ILC vertex detector R

1
Elementary Particle Physics Seminar Oxford, 6
November 2007
ILC vertex detector RD optimising the detector
design for physics
Sonja Hillert (Oxford)
2
Outline of this talk

• Role of the vertex detector for extracting ILC
  physics
• Measuring quark charge vertex charge and charge
  dipole procedure
• Sensitivity of vertex charge reconstruction to
  detector design fast MC results
• LCFI Vertex Package software tools for full MC
  studies
• Optimising the vertex detector design
• Recent results of LCFI vertex detector RD on
  sensors mechanical support

3
Typical event processing at the ILC
4
Dependence of physics reach on detector
performance

• Flavour tag needed for event selection and
  reduction of combinatoric backgrounds
• Quark charge sign determination used for
  measurement of ALR,
• angular correlations (? top polarisation)
  vertex detector performance crucial
• Examples
• Higgs branching ratios
• classical example of a process
• relying on flavour tag
• ee- ? ZHH
• 4 b-jets in final state requiring
• excellent tagging performance
• could profit from quark charge
• sign selection

5
Processes requiring quark sign selection ee- ?
b bbar

• ee- ? bb indirect sensitivity to new physics,
  such as extra spatial dimensions, leptoquarks,
• Z, R-parity violating scalar particles
  (Riemann, LC-TH-2001-007, Hewett PRL 82 (1999)
  4765)
• quark charge sign selection to large cos q
  needed to unfold cross section and measure ALR

6
Processes requiring quark sign selection ee- ?
t tbar

• ee- ? tt demanding for vertex detector
• multijet event final state likely to include
  soft jets
• some of which at large polar angle
• flavour tag needed to reconstruct the W bosons
  and
• top-quarks
• quark charge sign selection will help to reduce
• combinatoric backgrounds
• top decays before it can hadronise polarisation
  of top quark
• can be measured from polarisation of its decay
  products
• best measured from angular distribution of
  s-jet (quark charge)

7
Requirements

• To measure quark charge efficiently one needs
• an excellent vertex detector
• pixel-based system
• few micron point resolution (lt 5 mm)
• small inner layer radius ( 15 mm)
• good polar angle coverage
• low mass support structure (lt 0.1 X0)
• mechanical stability
• appropriate high-level reconstruction software,
  e.g.
• topological vertex finding
• flavour tagging
• vertex charge reconstruction (charged hadrons)
• charge dipole reconstruction (neutral B hadrons)

8
Vertex charge reconstruction

• b-jets contain a complex decay chain, from which
  the charge has to be found
• in the 40 of cases where b quark hadronises to
  charged B-hadron,
• quark sign can be determined by vertex charge

• need to find all stable tracks from
• B decay chain
• define seed axis
• cut on L/D (normalised distance
• between IP and projection of track POCA
• onto seed axis)
• tracks that form vertices other than IP
• are assigned regardless of their L/D

• need vertex finding as prerequisite (definition
  of seed axis)
• in most analyses, only calculate charge for jet
  of specific flavour need flavour tagging
• probability of mis-reconstructing vertex charge
  is small for both charged and neutral cases

9
Charge dipole procedure

• For some neutral vertices, quark charge can be
  obtained from
• the charge dipole formed by B- and D-decay
  vertex.
• ghost track vertexing algorithm (aka ZVKIN)
• developed at SLD, was shown to yield higher
  purity for
• charge dipole than standard ZVTOP code
  (ZVRES, cf p12)
• advantage one-prong vertices identified by
  vertex finder
• ? increased efficiency, especially at short B
  decay lengths
• at ILC, charge dipole procedure still to be
  explored

10
Performance of charge reconstruction leakage
rates

• define leakage rate l0 as probability of
  reconstructing neutral hadron as charged
• performance strongly depends on low momentum
  tracks
• largest sensitivity to detector design for
  low jet energy, large cos q

preliminary results from fast MC study (SGV)
11
Using vertex charge for detector optimisation

• Using fast MC SGV, studied dependence of leakage
  rate on vertex detector design
• compared detectors with different inner layer
  radii (? beam pipe radii)
• varied amount of material per detector layer
  (factor 4 compared to baseline)
• translated into integrated luminosity required
  to obtain physics results of same significance
• for processes requiring independent quark
  charge measurement in 2 jets,
• increase of beam pipe from 15 to 25 mm has
  sizeable effect (factor 1.5 2)

preliminary results from fast MC study (SGV)
12
The LCFI Vertex Package

• The LCFIVertex package provides, in a full MC
  and reconstruction framework
• vertex finder ZVTOP with branches ZVRES and
  ZVKIN (new in ILC environment)
• flavour tagging based on neural net approach
  (algorithm R. Hawkings, LC-PHSM-2000-021)
• includes full neural net package flexible to
  allow change of inputs, network architecture
• quark charge determination, currently only for
  jets with a charged heavy flavour hadron
• first version of the code released end of April
  2007
• code, default flavour tag networks and
  documentation available from the ILC software
  portal
• http//www-flc.desy.de/ilcsoft/ilcsoftware/LCF
  IVertex
• next version planned to be released this Friday
• minor corrections, e.g. to vertex charge
  algorithm further documentation
• diagnostic features to check inputs and outputs
• new vertex fitter based on Kalman filter to
  improve run-time performance

13
ZVTOP vertex finder, Pt-corrected mass

• two branches ZVRES and ZVKIN (already mentioned
  when discussing charge dipole)
• The ZVRES algorithm (D. Jackson, NIM A 388
  (1997) 247)
• very general algorithm that can cope with
  arbitrary multi-prong decay topologies
• vertex function calculated from Gaussian
  probability tubes representing tracks
• iteratively search 3D-space for maxima of this
  function and minimise c2 of vertex fit

14
Flavour tagging approach

• Vertex package provides flavour tag procedure
  developed by R. Hawkings et al
• (LC-PHSM-2000-021) as default
• number of vertices found determines which
• NN input variables are used
• if secondary vertex found MPt , momentum
• of secondary vertex, and its decay length and
• decay length significance
• if only primary vertex found momentum and
• impact parameter significance in R-f and z for
  the
• two most-significant tracks in the jet
• in both cases joint probability in R-f and z
  (estimator of
• probability for all tracks to originate
  from primary vertex)
• flexible permits user to change input
  variables, architecture and training algorithm of
  NN

15
Flavour tagging performance
Z-peak
Z-peak
500 GeV
500 GeV
16
Diagnostic features

• plan to make available inputs and outputs for
  ZVRES flavour tag (later vertex charge)
• nearly complete LCFIAIDAPlot module for
  flavour tag diagnostics based on AIDA
• input and output variables of the flavour tag
  neural nets, separately for b-, c-, light jets
• graphs of purity vs efficiency and flavour
  leakage rates (i.e. efficiencies of wrong
  flavours)
• vs efficiency separately for the 1-, 2- and
  3-vertex case
• JAS3-macro to plot these easily
• optionally raw numbers of jets vs NN-output,
  AIDA tuple with flavour tag inputs written out

example inputs to flavour tag
17
New vertex fitter Kalman filter

• Motivation improve run time performance by
  replacing the space-holder
• Least-Squares-Minimisation (LSM) fitter of
  first release
• Kalman filter code by S. Gorbunov, I. Kisel
  interfaced to Vertex Package
• successfully tested
• find same flavour tagging performance
• as with LSM-fitter
• resulting improvement in run time performance
• overall run time of Vertex Package is
• reduced to 25 of the 1st-release value

18
Towards a realistic simulation

• Current simulations are based on many
  approximations / oversimplifications.
• The resulting error on performance is at
  present unknown and could be sizable,
• especially when looking at particular regions
  in jet energy, polar angle (forward region!)
• Issues to improve
• Vertex detector model replace model with
  cylindrical layers by model with barrel staves
• GEANT4 switched off photon conversions for time
  being (straightforward to correct)
• hit reconstruction using simple Gaussian
  smearing at present realistic code exists only
• for DEPFET sensor technology, not for CPCCDs
  and ISIS sensors developed by LCFI
• track selection
• KS and L decay tracks suppressed using MC
  information
• tracks from hadronic interactions in the
  detector material discarded using MC info
• only works for detector model LDC01Sc (used for
  code validation) at present
• current default parameters of the code optimised
  with fast MC or old BRAHMS (GEANT3) code
• default flavour tag networks were trained with
  fast MC

19
Examples of impact of simplifications

• effects of simplifications can be sizeable
• note photon conversions and hadronic
• interactions in detector material can
• efficiently be corrected for
• currently making initial checks needed for
• implementing these corrections

c
20
Further development of the Vertex Package

• Areas of relevance for wider user community
• integration into ALCPG software framework
  org.lcsim drivers under development in the US,
• to be released as soon as possible (N. Graf)
• consistent IP treatment, based on per-event-fit
  in z and on average over N events in Rf
• Vertexing
• explore use of ZVKIN branch of ZVTOP for flavour
  tag and quark charge determination
• optimise parameters
• study performance at the Z-peak and at sqrt(s)
  500 GeV
• explore how best to combine output with that of
  ZVRES branch for flavour tag
• use charge dipole procedure (based on ZVKIN) to
  study quark charge determination for
• (subset of) neutral hadrons

21
Improvements and extensions

• Areas of relevance for wider user community
  contd
• Flavour tagging explore ways to improve the
  tagging algorithm, e.g. through use of
• different input variables and/or different
  set-up of neural nets that combine these
• improvements to MPt calculation using
  calorimeter information, e.g. from high-energy p0
• vary network architecture (number of layers
  nodes, node transfer function), training
  algorithm
• explore new data mining and classification
  approaches (e.g. decision trees, )
• Vertex charge reconstruction
• revisit reconstruction algorithm using full MC
  and reconstruction (optimised with fast MC)
• Functionality specifically needed for vertex
  detector optimisation
• Correction procedure for misalignment of the
  detector and of the sensors will need to be
• developed, adapted or interfaced (see
  optimisation of the detector)

22
Optimising the detector design

• Simulations serve to estimate performance of
• benchmark quantities impact parameter
  resolution, flavour tag, vertex charge
  reconstruction
• reconstruction of physics quantities obtained
  from study of benchmark physics processes
• Vertex detector-related software cannot be
  developed in isolation
• vertexing, flavour tag, vertex charge recn
  performed on a jet-by-jet basis (depends on jet
  finder)
• strong dependence on quality of input tracks
  (i.e. hit and track reconstruction software)
• physics processes to optimise calorimeter also
  depend on tagging performance (e.g. ZHH)
• Study of benchmark physics processes
• performed in close collaboration with the two
  main ILC detector concept study groups
• SiD and ILD (formerly GLD / LDC)
• ILCSC has issued call for Letters of Intent, to
  be submitted 1 October 2008
• These will be followed by more detailed
  Engineering Design Studies to be completed by 2010

23
Benchmark Physics Studies

• Benchmark physics processes should be typical of
  ILC physics and sensitive to detector design.
• A Physics Benchmark Panel comprising ILC
  theorists and experimentalists has published
• a list of recommended processes that will
  form the baseline for the selection of processes
• to be studied in the LoI- and engineering
  design phases.
• Following processes were highlighted as most
  relevant by the experts (hep-ex/0603010)

particularly sensitive to vertex detector design
24
Parameters and aspects of design to be optimised

• The Vertex Package, embedded into full MC and
  reconstruction frameworks,
• permits the following aspects of the vertex
  detector design to be optimised
• Beam pipe radius
• Sensor thickness, material amount at the ends of
  the barrel staves
• Material amount and type of mechanical support
  (e.g. RVC, Silicon carbide foams)
• Overlap of sensors linked to sensor alignment,
  tolerances for sensor positions along
• the beam perpendicular to it
• Arrangement of barrel staves
• Long barrel vs short barrel plus endcap geometry
• Study of trade-offs, involving variations of
  more than one parameter, should be aimed at
• Physics simulation results will be only one of
  the inputs that determine the detector
• design the more decisive input may well be
  provided by what is technically feasible.

25
LCFI sensor development introduction

• LCFI pursuing development of two sensor
  technologies for ILC vertex detector
• Column Parallel CCD (CPCCD)
• CPC1 (first CPCCD)
• CPC2 second generation, large area device
• In-situ Storage Image Sensor (ISIS)
• proof-of-principle device (ISIS1)
• effort towards ISIS2

CPC1
CPC2
ISIS1
26
Column Parallel CCD principle

• Main sensor technology developed by LCFI
• Every column has its own amplifier and ADC
• Readout time shortened by 3 orders of magnitude
  compared to classic CCD
• All of the image area is clocked, complicated by
  the large gate capacitance
• Optimised for low voltage clocks to reduce power
  dissipation

27
CPC2 devices
ISIS1
Busline-free CPC2
CPC2-70
104 mm
CPC2-40
CPC2-10

• Three device sizes 10, 40 and 70 mm length
  (requirement for inner layer device 100 mm)
• Some devices designed to reach high speed (up to
  50 MHz operation)
• 2-level metallisation permits using whole
  image area as distributed bus line
• Charge to voltage conversion can happen on CCD
  (50 channels) or on readout chip (other 50)

28
CPC2 test results
20 MHz System noise 110 e- RMS
10 MHz System noise 75 e- RMS

• short (CPC2-10) device, high-speed design,
  tested with 55Fe signal (1620 e-, MIP-like)
• X-ray hits seen up to 45 MHz important
  milestone
• CPC2 works with clock amplitude down to 1.35 Vpp
• in standalone tests (w/o readout chip, using
  2-stage source follower outputs)
• at 10 MHz achieve noise level of 75 e- RMS
  (CMOS driver chip)
• tests continuing

29
Readout chips CPR1 and CPR2
Voltage and charge amplifiers 125 channels
each Analogue test I/O Digital test I/O 5-bit
flash ADCs on 20 µm pitch Cluster finding logic
(2?2 kernel) Sparse readout circuitry FIFO
Bump bond pads
CPR1
CPR2

• both chips made on 0.25 µm CMOS process (IBM)
• front-end amplifiers matched to the CCD outputs
• additional test features in CPR2
• CPR2 includes data sparsification

Wire/Bump bond pads
30
Readout chips results
CPR1, 1 MHz
CPR2 sparsification

• Voltage outputs
• non-inverting (negative signals)

• Charge outputs
• inverting (positive signals)

• CPR1 in charge channels expected gain observed,
  constant over device
• in voltage channels gain drops from edge to
  the centre of the device (not understood)
• CPR2 same gain drop in voltage channels as in
  CPR1, charge channels dont work
• errors in sparsification for cluster
  distances lt 60 100 pixels extensive tests with
  measured
• and simulated input led to major re-design
  of next generation chip CPR2A

31
Clock driver for CPCCD

• Challenge providing 2 Vpp clocks at 50 MHz for
• CPCCD (capacitance 40 nF / phase) 20 A peak
  current
• Further requirements
• low power dissipation
• must be close to CCD (to reduce parasitic
  inductance)
• must not add too much to material budget
• may have to work at low temperature (down to
  -100 oC)

• 2 approaches transformer (fallback), custom
  ASIC (baseline)
• performed tests with 161 transformer integrated
  into PCB (above)
• and with first version of driver chip CPD1
  (right)
• 1 chip drives 2 phases, up to 3.3 V clock swing
• 0.35 mm CMOS process, chip size 3 x 8 mm2
• 8 independent clock sections
• careful layout on- and off-chip to cancel
  inductance
• bump-bondable

32
Integration of CPC, CPR and CPD

• a lot more work needed to arrive at ladders
• (design of ladder end left)
• but an integrated system of CPC, CPR and CPD
• exists and works up to frequency of 9 MHz
• (speed limited by CPR2)

33
LCFI Mechanical Studies

• LCFI mechanical work comprises
• support structure prototyping
• (RVC-, SiC foam, carbon fibre, shells)
• cooling studies
• conceptual design

example design of a foam ladder (cross section)
SiC, samples of 8 and 6 relative density
obtained Plot deviation under temperature cycling
34
Summary

• ILC physics will require an excellent vertex
  detector as well as adequate reconstruction
  software.
• Studies of the performance of vertexing, flavour
  tagging, quark charge reconstruction,
• and studies of benchmark physics processes are
  used to optimise the vertex detector design.
• The LCFI Vertex Package provides software tools
  to perform such optimisation using
• GEANT4-based simulation and full
  reconstruction software (including e.g. pattern
  recognition).
• Further development of these tools is needed for
  realistic detector assessment and comparison.
• The development of CPCCD-sensors, CPR readout
  chips and CPD drivers within the
• LCFI collaboration is far advanced. In
  parallel, mechanical work is progressing well.
• In lab tests, CCDs were driven with amplitudes
  as low as 1.35 Vpp (design spec 2 Vpp)
• and signals observed up to frequencies of 45
  MHz (design spec 50 MHz)
• A combined system of CPC2-CPR2-CPD1 was
  successfully operated up to 9 MHz.

35
Additional Material
36
D. Jackson, NIM A 388 (1997) 247
The ZVTOP vertex finder

• two branches ZVRES and ZVKIN (also known as
  ghost track algorithm)
• The ZVRES algorithm very general algorithm
• that can cope with arbitrary multi-prong
  decay topologies
• vertex function calculated from Gaussian
• probability tubes representing tracks
• iteratively search 3D-space for maxima of this
  function
• and minimise c2 of vertex fit
• ZVKIN more specialised algorithm to extend
  coverage to b-jets with
• 1-pronged vertices and / or a short-lived
  B-hadron not resolved from the IP

• additional kinematic information
• (IP-, B-, D-decay vertex approximately
• lie on a straight line) used to find
• vertices
• should improve flavour tag efficiency
• and determination of vertex charge

37
CPC2 Clock Driving

• CPC1 did not have optimal drive conditions due
  to the single level metal
• Novel idea from LCFI for high-speed clock
  propagation busline-free CCD
• 50 MHz achievable with suitable driver in
  CPC2-10 and CPC2-40 (L1 device)
• Transformer or ASIC driver

1 mm
38
In-situ Storage Image Sensor (ISIS)

• Operating principles of the ISIS
• Charge collected under a photogate
• Charge is transferred to 20-cell storage CCD in
  situ, 20 times during the 1 ms-long train
• Conversion to voltage and readout in the 200
  ms-long quiet period after the train (insensitive
  to beam-related RF pickup)
• 1 MHz column-parallel readout is sufficient

39
In-situ Storage Image Sensor (ISIS)
5 µm
Global Photogate and Transfer gate

• The ISIS offers significant advantages
• Easy to drive because of the low clock
  frequency 20 kHz during capture, 1 MHz during
  readout
• 100 times more radiation hard than CCDs (less
  charge transfers)
• Very robust to beam-induced RF pickup
• ISIS combines CCDs, active pixel transistors and
  edge electronics in one device non-standard
  process
• Presently discussing with 3 vendors for the
  ISIS2 development
• Looking at modified 0.18 µm CMOS process with
  additional buried channel and deep p implants
• Proof-of-principle device (ISIS1) designed and
  manufactured by e2V Technologies

ROW 1 CCD clocks
ROW 2 CCD clocks
On-chip logic
On-chip switches
ROW 3 CCD clocks
ROW 1 RSEL
Global RG, RD, OD
RG RD OD RSEL
Column transistor
40
Tests of ISIS1

• Tests with 55Fe X-ray source
• ISIS1 without p-well tested first and works OK
• Correct charge storage in 5 time slices and
  consequent readout
• Successfully demonstrated the principle
• New ISIS1 chips with p-well have been received,
  now under tests
• ISIS1 without p-well is now in a test beam in
  DESY