1. The LHCb VErtex LOcator
Tracking, Vertexing and Triggering
in a
harsh radiation environment
Doris Eckstein, CERN
May 29, 2003
9th Pisa meeting on advanced detectors

2. 9th Pisa meeting on advanced detectors
The LHCb Experiment
Dedicated to the study of CP violation in the B system
LHC: -pp 14TeV
-full spectrum of B hadrons ( )
-high intensity:
LHCb: -single arm spectrometer
mrad angular acceptance
-recently optimised to minimise
material
Vertex
Locator
May 29, 2003
9th Pisa meeting on advanced detectors

3. 9th Pisa meeting on advanced detectors
The requirements for the VELO
~1m
Interaction
region
Downstream
Reconstruction of pp interaction vertex
wide spread of interaction region in z (sz=5.3cm)
many stations around z=0
Reconstruction of b-hadron decay vertex
short track extrapolation distances
measure at smallest radii
minimal multiple scattering
minimise material between interaction and
first measured point
VELO is the tracker before the LHCb magnet
Angular coverage of full downstream detector
angular range
21 Silicon stations allowing to measure at least 3 hits/track
2 R- and 2 F-measuring sensors per station
overlap (acceptance, alignment)
May 29, 2003
9th Pisa meeting on advanced detectors

4. 9th Pisa meeting on advanced detectors
The VELO Design
Mechanical design as consequence of these criteria
VELO sensors as close as possible to beam
no beam pipe, sensors ~7mm away from beam
Injection: larger aperture required retraction by 30mm
Protect sensors against RF pickup from the LHC beam
Protect the LHC Vacuum from possible outgasing of detector modules
Place sensors in a secondary vacuum: Roman pots
May 29, 2003
9th Pisa meeting on advanced detectors

5. 9th Pisa meeting on advanced detectors
Secondary Vacuum – RF Foil
outer corrugations
beam
inner corrugations
F sensors
R sensors
Made from 250mm thick Al
Inner corrugations :
Minimal material before the first
sensor is hit
Outer corrugations:
allow for overlap of detector halves
for full azimuthal coverage and for
alignment
Prototyping at NIKHEF
method: Hotgas Forming
full size foil
vacuum tight and stiff
May 29, 2003
9th Pisa meeting on advanced detectors

6. 9th Pisa meeting on advanced detectors
More requirements for the VELO
Rejection of multiple interactions in L0 Trigger
additional VETO stations upstream of
interaction point
Fast stand-alone tracking and vertexing for L1 Trigger
motivates R-and F-measuring sensors
Design allows to optimise resolution vs.
number of channels
Baseline design of sensors:
Active area 8mm to 42 mm
R measuring sensors:
division into 45o sectors
F measuring sensors:
inner/outer region
increasing pitch from inner towards outer radii
2nd metal layer to route signal to chips
May 29, 2003
9th Pisa meeting on advanced detectors

7. 9th Pisa meeting on advanced detectors
Second Level Vertex Trigger
Forward flight direction of B: Rz impact parameter
First step: 2D
-build Triplets of clusters in R sensors
-form tracks in Rz
-fill z-vertex histogram
-preselect large impact parameter
tracks in Rz
-match to m
Second step: 3D
-preselected (5-10) 2D tracks
-add information from F sensors for
3D reconstruction
-match to L0 and TT
z-vertex histogram
xy-vertex
Example: 2D tracks in 45o
sz~70mm
sx,y~30mm
May 29, 2003
9th Pisa meeting on advanced detectors

8. 9th Pisa meeting on advanced detectors
Optimisation of VELO sensor design
New 45o
Old 90o
200mm sensor
L1 Trigger: speed, number of ghost tracks
sector division
Clustering/tracking efficiency: Signal to Noise
strip length
Options of design studied (keeping constant number of strips)
Different strip pitches
Does impact parameter
resolution suffer?
max. 5%
design chosen with gradual
increase of pitch (40mm
to 103mm)
Resolution (mm)
May 29, 2003
9th Pisa meeting on advanced detectors

9. 9th Pisa meeting on advanced detectors
Even more requirements for the VELO
This detector has to operate in an extreme radiation environment
Middle station
Far station
Maximum irradiation per station:
5x1012 to 1.3x1014 neq/cm2/year
Strongly non-uniform dependence on R and station (z)
Maintain a good S/N performance for at least 2 years (replacement)
Extensive R&D program to select
Sensor and Front-End chip
May 29, 2003
9th Pisa meeting on advanced detectors

10. 9th Pisa meeting on advanced detectors
Prototype testing in Lab and Test beam
Test beam: CERN SPS (120 GeV p and m)
Irradiated
Not irradiated
Irradiated
Not irradiated
DELPHI-ds sensor
Irradiated/Not irradiated
PR03 sensors
May 29, 2003
9th Pisa meeting on advanced detectors

11. 9th Pisa meeting on advanced detectors
Tests of the p-on-n prototype
Efficiency of cluster reconstruction close to track
Box size ~ efficiency
PR02 F-sensor: routing lines in outer region
none in inner region
More efficient in inner region
Less efficient in irradiated region
May 29, 2003
9th Pisa meeting on advanced detectors

12. 9th Pisa meeting on advanced detectors
P-on-n vs n-on-n
P-on-n:
-Fraction of charge in routing line
reaches 20% in outer region
-5% in inner region
-Detector has undepleted and insulating
layer after irradiation
-Expected to be less for n-on-n
Compare efficiency for p-on-n and
n-on-n for different depletion depth
P-on-n efficiency degrades fast
N-on-n efficiency ~100% for only 60% depletion depth
Chose n-on-n for VELO
May 29, 2003
9th Pisa meeting on advanced detectors

13. 9th Pisa meeting on advanced detectors
Front-End chip decision
Two parallel developments: SCTA_VELO (DMILL) and Beetle (0.25mm CMOS)
Features: 128 input channels
40MHz sampling (LHC clock)
Hybrids equipped with 16 chips tested in test beam
Decision taken at beginning of this year to use Beetle
Performance equally good
Availability, radiation hardness and usage in LHCb
SCTA_VELO
Beetle1.1
May 29, 2003
9th Pisa meeting on advanced detectors

14. 9th Pisa meeting on advanced detectors
Beetle chip tests
Currently Beetle1.2 under study
Test set-up in Lab with one chip reading out a n-on-n 200mm thick
prototype sensor
Sensor close to final design
Measure S/N with Sr source
Prepare for test beam:
Hybrid with 16 Beetle1.2
chips reading out a full
sensor
MPW submission of improved
Beetle1.3 soon
May 29, 2003
9th Pisa meeting on advanced detectors

15. 9th Pisa meeting on advanced detectors
Summary & Outlook
VELO design is close to completion
Important decisions: finalised sensor design
choice of Front-End chip
Successfully tested module prototypes consisting
of Sensor, Hybrid and 16 chips
Plan to have first module end of 2003
Complete VELO in 2006
May 29, 2003
9th Pisa meeting on advanced detectors