LHCb Vertex Locator (VELO) Lars Eklund École Polytechnique Fédérale de Lausanne Liverpool University Vrije Universiteit Amsterdam

LHCb Vertex Locator (VELO)

Lars Eklund

École Polytechnique Fédérale de Lausanne

Liverpool University

Vrije Universiteit Amsterdam

Vertex 05, November 7, 2005Lars Eklund, CERN 2

Outline

• Introduction to LHCb and VELO

• Design of the VELO

• Performance plots

• Towards a beam test in 2006

• A more specific example:

– Digital filtering for x-talk corrections


LHCb

Tracker

ECAL HCAL Muon chambers

Magnet

Angular acceptance

15 - 300mrad

Proton interaction region

RICH 2

RICH 1

Trigger tracker

Dedicated b-physics experiment at LHC

• 1012 bb-pairs per year

• CP violation and rare b-decays

• Correlated boost of the bb-pairs

• Single forward spectrometer

The Vertex Locator


VELO – the experimental challenge

1. Trigger on the B decay of interest (Velo part of software trigger)

2. Suppress multiple interactions (Pile-up veto part of hardware trigger)

3. Reconstruct decay as a function of time

bt

Bs K

K

,K

Ds

Primary vertex

p p

Since the oscillations are fast, it requires excellent vertex resolution

Measurement of BS oscillations


The Vertex Locator (VELO)

~1m

Interaction region Downstr

eam21 Tracking stations

• Four ½ disks sensors per station

• Silicon micro-strip technology

• R-Φ geometry

Minimalist view of the VELO

Minimise material

• No conventional beam-pipe

• Sensors are operated in vacuum

• 250 μm Al foil to separate VELO from the beam vacuum

Minimise extrapolation distance

• First active element at R = 8.2 mm

• Retractable detector halves

• 30 mm at LHC filling

Pile-up veto

2 backward stations in the H/W trigger


Velo Sensors

–measuring sensor

(radial strips with an stereo angle)

Non uniform radiation environment

•1.3 * 1014 neq/cm2/year at R = 8 mm

• 5 * 1012 neq/cm2/year at R = 42 mm

• n+ in n-bulk sensors

• second metal layer for signal routing

• 2048 micro strips per sensor

• 40 – 100 μm pitch

R-measuring sensor (concentric strips)

42 mm

8 mm

FE

chip


Detector module

Cooling cookies

Pitch adaptors

Silicon sensor

TPG/carbon fibre substrate with laminated kapton circuit

Carbon fibre paddle

Front-end chip

(Beetle1.5)

PRR in December


Detector modules - performance

Operational window:

• Sufficient cluster efficiency (> 99 %)

• Acceptable noise occupancy (< 0.1 %)

• Low fake cluster rate in next time bin (< 25 %)

Cluster efficiency vs. S/N cut

Results test beam November 2004 – 300 μm thick Φ measuring sensor

Noise occupancy vs. S/N cut

Overspill vs. S/N cut


Cooling, vacuum and positioning

Beam pipe

Detector halves retractable by 30 mm

CO2 cooling manifold

Primary (beam) vacuum

Secondary (detector) vacuum

RF foil (250 μm Al)

Vacuum bellows

(to allow the retraction)


Mechanics – status

Vacuum and positioning assembly at NIKHEF

Vacuum bellow


DAQ chain

front-end ASIC

2 m low mass cable

60 m twisted

pair

ADC and pre-processing

FPGA

Radiation dose: ~ MRad

pre-compensating

cable driver

Radiation dose: ~100 kRad Radiation free area

readout network

PC farm

Radiation free area

Beetle 1.5

• analogue readout

• 128 channels

• 4 serial links

• 900 ns readout time

Kapton cables

• low mass

• flexible

• vacuum compatible

Analogue repeaters

• compensates cable response

• COTS components

• 5632 links

Cat 6 cable Gigabit Ethernet

• commercial components

TELL1

• ADC (10 bit, 40 MHz)

• digital filter

• pedestal and common mode noise subtraction

• strip re-ordering

• clustering

PC farm

• Linux cluster

• software trigger

• permanent storage


Performance

IP = 14m+35m/pT

Impact parameter resolution

Bs vtx resolution (mm)


Towards the final system…

• Components are or will soon be in production• Module production will start in December 2005• Assembly will start January 2006

– At CERN

• System test April 2006– Whole detector half powered and configured– 10 modules read out

• Beam test 2006– Whole detector half in the beam test– Fully system verification: Detector modules, control system,

DAQ, reconstruction software, alignment …


Two systems issues

1.Interaction vertices in the silicon sensors and halo tracks

2. Digital filtering of the analogue data link


Test beam set-up

scintilator trigger

proton beam

interaction in the sensor

• Detector half operated in vacuum

• Use silicon sensors as targets

• Look for interactions in the silicon

• Reconstruct tracks and align

Partially commissioned at installation


Need stand-alone tracking

Tracks from interaction point (R=0) are linear in the R-z plane

Pattern recognition assume tracks from close to R = 0

Need for a stand-alone reconstruction package

• Handles vertices anywhere

• Feeds tracks to the alignment algorithm

z [mm]

R [

mm

]Event display of an interaction in the silicon sensor.

Tracks highly non-linear in the R-z plane


Spin-off: halo tracks in LHCb

Velo Left

Velo Right

cartoon event display of the VELO

Solution: Use beam halo tracks and interactions in the silicon sensors for alignment in LHCb

• Re-use algorithms from the beam test 2006

Velo divided into four weakly coupled parts in terms of alignment:

• Backward-Left, Backward-Right, Forward-Left and Forward-Right

• very few tracks traverse more than one part

L-B

R-B

L-F

R-F


Two systems issues

1. Halo tracks and interaction vertices in the silicon sensors

2.Digital filtering of the analogue data link


Digital filtering – the problem

32 channels serial data (800 ns)

signal noiseData is transferred on four serial links per front-end ASIC:

header

25 ns

Signal travels through:

• front-end hybrid

• kapton cables (2 types)

• vacuum feed-through

• Amplifier (+ PCB)

• 60 m TP cable

Blue: no signal (pedestals)

Red: signal in two channels


Modelling the problem

Consider each output as a number series, where n corresponds to the channel number

x[n] : “True” data at the input (from sensor or test pulse)

w[n] : Raw ADC values

y[n] : Corrected values

H : Transfer function of the ASIC and the analogue link

G : Transfer function of the digital filter

Sensor, ASIC and the

analogue linkDigital filter

x[n] w[n] y[n]

H(z) G(z)

Method: (Assuming LTI: Linear Time Invariant System)

1. Determine H from the data (from beam particles or calibration pulses)

2. Find ‘G’ such that G*H = 1, implying that y[n] = x[n]


The impulse response

][][][ nkxkhnw

The transfer function H is characterised by the series h[n]:

h[n] can be determined by injecting a single pulse:

00

01][][

nif

nifnnx

Obtaining a Delta function δ:

1. Internal calibration pulses

• Feature of the Beetle chip

2. From track data

• Point a track on the sensor

• Select tracks centered on strips

• Assume no charge sharing

3. look at the signal in adjacent channels

4 ASICs, 16 serial links: 32 different impulse responses

h[+1]

h[-1]

x-talk measurements from TB November 2004


Determining the filter algorithm (1)

Sensor, ASIC and the

analogue linkDigital filter

x[n] w[n] y[n]

H(z) G(z)

Postulate:

• The impulse response h[n] = 0, except for N time-bin

• g[n] = 0, except for M time-bins (Finite Impulse Response filter = FIR).

][][][ nkwkgny

][][][ nkxkhnw

requires that

][][][ nkhkgnand

][][ nxny

][][ nnx

2

1

2

1

1 ][][][

M

M

NM nkhkgn

)(0

0

0)(

0

1

0

][1

MNnif

nif

nMNif

nMNwhere

The infinite sum is truncated to M + N – 1 conditions:


Determining the filter algorithm (2)

The truncated sum gives:

• N + M – 1 constraints

• M unknowns (the g[n])

Solve these over constrained equations with the least square method

corrected x-talk from TB November 2004

4 ASICs, 16 serial links: 32 different impulse responses

Boundary problems:

• 32 channels per link

• exceptions for channels 0 and 31

• missing information

• The data header

• specific corrections


Effects on the resolution

Track residuals:

• reversed readout order for small radii

• Asymmetric (forward/backward) x-talk

• step in residuals

Resolution:

• Smearing due to x-talk

• Odd/even channel dependence

• ~1 μm improvement

Results test beam November 2004 – 200 μm thick R measuring sensor (7 degree track angle)


Summary

• The Vertex Locator of LHCb uses silicon micro-strip sensors with R-Φ geometry

• Operated in vacuum with retractable detector halves• Shows good efficiency and noise occupancy

performance in the beam test• Is in production – assembly will start soon• System issues

– System test and beam test 2006

• Specific example– Corrections of the data link

Documents

LHCb Vertex Locator (VELO) Lars Eklund École Polytechnique Fédérale de Lausanne Liverpool University Vrije Universiteit Amsterdam