1. Simulated vertex precision
VERTEX LOCATOR
Presented by:
Malcolm John (CERN) on behalf of the LHCb VELO collaboration
April 2006
Presented at:
The International Symposium on Detector Development (SNIC) April 2006
Detector overview
LHCb is an experiment under construction at point-8 on the LHC collider at CERN. It is conceived to take advantage of the large number of B-mesons (1012/year) produced in the 14TeV proton-proton interactions.
The study of B-mesons depends on efficient resolution of displaced vertices. B-mesons produced at LHC fly several millimeters before decay. Precise vertexing is required to resolve a primary collision vertex and the secondary B-decay vertex.
~1 metre
VELO vacuum tank is integrated into the LHC beam-pipe
Closest active element approaches to within only 8.2mm of the LHC beam
Silicon sensors, front-end analogue electronics and associated cooling units are housed in a secondary vacuum contained by an aluminum RF-shield
Each entire VELO-half is mobile. It is required to retract away from the beam-line by 3cm during beam-filling
3cm retraction
During beam-fill B- - + µ-
Look for displaced vertices whose momenta sums to the B mass
Primary vertex:
many tracks
(typically )
B decay vertices:
a few tracks
(typically 2 to 5) D0 µ
100 µm to 10 mm B0
At LHCb, the VELO has three vital roles:
Trigger on a B decay of interest (the VELO is part of the online software trigger)
Suppress multiple interactions (A pile-up veto in the hardware trigger)
Track reconstruction: used to seed the tracking in the rest of LHCb.
VELO tracking can also provide an absolute measurement of the LHC beam’s movement – even when the VELO-halves are retracted
4. Precise vertexing and reconstruction of decays as a function of the B meson’s time of flight (offline reconstruction)
Physics motivation
An example is the measurement of BS-mixing. Because the BS oscillate very fast, excellent vertexing is required to resolve the time (and, because of the boost at LHC energies, distance) -dependent decay distribution
Simulated vertex precision
Baseline VELO design
21 stations. Each station measures r and 
Left-Right staggered in Z to allow for a small overlap region (alignment)
Inner (<16mm) and outer region
‘Dog-leg’ to give a stereo angle
Pitch: 35 to 100 microns
4x45o sectors (Provides crude  )
Pitch: 40 to 100 microns
Silicon sensor. Choose:
n+ in n-bulk sensors
200 – 300 microns thick
n-on-n shows most tolerance to radiation damage
Needs cooling (< 0º) to minimise radiation damage
Bias voltage < 600V (after 3 years of radiation)
Second metal layer for signal routing
2048 micro strips per sensor (Over channels in total)
35 – 100 μm pitch (To keep occupancy reasonable at low r)
Beetle FE chip pulse shape from testbeam data
Non uniform radiation environment
1.3 * 1014 neq/cm2/year at r = 8 mm
5.0 * 1012 neq/cm2/year at r = 42 mm
Front-end analogue read-out : Beetle chip
Fast (LHC bunch-crossing freq. = 25ns)
Low noise
Silicon design
-sensors
r-sensors
Readout lines
Silicon strips
RMIN = 8.2mm
RMAX = 42.1mm
Silicon side
Beam side
The VELO vacuum box is required to protect the VELO from RF pickup from LHC beams, to provide a wake-field guide, and to protect LHC vacuum from out-gassing from VELO
To avoid inelastic deformations The secondary vacuum in the box is maintained to within 5 mbar of the primary LHC vacuum.
Careful R&D has designed an inner foil that minimizes the amount of material seen by particles coming from the interaction point yet allows the left and right halves of the VELO to overlap (needed for alignment)
This RF shield varies in thickness from 300m down to 150m at the very contorted part close to the beam.
Vacuum box
The RF-shield accounts for half the radiation length of the VELO (shown in pie-chart) for particles in the LHCb acceptance!
Cut-away of the RF-box showing the position of the silicon-sensor modules within 1
Module production
Hollow carbon-fiber paddle
After 10years of R&D, the 42 VELO modules are now entering production
A complete VELO half is expected ready for beam-tests after the summer
Installation in the experimental hall is due early in 2007
Precision-milled feet set in carbon-fibre base
Aluminum “cookies” are the heat-sink for the CO2 cooling system. The cookies are mounted at the bottom of the hybrid
Bonding Front-end Beetle chips to hybrid
Pitch adaptor fabrication.
PAs bond to the sensor at one end and to the Beetle chip at the other
receive and validate components
populate hybrids with Beetle FE chips and pitch adaptors (PA)
bond back-end of chips to the hybrid and the front-end to the PAs
glue r-measuring and f-measuring sensors on double-hybrids
bond PAs to sensors
glue double-hybrid to carbon-fibre paddle and base
final QA, metrology and burn-in
Silicon detectors (r,f)
Hybrid (Beetle FE, PA)
PCB substrate
Cooling unit
Carbon-fibre paddle
Precision-made base
silicon  0ºC
cookies -10ºC
By September 2006
Mechanics: installation
The VELO installation is split into two projects:
Assemble/commission silicon sensors, readout and electronics outside the experimental hall. Metrology, and beam-tests will be performed on each VELO-half (box right)
Install vacuum mechanics in the experimental hall for LHC integration and bake-out. Testing of the primary and secondary vacuums
Vacuum-tight bellows:
Allows the retraction whilst maintaining vacuum-tightness
(Trial-) installation a detector hood containing its module support plate into the vacuum tank
Vacuum tank shipping to CERN now