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Vertex Detectors and the Linear Collider


General principles of vertex detector design, and prospects for ILC ... benchmark for charm tag (A Sopczak, A Finch, H Nowak LCWS 2004) ... – PowerPoint PPT presentation

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Title: Vertex Detectors and the Linear Collider

Vertex Detectors and the Linear Collider
  • Chris Damerell
  • Rutherford Appleton Lab
  • What is their purpose? (still somewhat
    contentious, long after LCWS 1991!)
  • General principles of vertex detector design,
    and prospects for ILC
  • What specific technology to use?
  • How not to be blown away by 109 pixels
    electromagnetic interference, signal sampling,
    and other issues
  • How can we get to ILC physics, from where we
    are now?

What are vertex detectors for?
  • Dave Burke, LCWS 1991
  • Particle flow almost reveals the underlying
    Feynman diagram

  • Vertex detector will tag b and c jets and tau
    leptons with high efficiency (reducing
    background, both combinatoric and from other
    multi-jet processes)
  • Will also efficiently perform heavy quark sign
    selection, via measurement of the vertex charge
    (net charge of displaced vertex) new from SLD

  • Particle flow plus efficient b and c tag plus
    heavy quark sign selection permits fully
    reconstructed production and decay distributions
    for a sizable fraction of Z0 and W decays
  • Z0 15 b-bbar 12 c-cbar 27
  • W 31 c-sbar
  • leading to
  • enhanced information and increased statistics
    wrt leptonic channels
  • different physics reach (eg for Z0 , coupling
    asymmetries Ab 0.94, Ac 0.67, where Al 0.15)
  • For top quark decays, a powerful tool to measure
    the top quark polarisation

  • Not a case of hadronic versus leptonic channels
    both are valuable
  • Gudi Moortgat-Pick We have about 100
    parameters in the MSSM and need all observables
    we need to explore the angular distributions in
    all decay channels not only in the leptonic
  • This is largely uncharted territory, since the
    potential of vertex charge has not been widely
    recognised, but has recently become established
    by successful physics analyses in SLD
  • Look at a few physics examples, starting with the
    simplest SM process, having the largest
    cross-section of all

  • ee- ? q qbar differential cross-sections for L
    and R-polarised electrons at max sqrt(s), as
    probe of BSM processes
  • Sabine Riemann, Fermion-pair production at a
    linear collider a sensitive tool for new
    physics searches LC-TH-2001-007
  • Sensitive to Z, leptoquarks, R-parity violating
    scalar particles, and extra spatial dimensions
  • Requires efficient quark charge sign-selection
    out to large cos theta

Sabine Riemann one example for large ED,
sqrt(s) 0.5 TeV
For these channels, sensitivity extends to MD
around 5 TeV For muon pairs, effects are much
weaker (not measurable even with 1 ab-1 of data)
Chargino and neutralino angular correlations
  • G Moortgat-Pick and H Fraas PRD 59 (1999) 015016
  • SY Choi, HS Song and WY Song PRD 61 (2000)
  • G Moortgat-Pick et al hep-ph/0002253 (2000)
  • TESLA TDR (2001) III-64
  • Publications so far have focused on leptonic
    channels eg
  • Quark pairs were disregarded since the
    possibility of quark sign selection was not yet
  • Adding b-bbar and c-cbar will increases
    statistics and provide additional physics
    sensitivity (soon to be studied by G

Momenta in ee- rest frame for associated
neutralino production and in the rest frame of
the decaying neutralino Following analysis
from G Moortgat-Pick, A Bartl, H Fraas and W
Majerotto hep-ph/0002253 (2000)
Production process Fwd-Bkwd asymmetries plotted
for the decay electron in the process as fn of
the gaugino parameter M1. Left and right plots
correspond to different masses of the -/0/
means left-/un-/right-polarised beam, electrons
listed first, positrons second These asymmetries
strongly constrain the selectron masses as well
as the mixing properties of
Top quark polarisation a general tool for
  • top quark decays before hadronization, and long
    before its spin can flip
  • So, polarisation of top quark at production is
    reflected in its decay
  • not true for any other quark, where
    depolarisation effects during fragmentation wash
    out the quark helicities
  • In t ? b W, W ? c sbar, the top polarisation is
    best measured from the angular distn of the
    s-quark jet E Ruiz Morales and M Peskin Proc
    LCWS 1999 (Sitges) and hep-ph/9909383
  • To distinguish between t and tbar, need to tag b
    and c jet, and sign-select at least one of them
    (so quark sign selection doesnt need to be

sbar jet has 1-cos theta angular distribution
wrt top polarisation direction, as does l in the
leptonic W decays In complex events, expect the
fully reconstructed hadronic decays to have lower
background. In general, aim to use both leptonic
and hadronic W decays Measurement of top
polarization in scalar top and scalar bottom
decays can be used to determine fundamental SUSY
parameters tan b and the trilinear couplings
At and Ab E Boos et al, hep-ph/0303110
  • Using all observables, the ILC makes available
  • Production angular distributions
  • Decay distributions
  • Beam polarisation asymmetries
  • can then search for anomalous top production and
    decay form factors, including effects of
  • t anomalous magnetic moment
  • t?bR W decay (forbidden in V-A theory)
  • anomalous t-tbar-Z couplings
  • Michael Peskin LC lectures, March 2002 and TESLA
    TDR III-146

  • If no light Higgs, provides a means to probe
    strong EWSB at the ILC, via the process
  • Tim Barklow, Proc Snowmass 1996 p 819
  • E Ruiz Morales and M Peskin, Proc LCWS
    1999 (Sitges) 461
  • Need fully reconstructed top quarks, for
    suppression of competing processes
  • Sensitive to new resonances at large sqrt(s)
    vector resonance (techni-rho) coupling to
    helicity conserving amplitudes and scalar
    resonance (techni-sigma) to helicity-flip
  • Spin of these resonances can be probed by
    measurement of top quark polarisation (again, by
    angular distributions of s-quark jets)

Other physics examples
  • Higgs branching ratios very important but not
    very challenging technically
  • Higgs decay angle distributions (where H decays
    to b-bbar, c-cbar or W-W)
  • Higgs self-coupling
  • SUSY CP asymmetries
  • Form T-odd triple product,
    needs fermion sign selection)
  • A Bartl et al, hep-ph/0306304
  • Sensitivity of b-bbar compared to mu pairs
    needs event sample of only 1000 staus compared to
    105 (benefiting from the larger branching
    fraction, and fact that
  • Ab 6.4 x Ae )
  • ----------------------------------------------

  • benchmark for charm tag (A Sopczak, A Finch, H
    Nowak LCWS 2004)
  • Particularly challenging for small dm between
    neutralino and stop masses theoretically
    motivated talk in SUSY session by Caroline
    Milstene, and Jonathan Fengs colloquium
  • The totally unexpected what we are all hoping
  • The universe is not only queerer than we
    suppose, but it is queerer than we can suppose.
    JBS Haldane

Sincere thanks to many experts who kindly helped
with this part of the talk Juan Antonio Aguilar
Saavedra, Dave Bailey, Alfred Bartl, Nicolo de
Groot, Stefan Hesselbach, Olaf Kittel,
Akiya Miyamoto, Gudi Moortgat-Pick, Michael
Peskin, Sabine Riemann, Andre Sopczak, Su Dong,
Peter Zerwas
General design principles
NA32, 1985
  • Fixed target experiments were much easier!
  • For the collider environment, the adequate
    vertex detector has yet to be built hence
    numerous options and constant upgrades

SLD, thanks to Su Dong
(No Transcript)
  • For the ILC, we can certainly do much better
  • Rbp 12-15 mm, cf 25 mm at SLD
  • Layer thickness 0.05-0.1 X0 , cf 0.4 X0 at SLD
    20 mm of Si is 0.02 X0
  • Point measurement precision at least as good as
    at SLD (approx 3 mm)
  • Resulting impact parameter precision will be
  • Compared with, at SLD
  • mm
  • mm
  • really helped by more open geometry, with longer
    lever arm provided by 5 layers compared to 3

Generic long-barrel detector (TESLA TDR)
SiD vertex detector design concept Norm Graf
(No Transcript)
Measuring flavour ID at ILC
TESLA TDR Purity means for quarks from Z0
decays its only one benchmark Case of charm
tag with low light-quark background is
interesting, eg for adding flavour tag to
reconstructed Ws, to enable top polarization
Measuring vertex chargeAt SLD
Population in bins with charge /- 2 is very
sensitive to tiny tracking inefficiencies a
valuable monitor, important for particle-flow
measurement of jet energies In SLD, this study
established that 1.5 apparent tracking
inefficiency was real, not an error in the MC
track multiplicity
SLD Tom Wrights thesis SLAC-R-302 (2002)
SLD measurement of parity violation parameters
Ab and Ac PRL 94 (7 March 2005) 091801
  • Measurement of the branching ratio of the Z0 into
    heavy quarks (PRD, to be published)
  • 1/40 of LEP sample, yet measurement of Rc is by
    itself more precise than the current world
  • -----------------------------------
  • Hope to do much better at ILC. Challenge is to
    associate each PV track with the PV, and each
    SV/TV track with the decay vertex Lowest
    momentum track in b decay chain is 1 GeV/c, so
    multiple scattering is important need
    small-radius, thin-walled beampipe and thin
    detector layers
  • Full Geant 4 description of vertex detector, with
    correct treatment of cluster finding and
    realistic backgrounds is essential. This forms
    part of LCFIs MIPs to Physics project. Hoping
    to broaden the study beyond our collaboration
    discussions Wednesday morning at SLAC
  • Meanwhile, we have some indications using fast
    Monte Carlo SGV

Prospects for vertex charge at ILC
Sonja Hillert, LCFI preliminary study could
get better or worse - dont yet believe these
  • Primary requirement minimise leakage of
    neutral Bs into charged sample
  • Charged sample (40) and part of neutral
    sample (20) will give clean quark sign
  • Similar for charm, but in many cases (low
    background from light quarks) one can push tag
    efficiency by including 1-prongs, which are ideal
    for quark sign selection
  • Sign-selection effic per b or c jet 60. For
    individual W decays, you have one b and possibly
    one c for Z decays, use of both jets leads to
    84 overall effic.
  • For t -gt b c sbar, both b and c jets can be used
    to sign select, again leading to 84 overall
  • Fot t-tbar, up to 4 jets available for sign
    selection need only one good one, 97 overall
    efficiency may be possible
  • Use of charged kaons provided a further boost in
    SLD to sign select charm quarks in c jets or in
    the b decay chain. Any chance for ILC? Bob
    Wilson has done some thinking about this. Also,
    advice from Jerry VaVra would be great.

A vital parameter the beampipe radius
  • In 1981, LEP was hell-bent on a 10 cm RADIUS
  • (Villars workshop 1-7 June 1981)
  • Disappointed, I followed other examples and
    turned to the New World
  • SLC? Whats the beampipe radius?
  • About the size of a drinking straw!

  • Such a time dependence is not inevitable at
    LEP it went the other way!
  • Their R_bp was reduced from 10.6 cm in 1991 to
    5.6 cm in 1995
  • Maybe the ILC machine design will be a balance
    between European conservatism, American
    optimism and Asian realism, hence more stable
  • In Europe, Nick Walker (ECFA workshop,
    Obernai, 1999) promised us a radius of 1.5 cm
    but not a millimetre less!
  • Dont let him forget this promise!

  • Really important for the machine people to
    design the FF system for minimal beampipe radius
    (though they cant of course reduce the pair
  • What if 10 mm Rbp had been possible at LEP and
  • SLD would very probably have measured the B_s
    mixing parameter and we are still waiting for
  • At LEP, had they been able to flavour-tag
    every jet cleanly, would have reached a
    definitive conclusion about a Higgs boson in
    their mass range, with only a handful of events,
    on ZERO background
  • As with any microscope, getting close helps.
    The physics potential has still to be investigated

What vertex detector technology for the ILC?
Groups CAP Birmingham
U PNSensor (Munich) CPCCD Bonn
U Strasbourg U FPCCD DESY Tohoku Gakuin
U HAPS Glasgow U Toyama College of ISIS
edge readout Insubria U Maritime
Tech ISIS distributed readout KEK U of
Mining and MAPS transverse readout Lancaster
U Metallurgy, Krakow MAPS-digital
LBNL Warsaw U SoI Liverpool U Yale
U Macro-pixel/Micro-pixel Mannheim U
U of Hawaii sandwich MPI Munich
(Halbleiterlabor) Nijmegen U NIKHEF
(Amsterdam) Oregon U Oxford U
Both lists are probably incomplete apologies!
Sensor operating principles
3.2 M-shell plasmons per mm (17 eV) 0.6 L-shell
plasmons per mm (120 eV) 0.01 K-shell plasmons
per mm (1.5 keV)
4 primary collisions/mm with wildly fluctuationg
energy loss Final thermalisation yields one e-h
pair per 3.6 eV deposited
(No Transcript)
Minority carrier diffusion length 200
mm ------------------------------ 0.1 mm
What epi-layer thickness? Prefer it thin, to
avoid losing precision for angled tracks But not
too thin, or lose tracking efficiency 20 mm is
about right
  • Imagine p and pmaterial brought into contact
    at same potential
  • Holes pour from p, leaving a negative
    space-charge layer (depletion) and forming a
    positive space charge layer in the p material
  • This space-charge must of course sum to zero,
    but it creates a potential difference, which
    inhibits further diffusion of majority carriers
    from p to p and incidentally inhibits diffusion
    of minority carriers (electrons) from p to p
  • This barrier is thermally generated, but the
    penetration coefficient is temperature
    independent, and is simply the ratio of dopant
    concentrations. eg 0.1/1000, so 10-4 - this
    interface is an almost perfect mirror!

  • We can repeat this on the top surface here
    the p-well can be used to implant structures
    (notably n-channel transistors), monolithic
    with respect to the detector layer below
  • Positively biased n implants (reverse-biased
    diodes) serve to collect the signal charges,
    partly by diffusion, partly by drift in depleted
    regions created in the p-type epi layer
  • Overlaying dielectric layers, and
    photolithographically patterned metal layers
    complete the toolkit for interconnecting the
  • Here you have the essentials of a MAPS pixel

  • Another variant same below, but positively
    biased n-layer above, creating a full-area
    depletion region.
  • Signal charge is pulled to a potential energy
    minimum buried channel about 1 mm below the
  • Metal or polysilicon gates control the
    location of stored charge in 1-D. Channel stop
    implants create orthogonal potential barriers
  • Signal charge packets can be transported in
    turn to an output node, similar to that already
    seen on the MAPS device.
  • This is a charge-coupled device or CCD
  • To learn about all the beautiful options for
    ILC vertex detectors, read the slides from
    Session H last Saturday morning (12 talks)

How not to be blown away by 109 pixels
  • (a) Electromagnetic interference
  • Electron/positron beams traversing the IR
    radiate massive RF power (wakefields)
  • Numerous imperfections (thin walled sintered
    Be beampipe, non-welded flanges, ports for
    BPMs,kicker magnet circuits, beam-size monitors,
    vac pumps, ) provide leakage paths for RF
  • A linear collider is intrinsically more hostile
    in terms of beam-induced RF than storage rings
  • The vertex detector (in which 109 unamplified
    signal charges of 1000 e- are transformed to
    voltage on the gates of tiny transistors within
    1 mm of the beampipe) is more liable to
    disturbance by this RF than most detector systems
  • Beam-induced pickup disrupted the SLD vertex
    detector electronics for several ms after each
  • Dangerous to assume it will be quieter at a
    machine with 10 times the energy and 104 times
    the luminosity of SLC, needing far more
    instrumentation to preserve its performance
  • Problems may not be primarily related to RF
    from the beam control systems, such as kicker
    magnet pulsing circuits active during the train
    may also be dangerous

Typical example ideal CCD
Reality, during the bunch train
From SLD experience, signal charges stored in
buried channel are virtually immune to
disturbance by pickup. They were transferred in
turn to the output node and sensed as voltages
between bunches, when the RF had completely died
away Could this also be done at ILC?
  • Bunch train 2820 bunches _at_ 337 ns, every 200 ms
  • If signals were accumulated throughout a bunch
    train, background would be totally unacceptable
    (except maybe for FPCCD of GLC Group)
  • Seemingly need to read the detector 20 times
    during train, at 50 ms intervals
  • This may be like trying to hear someone whisper
    on a railway platform while an express train is
    roaring by. Why not simply wait?
  • All detector options considered till recently
    suffered from this problem
  • After ECFA workshop in Montpellier, November
    2003, we came up with a new idea
  • David Burt of e2V told us Its been done!
    Even better!

ISIS Imaging Sensor with In-situ Storage
  • Pioneered by W F Kosonocky et al IEEE SSCC
    1996, Digest of Technical Papers, p 182
  • Current status T Goji Etoh et al, IEEE ED 50
    (2003) 144
  • Frame-burst camera operating up to 1 Mfps,
    seen here cruising along at a mere 100 kfps
    dart bursting a balloon
  • Evolution from 4500 fps sensor developed in
    1991, which became the de facto standard high
    speed camera (Kodak HS4540 and Photron FASTCAM)
  • International ISIS collaboration now
    considering evolution to 107 108 fps version!

  • charge collection to photogate from 20 mm
    silicon, as in a conventional CCD
  • signal charge shifted into storage register
    every 50ms, to provide required time slicing
  • string of signal charges is stored during bunch
    train in a buried channel, avoiding
    charge-voltage conversion
  • totally noise-free charge storage, ready for
    readout in 200 ms of calm conditions between
  • The literature is littered with failed

  • Correlated Double Sampling
  • CDS is a vital part of the strategy to avoid a
    data deluge, one which most technologies for the
    ILC vertex detector claim to employ
  • Even if reading between trains, fluctuations in
    transistor noise and detector-related pickup
    sources will be present, as seen at SLD
  • CDS - term given in early 70s to pedestal
    subtraction in CCDs used in astronomy and
    elsewhere to sense very small signals, where
    reset noise and 1/f noise would otherwise have
    imposed severe performance restrictions
  • Same functionality was achieved in late 70s in
    CCD-based particle detectors, where the sparse
    data permitted resetting only between rows, hence
    faster sampling
  • ERF - Extended Row Filter, was an important
    refinement added in SLD
  • Beware of imitations

(No Transcript)
Extended Row Filter (ERF) suppresses residual
noise and pickup
SLD experience
Without ERF, rate of trigger pixels would have
deluged the DAQ system
Read out at 5 MHz, during quiet inter-bunch
periods of 8 ms duration Origin of the pickup
spikes? We have no idea, but not surprising given
the electronic activity, reading out other
detectors, etc
How can we get to ILC physics, from where we are
  • Currently many groups are pursuing an expanding
    range of options for the ILC vertex detector
  • All use silicon pixels, but there the similarity
  • How to converge on (hopefully) two technologies
    for two large detectors?
  • The ILC vertex detector community has informally
    undertaken to provide working ladders in test
    beams, circa 2010 ( d)
  • Overall detector collaborations should evaluate
    their results carefully, considering all
    performance criteria, including efficiency,
    spatial resolution, material budget at small
    angles due to mechanical supports and electronics
    at ends of ladders, robustness wrt EMI, etc

Dont believe what any of us claim we can deliver!
  • Only then should they make their technology
    choices, deciding between long barrels, short
    barrels plus endcaps, etc etc
  • Everyone who has participated in the RD should
    be welcome to join one of the winning
    technologies for detailed design, construction,
    commissioning, and extracting the physics
    (ideally, a rerun in miniature of the ITRP
  • Several losers will change direction and
    find wonderful applications for their
    technologies, and may gain more than they lose!
  • In the meantime, we should continue building a
    world-wide community who would all enjoy working
  • I suggest we increase the frequency of our
    inexpensive world-wide phone conferences to maybe
    twice a year
  • Arlington TX 8 Jan 2003, Mumbai 14 Dec 2003
  • Overall detector collaborations should design
    for convenient access to the vertex and other
    small-radius equipment for several reasons,
    including future upgrades. A losing
    technology, could prove to be a long-term winner
    (as happened with CCDs at SLC)
  • The SLD vertex detector was considered a jewel
    in the crown. This may also be true at ILC
    the physics potential, combined with particle
    flow for jet energy measurement, goes far beyond
    what can be done at LHC

  • Can we be sure that silicon pixels will provide
    the best solution?
  • Do you remember the front running technology
    for SLC in 1982?
  • CCDs were regarded as a risky outsider.
  • If you come up with a revolutionary new idea,
    please do follow it up! Dont be discouraged by
    the so-called experts!
  • In the 70s, when most expertise in silicon
    detectors was in the hands of nuclear science
    people with string-and-sealing wax production
    facilities, the construction of semiconductor
    detectors was more like cookery than science
  • To these experts, the concept of CCDs as particle
    detectors seemed outrageous

SLC Experiments Workshop 1982
delicate device in a beam and you will ruin
it". "Will work if you collect holes, not
electrons". "Far too slow to be useful in an
experiment". "It's already been tried didn't
work". "It will work but only with 50
efficiency". "To succeed, you will have to learn
to custom-build your own CCDs investment
millions". "At room temp it would be easy, but
given the need to run cold, the cryogenic
problems will be insurmountable". "May work in a
lab, but the tiny signals will be lost in the
noise (RF pickup etc) in an accelerator
environment". However, Wrangy Kandiah from AERE,
Veljko Radeka and Pavel Rehak from BNL, Joe
Killiany from NRL, Herb Gursky from Harvard
Smithsonian, Emilio Gatti from Milano and a few
others were supportive