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ILC vertex detector R

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Title: 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
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