1. The LHCb VELO and its use in the trigger
Thomas Ruf
Vertex September 2001
Introduction
Silicon R&D
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 Vacuum Vessel ~1 m Introduction
 silicon sensors are placed in vacuum
Vacuum Vessel
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
Number of silicon sensors: 104 Area of silicon: m2 Number of channels: ,800
~1 m

4. VELO Setup One detector half: p p forward Introduction
Positioning and number of stations is defined
by the LHCb forward angular coverage of
15mrad <  < 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. Introduction
Radiation
Sensors have to work in a harsh radiation environment:
max. fluences: x x 1014 neq / cm2 / year
N*r-a
a = 1.6  2.1
neq = damage in silicon equivalent to neutrons of 1 MeV kinetic energy

6. Introduction
Sensor Design
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 beam: 8 mm (sensitive area)  LHC machine
Outer radius is constrained by the practical wafer size: 42 mm

7. Sensor Design Strips are readout by using a double metal layer
Introduction
Sensor Design
Strips are readout by using a double metal layer
Analog readout for better hit resolution and monitoring
2048 strips / sensor  16 FE chips

8. Silicon R&D Tested Prototypes 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 ?
Tested Prototypes
Double sided DELPHI sensor
thickness: 310 m

9. Silicon R&D Tested Prototypes 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 ?
Tested Prototypes
Hamamatsu n-on-n
thickness: 300 m

10. Silicon R&D Tested Prototypes VELO Design Challenges
Varying strip lengths
Double metal layer
Regions of fine pitch
Large and non-uniform irradiation
VELO Technology Options
Thickness
Oxygenation
Cryogenic Operation
Segmentation p or n strips ?
Tested Prototypes
MICRON p-on-n
thickness: 200/300 m

11. Results about irradiated sensors
Silicon R&D
R&D Results
~5% of VELO sensors tested with beam
First confrontation with alignment issues
40MHz readout chip SCT128A
Resolution vs angle and pitch
See talk of M. Charles
Best resolution: 3.6 m
Trigger performance
simulation
lhcb-vd/TDR/TDR_link.htm
May 2001
Results about irradiated sensors

12. Sensor readout with 25ns electronics (SCT128A)
Silicon R&D
n-on-n Prototypes
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

13. Silicon R&D
p-on-n Prototypes
After irradiation, silicon needs to be fully depleted, otherwise:
Resolution degrades
Charge Collection Efficiency
Resolution [mm]

14. Silicon R&D
p-on-n Prototypes
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

15. VELO Technology Choices
Silicon R&D
VELO 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
Fully depleted for >2 years, Ubias < 400 V Prototype 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

16. LHCb Trigger System
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 MHz Fixed latency: 4.0 ms p
L1: vertex trigger
1 MHz  40 kHz L1 buffer = 1927 events
L0 YES: Data is digitized, but stays in memory of individual detector electronics
L1 YES: Digitized data is read-out, event building  CPU farm
L2: refined vertex trigger
40 kHz  kHz
L3: online event selection
5 kHz  200 Hz

17. L1 (Vertex) Trigger Input rate: 1 MHz Output rate: 40 kHz
LHCb Trigger
L1 (Vertex) Trigger
Input rate: MHz
Output rate: 40 kHz
Maximum latency: ~2 ms
Purpose:
Select events with detached secondary vertices
Needs:
Standalone tracking and vertex reconstruction
~ 1.5 tracks in the seeding region (innermost 450 sectors of R-sensors)
 track reconstruction is straightforward
Occupancy < 1% per channel

18. Algorithm 2d track reconstruction using only R-hits Primary vertex:
L1 Vertex Trigger
Algorithm r
2d track reconstruction using only R-hits
Efficiency: 98% z
triplets
Primary vertex:
Histogram of 2-track crossings  z F-segmentation  x,y
sz,y  70 mm sx,y  20 mm
Impact Parameter
Reject events with multiple interactions
Test beam
3d reconstruction for tracks with large impact parameter
Efficiency: 95%
Search for 2 tracks close in space
Calculate probability to be a non b-event based on impact parameter and geometry

19. Implementation Basic Idea: 150-250 processors SWITCH CPU farm
L1 Vertex Trigger
Implementation
Readout Unit
processors
Basic Idea:
SWITCH
CPU farm
Digitizer Board: zero-suppression, common mode correction, cluster finding
1 MHz
x100
x24
2d torus topology based on SCI shared memory
Challenge: 4 Gb/s and small event fragments of ~170bytes
680 Mb/s, limited by number of senders

20. Execution Time of Algorithm
L1 Vertex Trigger
Execution Time of Algorithm
450 MHz Pentium III running Windows NT
Bp+p-
Minimum bias
L1 latency
Expect CPU’s to be 10 times faster in 2005

21. Minimum bias retension
L1 Vertex Trigger
Physics Performance
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 point 40kHz
Signal efficiency
Link L0 objects: large pt (e, m, h) with VELO tracks.
Investigate effect of possible momentum information.
Recent developments:

22. Super Level 1 Example: Matching with HCAL clusters
L1 Trigger New ideas
Super Level 1
matching efficiency
Bp+p : 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p+p-
BJ/y(mm)Ks
Bp+p- TP
BJ/y(mm)Ks
L1 rate
In general, gain back factor 2, in some channels even more.

23. Mini Level 1 For final answer: See Trigger TDR in 2002
L1 Trigger New ideas
Mini Level 1
Silicon tracking station 30% additional data to be send to L1 farm
momentum resolution: s(pt)/pt  20%
 B0s  Ds-(K+K--) K+  B0d  +-
For final answer:
See Trigger TDR in 2002

24. LHCb chooses luminosity for maximum number of single interactions
LHCb Trigger
L0 Pile Up VETO
Input rate: MHz
Output rate: 1 MHz
Latency: ms
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
Number of inelastic interactions/bunch crossing as a function of luminosity
LHCb chooses luminosity for maximum number of single interactions
Cross-section
Luminosity
Bunch crossing frequency  30 MHz
with
At L=2x1032cm-2s-1: P>1/ P1  24%
b-events: P>1/ P1  41%

25. Concept ZB ZA RB RA ZPV 2 silicon R-stations upstream of VELO B A
L0 Pile Up VETO
Concept
RB [cm]
RA [cm]
True combinations
All combinations
ZPV [cm] ZB ZA RB RA
ZPV
2 silicon R-stations upstream of VELO
B A
If hits are from the same track:
 ZPV
mask hits belonging to P1
search next highest peak P2 (peak size S2)
classify: if S2<Slimit then single,
else multiple
build a ZPV histogram
search highest peak P1

26. Implementation 2 R-stations upstream of the VELO
L0 Pile Up VETO
Implementation
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, …
Performance:
Gain of 30-40% of single bb-events at optimal luminosity

27. 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.