigger

1
The LHCb VELO and its use in the trigger
Thomas Ruf
Vertex 2001 23-28 September 2001

• Introduction
• Silicon RD
• Second Level trigger
• Pile up VETO trigger

2
Introduction
VErtex LOcator of the LHCb experiment
LHCb Systematic studies of CP in the beauty
sector by measuring particle -
antiparticle time dependent decay rate
asymmetries.

• LHCb requires

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

3
VELO Overview
Introduction

• 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

4
VELO Setup
Introduction
Positioning and number of stations is defined by
the LHCb forward angular coverage of
15mrad lt ? lt 390mrad together with the
inner/outer sensor dimensions

• Also need to account for
• spread of interaction region s 5.3 cm
• partial backward coverage for improved primary
  vertex measurement

One detector half
Interaction region
p
p
forward

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

5
Radiation
Introduction

• Sensors have to work in a harsh radiation
  environment
• max. fluences 0.5 x 1014 - 1.3 x 1014 neq /
  cm2 / year

Nr-a a 1.6 ? 2.1
neq damage in silicon equivalent to neutrons of
1 MeV kinetic energy
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Sensor Design
Introduction

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

• Advantages of Rf geometry
• Resolution Smallest strip pitch where it is
  needed, optimizing costs/resolution.
• Radiation Short strips, low noise, (strixels,
  40mm x 6283mm) close to beam
• L1 Segmented R-sensor (45o) information is
  enough for primary vertex reconstruction and
  impact parameter measurement.

• Inner radius is defined by the closest possible
  approach of any material to the beam8 mm
  (sensitive area) ? LHC machine
• Outer radius is constrained by the practical
  wafer size 42 mm

7
Sensor Design
Introduction

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

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Silicon RD

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

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

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Silicon RD

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

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

Tested Prototypes
Hamamatsu n-on-n
thickness 300 ?m
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Silicon RD

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

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

Tested Prototypes
MICRON p-on-n
thickness 200/300 ?m
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RD Results
Silicon RD
First confrontation with alignment issues
5 of VELO sensors tested with beam
40MHz readout chip SCT128A
Resolution vs angle and pitch
See talk of M. Charles
Best resolution 3.6 ?m
Trigger performance
simulation
http//lhcb-vd.web.cern.ch/lhcb-vd/TDR/TDR_link.h
tm
May 2001
?Results about irradiated sensors
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n-on-n Prototypes
Silicon RD
Variable irradiation with 24 GeV protons at the
CERN-PS
Sensor readout with 25ns electronics (SCT128A)
Repeater card
S/N 21.5
3-chip hybrids
Most irradiated region corresponds to 2 years of
LHCb operation for the innermost sensor part.
Temperature probes
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p-on-n Prototypes
Silicon RD
After irradiation, silicon needs to be fully
depleted, otherwise

• Resolution degrades

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p-on-n Prototypes
Silicon RD
After irradiation, silicon needs to be fully
depleted, otherwise

• Charge is lost to double metal layer

2nd metal layer
Size of boxes proportional to CCE
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VELO Technology Choices
Silicon RD

• 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

• Fully depleted for gt2 years, Ubias lt 400
  VPrototype n-on-n sensors even for 4 years
• With n-on-n, can accept 40 under-depletion
  ? extending the lifetime even further

• Operation model
• 100 days constant fluence, T-50C
• 14 days at 22oC

Prototype n-on-n sensors behaved much better than
expected for standard silicon
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LHCb Trigger System
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L1 (Vertex) Trigger
LHCb Trigger
Input rate 1 MHz Output rate 40
kHz Maximum latency 2 ms
Purpose Select events with detached secondary
vertices Needs Standalone tracking and vertex
reconstruction
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Algorithm
L1 Vertex Trigger
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Implementation
L1 Vertex Trigger
Challenge 4 Gb/s and small event fragments of
170bytes
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Execution Time of Algorithm
L1 Vertex Trigger
450 MHz Pentium III running Windows NT
Expect CPUs to be 10 times faster in 2005
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Physics Performance
L1 Vertex Trigger
Technical Proposal
With new Event Generator, performance degraded by
a factor of 2 !
Minimum bias retension

• Main limitations
• No momentum information ?significance of impact
  parameter
• No particle ID

Working point40kHz
Signal efficiency

• Link L0 objects large pt (e, m, h) with VELO
  tracks.
• Investigate effect of possible momentum
  information.

Recent developments
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Super Level 1
L1 Trigger New ideas
matching efficiency B?pp- 78
for one p B?J/y(mm)Ks 96 for one m
HCAL
MUON
ECAL
Example Matching with HCAL clusters
?B dl 4 Tm, pt kick 1.2 GeV/c
New
B?pp-
B?J/y(mm)Ks
B?pp-
TP
B?J/y(mm)Ks
L1 rate
In general, gain back factor 2, in some channels
even more.
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Mini Level 1
L1 Trigger New ideas
Silicon tracking station 30 additional data to
be send to L1 farm
momentum resolution s(pt)/pt ? 20
? B0s ? Ds-(KK-?-) K? B0d ? ??-
For final answer See Trigger TDR in 2002
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L0 Pile Up VETO
LHCb Trigger
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).
Input rate 40 MHz Output rate 1
MHz Latency 4.0 ms
b-event rate 25kHz
Number of inelastic interactions/bunch crossing
as a function of luminosity
At L2x1032cm-2s-1 Pgt1/ P?1 ? 24 b-events
Pgt1/ P?1 ? 41
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Concept
L0 Pile Up VETO
If hits are from the same track
? ZPV

• mask hits belonging to P1
• search next highest peak P2 (peak size S2)
• classify if S2ltSlimit then single,
• else multiple

• build a ZPV histogram
• search highest peak P1

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Implementation
L0 Pile Up VETO

• 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 gt350k gates and gt600 I/O pins.
  Candidates Altera EP20K400, XILINX XC4000,

Performance Gain of 30-40 of single bb-events
at optimal luminosity
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Summary

• 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.