30 July 2003 Daresbury LC Opportunities – Chris Damerell1

Connections between Vertex Detector and Beam Delivery System

Chris Damerell

30 July 2003

Topics:

Beampipe radius and thickness

Beam-associated RF pickup

Radiation backgrounds

Access to inner detector region

‘Final Focus Lab’

Conclusions – the way forward

30 July 2003 Daresbury LC Opportunities – Chris Damerell2

Beampipe radius and thickness

Rbp = 12-15 mm (NLC, TESLA)

30 July 2003 Daresbury LC Opportunities – Chris Damerell3

Beampipe radius important where low mom trks need to be well measured (charm tag, B vertex charge)

Difficult to quantify in context of future TeV-scale physics, but there are numerous historical examples:

UA1 ‘top’ at 40 GeV (Proc. SLAC Summer Institute, 1984, p 45)

LEP Higgs

SLD Bs mixing

4 layer, dble5 layer

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Agreed that VX radius is, in certain extents, a free parameter that accelerator physicists can optimize

Andrei Seryi and John Jaros at Cornell workshop 14 July:

Based on conclusions of Chao study from 1.5 years ago …

Meanwhile, studies of Xella, de Groot, Wing and Kuhl reported in numerous workshops

One cannot argue that luminosity or backgrounds should be compromised

Collimator wakefields are

(A Seryi) so the large-L* option has some negative aspects

If large radius beampipe is price of this FF design, should at least be considered carefully

Aim to avoid a design path for which Rbp expands exponentially till startup …

2*L

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(a) difference between ‘startup’ and ‘physics’ beampipes ‘like night and day’

Dec 2, 1988 SLD Adv Gp discussed 16, 18 and 25 mm options

SLC backgrounds proved to be at or beyond the acceptable limit for the SLD drift chamber, so 25 mm was a wise choice

However, the loss of Bs mixing was a high price, as was the loss of a possible Higgs signal at LEP

When SLD was designed, even the synergy between vertexing and beam polarisation was generally dismissed …

How will all this play out at the future LC?

Detector was re-designed, starting December 1988

With RBP = 25 mm, it was constructed and installed within the following 3 yrs

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stabilise delicate inner section of beampipe with robust support shell of vertex detector

particularly important during transportation/installation of ‘R20 module’ in detector

will permit inner section of beampipe wall thickness of < 0.5 mm Be

How about the wall thickness? – coupled to the radius

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Beam-associated RF pickup

SLD experience

tiny signal charges stored safely in 307 million potential wells

DAQ system (PLL loop in optical converter) dislocated by every bunch

EM radiation was not leaking through steel/beryllium; believed associated with ceramic feedthroughs and imperfectly shielded coax cables

Warm machine

As at SLC, can afford to reset electronics after each bunch train

minor incursion into 8 ms DAQ period

low residual pickup during readout can be filtered out, as at SLD, by Correlated Double Sampling logic (q.v.)

this works cleanly for CCD and DPEFET option, not so for HAPS or MAPS. But possible add-ons …

Cold machine

must be actively reading throughout bunch train

150 BX per frame of layer-1

pickup immunity a major issue; must be tested in realistic conditions

30 July 2003 Daresbury LC Opportunities – Chris Damerell8

Correlated double sampling

• CDS is the term invented circa 1972 for the form of pedestal subtraction used to suppress reset noise in CCD front-end circuits

• Simplest CDS involves:

Reset sensing capacitor***measure V-out***transfer signal charge***re-measure V-out

Used to reduce the system noise from tens of e- to ~1 e- by suppressing the fluctuations in post-reset V-out

• DEPFET shares robust CDS capability with CCD, in LC application:

Read pedestal+signal***reset – ie remove signal Q***read pedestal alone

• However, MAPS CDS involves progressive signal integration over full frame period of 50 s or whatever, cf 20 ns for CCD CDS

• Problem could in principle be solved by incorporating 1-pixel CCD, or DEPFET structure, within the CMOS pixel

• CDS with = 50 s?

• SLD was OK in inter-train period with 200 s CDS sampling period, and ERF (q.v.)

• might get away with it at NLC, after some settling time

• might not work at TESLA due to RF activity within train

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Extended row filter, SLD

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Effectiveness of ERF in suppressing ‘noise’ hits (including pickup in operational conditions in SLD barrel between bunch trains)

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Radiation backgrounds

Estimates were stable for some years:

50 krad/year (e+e- pairs)

109 neutrons/cm2/year

these figures would probably be acceptable for all VTX options, with the caveat that R&D on radiation effects in pixel detectors is rudimentary, and there are still surprises (Olga Igonkina results reported at Cornell)

For gamma-gamma option, a bombshell reported at Cornell:

electron beams are highly disrupted by lasers

beams with large energy spread can’t be dumped cleanly

inner two layers of VTX have line-of-sight to main beam dump

see 1011 neutrons/cm2/year !!

may leave only the HAPS option standing, severely compromising LC heavy flavour physics for gamma-gamma

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NOTE: Lower neutron rate at IR than LHC

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Access to inner detector region

How to access inner detector system?? Bill Ash as late as 1989 swept away a dreadful plan with a brilliant new idea …

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Procedure agreed also for TESLA (and NLC detectors), despite initial concerns from Ron Settles – remember, LC tracker will be ~ 6 m long

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For the upgrade VXD2-VXD3, we removed a ton of cables

For LC VTX, will also eliminate all inner electronics – endcap tracking should be beautiful at last

Access to the ‘R20’ region will remain essential

SLD procedure is now universally (?) accepted

Entire operation took 2 months,

late October-December 1991

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‘Final Focus Lab’

Suggest ‘no beam’ FF lab somewhere in the world (eg Daresbury)

will have an unprecedented density of high tech instrumentation

800 million channels of some silicon pixel technology, read every 50 s during the bunch train (TESLA) may be non-trivial!

more generally, issues of mechanical (eg vibrational), thermal and electromagnetic interference

currents on wires in beampipe won’t generate the highest frequency RF, but probably enough (Marty B, Marc R, Clive Field, Jerry VaVra)

ceramic feedthroughs, imperfectly screened coax, are main RF sources identified so far. These may be more distant in LC, but there are many other factors, like warm or cold machine, FONT or not, …

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Synergy with other science

• Pixel detectors are uniquely inter-disciplinary

• Example from ‘fall of the wall’ in structural biology (J Hajdu, TESLA colloquium)

• 120 Hz frame rate needed at LCLS (with 14 bit dynamic range)

• SNAP (600 Mpixels), XEUS, biological cell imaging (CPCCD mentioned 4 times in London meeting on 24 June, …

• Fast Gigapixel-scale imaging systems are widely needed for science, and the LC vertex detector community is making a strong contribution to their development

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Conclusions – the way forward

BDS design should help build the platform for unique heavy flavour physics capability at the LC (hence SUSY, extra dimensions, bosonic supersymmetry, etc)

BDS and VTX systems are ‘joined at the hip’, hence each can help the other, or render the other inoperable …

Historical precedents for both cases exist

don’t dismiss each problem with the escape clause:

“One can still use [one of CCDs/MAPS/HAPS/diamond detectors/…]”

some may be only dreams; all have their Achilles heels. Be careful not to get painted into a corner. The ‘paradigm shift’ from strips to pixels is in its infancy – limited experience with pixel-based vertex detectors is worrying!

Main issues/risks:

beampipe radius

radiation background, notably neutrons

pickup immunity (a major headache if TESLA)

Avoiding the inferno at heart of LHC gives the LC a major physics advantage (or complementary reach)

We should not erode this by an unbalanced design strategy – physics was lost at LEP and SLD through insufficient control of small-radius backgrounds

These lessons should have been learned. By working closely together, the BDS and VTX communities can prepare for stunning physics discoveries at the LC

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Neil Calder at Cornell workshop: supply the best 60 second answer.