21 March 2005LCWS 2005 – Chris Damerell 1 Vertex Detectors and the Linear Collider Chris Damerell...

21 March 2005 LCWS 2005 – Chris Damerell1

Vertex Detectors and the Linear Collider

Chris DamerellRutherford 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?


Dave Burke, LCWS 1991 Particle flow almost reveals the underlying Feynman diagram …

What are vertex detectors for?


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

2 2

2 f ff

f f

v aA

v a


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 case’

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


e+e- 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|

at max e e qq s


For these channels, sensitivity extends to MD around 5 TeVFor muon pairs, effects are much weaker (not measurable even with 1 ab-1 of data)

Sabine Riemann: one example for large ED, sqrt(s) = 0.5 TeV

e e bb

e e cc


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) 075004

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 apparent

Adding b-bbar and c-cbar will increases statistics and provide additional physics sensitivity (soon to be studied by G Moortgat-Pick)

0 02 1 l l


Momenta in e+e- 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)

02

0 01 2e e

1 1

1 1

(cos 0) (cos 0)

(cos 0) (cos 0)FBA


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 mixingproperties of

0 01 2e e

0 02 1 e e

Le

0 01 2 and

0 01 2( ) 71 GeV and ( ) 130 GeVm m


Top quark polarisation – a general tool for physics

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 doesn’t need to be super-efficient)


c

sbar

t

W+

b

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 and the trilinear couplings At and Ab [E Boos et al, hep-ph/0303110]

1t

01


e e tt

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

• tbR W+ decay (forbidden in V-A theory)

• anomalous t-tbar-Z couplings

Michael Peskin LC lectures, March 2002 and TESLA TDR III-146


e e tt

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 amplitudes

Spin of these resonances can be probed by measurement of top quark polarisation (again, by angular distributions of s-quark jets)

W W tt

and e e tt tt


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 )

----------------------------------------------

0e e Z HH

01 ff

.( )q f qp p p


benchmark for charm tag (A Sopczak, A Finch, H Nowak LCWS 2004) Particularly challenging for small m between neutralino and stop masses; theoretically motivated: talk in SUSY session by Caroline Milstene, and Jonathan Feng’s colloquium

The totally unexpected [what we are all hoping for]

‘The universe is not only queerer than we suppose, but it is queerer than we can suppose.’ JBS Haldane

0

0

e e tt

c

c


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

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

NA32, 1985


SLD, thanks to Su Dong



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 m of Si is 0.02% X0]

Point measurement precision at least as good as at SLD (approx 3 m)

Resulting impact parameter precision will be m

Compared with, at SLD:

m

m

[really helped by more open geometry, with longer lever arm provided by 5 layers compared to 3]

3 / 23.9 7.8 /( sin )r rz p

3 / 27.7 33 /( sin )r p

3 / 29.6 33 /( sin )rz p


Generic long-barrel detector (TESLA TDR)


SiD vertex detector design concept – Norm Graf



TESLA TDR‘Purity’ means for quarks from Z0 decays – it’s only one benchmarkCase of charm tag with low light-quark background is interesting, eg for adding flavour tag to reconstructed Ws, to

enable top polarization measurement

Measuring flavour ID at ILC


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 Wright’s thesis SLAC-R-302 (2002)

Measuring vertex chargeAt SLD


SLD –measurement of parity violation parameters Ab and Ac PRL 94 (7 March 2005) 091801

eL

eL

eR

eR

e e bb

e e cc


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 average-----------------------------------

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 LCFI’s ‘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 - don’t yet believe these figures!


Primary requirement – minimise ‘leakage’ of neutral Bs into charged sample

Charged sample (~40%) and part of neutral sample (~20%) will give clean quark sign selection

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 -> b c sbar, both b and c jets can be used to sign select, again leading to ~84% overall effic.

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.]


In 1981, LEP was ‘hell-bent’ on a 10 cm RADIUS beampipe … (Villars workshop 1-7 June 1981)

Disappointed, I followed other examples and turned to the New World

‘SLC? What’s the beampipe radius?’

‘About the size of a drinking straw!’

A vital parameter – the beampipe radius


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! Don’t let him forget this promise!


Really important for the machine people to design the FF system for minimal beampipe radius (though they can’t of course reduce the pair background)

What if 10 mm Rbp had been possible at LEP and SLC?

SLD would very probably have measured the B_s mixing parameter – and we are still waiting for that …

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?

Technologies GroupsCAP Birmingham U PNSensor (Munich)CPCCD Bonn U RALDEPFET Bristol U SLACFAPS Brunel U Strasbourg UFPCCD DESY Tohoku Gakuin UHAPS Glasgow U Toyama College of ISIS – edge readout Insubria U Maritime TechISIS – distributed readout KEK U of Mining and MAPS – transverse readout Lancaster U Metallurgy, KrakowMAPS-digital LBNL Warsaw USoI Liverpool U Yale UMacro-pixel/Micro-pixel Mannheim U U of Hawaiisandwich MPI Munich (Halbleiterlabor)

Nijmegen UNIKHEF (Amsterdam)Oregon UOxford U

Both lists are probably incomplete – apologies!


3.2 M-shell plasmons per m (17 eV)

0.6 L-shell plasmons per m (120 eV)

0.01 K-shell plasmons per m (1.5 keV)

~4 primary collisionsm with wildly fluctuationg energy loss

Final thermalisation yields one e-h pair per 3.6 eV deposited

Sensor operating principles



Imagine p and p+material 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 (accumulation)

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!

Minority carrier diffusion length

~ 200 m

------------------------------

~ 0.1 m

What epi-layer thickness?

Prefer it thin, to avoid losing precision for angled tracks

But not too thin, or lose tracking efficiency

20 m is ‘about right’


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 circuit

Here you have the essentials of a MAPS pixel detector


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 m below the surface

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 s after each bunch

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 @ 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 s 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 ‘It’s been done!’ Even better!


• 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!

ISIS: Imaging Sensor with In-situ Storage


• charge collection to photogate from ~20 m silicon, as in a conventional CCD

• signal charge shifted into storage register every 50s, 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 trains

• ‘The literature is littered with failed attempts …’


(b) 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’

/ 2nENC kTC



Extended Row Filter (ERF) suppresses residual noise and pickup:


SLD experience:

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

Without ERF, rate of trigger pixels would have deluged the DAQ system


How can we get to ILC physics, from where we are now?

Currently many groups are pursuing an expanding range of options for the ILC vertex detector

All use silicon pixels, but there the similarity ends

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 (+

• 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

Don’t 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 R&D 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 process)

• [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 together

• 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! Don’t 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


SOME EXPERT OPINIONS IN 1979

"Put such a 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

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21 March 2005LCWS 2005 – Chris Damerell 1 Vertex Detectors and the Linear Collider Chris Damerell...

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