Sonja Hillert, University of Oxford PPRP open session: LCFI, London, 8 th September 2004 p. 0 Linear Collider Flavour Identification (LCFI) - Part 1 -

Sonja Hillert, University of Oxford PPRP open session: LCFI, London, 8th September 2004 p. 1

Linear Collider Flavour Identification (LCFI) - Part 1 -

S. Hillert (Oxford) on behalf of the LCFI collaboration

Bristol U, Lancaster U, Liverpool U, Oxford U, RAL

PPRP open session, London, 8th September 2004

Overview

Physics Studies

Thin Ladder R & D


Introduction There is consensus in the High Energy Physics community that a TeV

scale e+e- linear collider (LC) has the first priority for the next major particle

accelerator, to operate with significant overlap with the LHC:

• “Reviews … point … to the conclusion that there is fundamentally new physics in the

energy range just beyond the reach of existing colliders.” (ICFA statement ’99)

• “The LC will extend the discoveries [to be made at the LHC] and provide a

wealth of measurements that are essential for giving a deeper understanding of

their meaning” (LC consensus document, 2004)

With the decision on the accelerator technology announced on 20 August,

world wide R&D effort will increase in speed and international collaboration

will intensify to reach a final design of the accelerator and the detectors.

“This is an extremely significant milestone. …

The UK should take a leading role in this one-off, global machine” (Ian Halliday)


The detector at the ILC

Compared to physics at the LHC, events at the ILC will be much cleaner;

much lower rates and background, known initial state;

Combining information from different subdetectors, we attempt to fully

understand the basic physics process on an event by event basis.

Requirements:

• continual, triggerless readout

• hermeticity

• highly granular tracking and calorimetry,

both inside a coil providing a high B-field

to resolve jets in multijet topologies

• vertex resolution for flavour identification


Application of a

particle flow algorithm

permits resolution of

events into the

jets corresponding to

the underlying quarks.

Use of the vertex detector permits us to distinguish the jets

generated by heavy quarks.

An example: e+e- t t

a typical e+e- t t event:

b

e+ e-

t

t

W

W+b

e.g. cq’ q’

s

qqe.g. s c


Vertex detector contribution to event reconstruction

The most interesting new processes (Higgs, SUSY, …) will be rich in heavy quarks.

Vertex topology and

effective mass of decay products

allows us to distinguish between

b and c jets.

Vertex charge allows us to

distinguish between quark and

anti-quark: b and b or c and c.


The LCFI R&D program

The linear collider flavour identification (LCFI) collaboration formed in 1998.

Since then we have carried out an extremely successful R&D program, aimed at finding viable

solutions for building a vertex detector whatever the machine choice would turn out to be.

The prototypes of sensors and readout chips developed by the end of the current funding

period would already have covered the major design specifications required by the warm

technology.

For the cold option, now chosen, we have developed two baseline designs, one of which

only emerged end of last year (cf. talk by Konstantin Stefanov).

The evaluation of which of these will be better matched to the requirements will need

further intensive R&D in close collaboration with international partners in academic

institutes and industry.

The LCFI program covers three closely connected areas of R&D:

physics studies, thin ladder R&D and detector development. The remainder of the

presentation will summarise our progress and future plans in each of these fields.


LCFI Physics studies

base, from which R&D goals are defined:

• How far do detector parameters like pixel size, ladder thickness, beam pipe radius,

readout time etc. need to be pushed for the measurements planned at the LC?

• What performance do the parameters which are technically achievable yield?

SLD experience: vertex detector is a powerful tool, crucial for LC physics goals;

besides b tagging it will allow

• high purity charm tagging (cf e.g. ICHEP’04 contribution 12- 0438)

provides a handle to unique physics in the TeV regime, complementary to LHC,

e.g. precision measurement of branching ratios in Higgs decays

• via vertex charge reconstruction: distinguishing between b and b, c and c

suppression of combinatorial background in multi-jet events

asymmetries: parity of Higgs boson;

CP asymmetries in SUSY processes

Bristol U

Lancaster U

Oxford U

RAL


Detector dependence of vertex charge reconstruction

“standard detector” characterised by: good angular coverage (cos = 0.96)

proximity to IP, large lever arm:

5 layers, radii from 15 mm to 60 mm

minimal layer thickness ( 0.064 % X0 )

to minimise multiple scattering

excellent point resolution (3.5 m)

standard detector is compared to

degraded detector: beam pipe radius 25 mm, 4 layers only; factor 2 worse point resolution

improved detector: factor 4 less material, factor 2 better point resolution

Vertex charge reconstruction studied in at ,

select two-jet events


Definition of vertex charge and of Pt-corrected mass

need to find all stable B decay chain tracks – procedure:

run vertex finder ZVTOP: the vertex furthest away

from the IP (‘seed’) allows to define a vertex axis

reduce number of degrees of freedom

cut on L/D, optimised for each

detector configuration, used to

assign tracks to the B decay chain

by summing over these tracks obtain

Qsum (charge), PTvtx (transverse momentum), Mvtx (mass)

vertex charge

Pt-corrected mass used as b-tag parameter


Vertex charge: results

conclusions:

purity flat out to efficiency of ~ 70%

for standard detector

significant detector dependence:

at b = 70% (MPt > 2.0 GeV):

b = 6%, (b) = 2%

result underlines the need for a small beam pipe radius,

previously indicated by impact parameter resolution ( LCWS ’04 result)

presented at Durham International LC Workshop 1- 4 September


Further improvement of tools

objected oriented frameworks, notably JAS3, will replace BRAHMS in the

medium term, but have not yet reached the maturity required to work on the questions

that need to be answered now

in the meantime, use fast MC program SGV (Simulation a Grande Vitesse):

• well-tested, flexible code

• includes ZVTOP package

complete, flexible neural network package has recently been developed at Bristol:

• improve on vertex charge result (e.g. by using a reconstruction similar

to an optimised procedure developed for SLD)

• extend studies to different energies and to flavour tagging;

results from this new tool expected soon


Physics studies: future plans

The physics studies are currently gaining momentum.

They are essential for the decisive phase of our R&D program which we

entered with the decision on the accelerator technology:

• Emphasis will shift from an idealised optimisation of the vertex detector alone

to the evaluation of tradeoffs between parameters of different subdetectors

as well as the accelerator.

• Different aspects of the global physics needs, such as hermeticity of the

forward calorimetry and degraded jet-energy resolution due to conversions

in vertexing and tracking detectors, will require compromises, for which

input from a vertexing point of view will be needed.

When joining one of the LC protocollaborations, LCFI is well suited to contribute

not only the vertex-detector system, but also the expertise needed to extract

from its data the physics, for which that detector is crucial.


Thin ladder development

concentric barrels of the vertex detector

consist of ladders comprising

• 1-2 CCDs

• substrate for mechanical support

• readout chips to process and

sparsify the data

ladders attached to Be support shell

requirements: little material ( ~ 0.1 % X0),

positional stability

initial idea: unsupported silicon under tension;

tests on thinned processed silicon showed bowing across

the width of the ladder not considered any longer

Bristol U

Oxford U

RAL

e2V


Semi-supported silicon: introduction

silicon, attached by adhesive pads to thin substrate, e.g. Beryllium,

stabilised by tension

difference in expansion coefficient between silicon and substrate can have

serious consequences

studied both by FEA and by measurements on physical models

LCFI developed purpose-built

laser ranging device (left),

allowing rapid ladder scans

at micron precision;

setup enclosed in cryostat


Semi-supported silicon: results

example: 30 m Si, 250 m Beryllium, 200 m thick glue pillars (silicone elastomer):

good qualitative agreement between measurement (left) and FEA simulation (right)

below silicon thickness of ~ 50 m, compressive load from Be substrate causes strong

buckling interest in other substrate materials: carbon fibre composites, ceramics, foams


The thin ladder development is an inexpensive, but complex field of R&D.

The effects of internal stresses in processed silicon require further investigation:

• Studies of a silicon-sellotape assembly modelling these effects are underway.

• Samples of large area thin CCDs supplied by e2V, will allow further measurements.

Linked to studies of these effects are open questions regarding attachment to

substrate. By the time the detector is going to be built, it may be possible to replace

the pads of adhesive by advanced microstructures, which keep the silicon under tension:

In the longer term, studies will expand from the central part of ladders to the

ladder ends. This topic will be closely connected to the development of drive electronics

to fit on the ladder end, linked by requirements on the material budget and on cooling.

Thin ladders: future plans


study based on single pions, generated using SGV

Impact parameter resolution

impact parameter in R at track perigee

increasing material budget has moderate effect, but

performance strongly suffers when beam-pipe radius is increased

detector geometries:

standard detector: 5 layers

(each 0.064 % X0)

at radii 15 mm to 60 mm

double layer thickness

beam-pipe with Ti-liner (0.07 % X0)

4 layers at radii 25 mm to 60 mm

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Definition of L/D

seed vertex (ZVTOP vertex candidate

furthest from IP) used to define the

vertex axis

consider all tracks initially passed to

ZVTOP and assign those to B decay

chain, which at point of closest

approach to the vertex axis have

• T < 1 mm: cleaning cut, only small

effect

• (L/D)min < L/D < 2.5: main cut,

optimised for each detector

configuration independently

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Improvement since LCWS – 1

MC: B _

MC: B+

MC: neutral

B hadrons

comparison of reconstructed Qsum distributions for the different generator level charges

LCWS new

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Documents

Sonja Hillert, University of Oxford PPRP open session: LCFI, London, 8 th September 2004 p. 0 Linear Collider Flavour Identification (LCFI) - Part 1 -