Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 0 Vertex Detector Contribution to ILC Physics Analyses

Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 1

Vertex Detector Contribution

to ILC Physics Analyses

Sonja Hillert (Oxford)

on behalf of the LCFI collaboration

3rd ECFA workshop on Physics and Detectors at the Linear Collider

Vienna, 14 – 17 November 2005

Introduction

the ZVTOP vertex finder

flavour tag

vertex charge reconstruction

the vertex package under development by LCFI


SM

e+ e-

t

t

W

W+

q q’

q’q

b

b

e.g. c s

e.g. s c

Questions the vertex detector can help answer

a typical e+e- t t event:without quark sign selection with perfect quark sign selection

Which type of event is one looking at?

identify quark flavours of all jets involved

Are jets stemming from quarks or antiquarks?

quark sign selection useful for e.g. studying

top polarisation, unfolding cross sections,

reducing combinatorial background

S. Riemann LC-TH-2001-007


Vertex detector input to physics analyses

In the event reconstruction, tracks are found from hits in the vertex detector,

the intermediate and the outer (barrel + forward) tracker.

Particle flow algorithms combine tracking and calorimeter information and

yield the four-momenta that form the basis for jet-finding.

Using track information on a jet by jet basis, the vertex detector provides:

- vertex finding: identify subsets of tracks

inside a jet, which are then fit,

yielding vertex position, momentum, mass, fit-2

- based on these and other quantities further obtain:

• flavour-tag: distinguish between b-, c- and light-flavour, gluon jets

• quark sign selection: distinguish between b and b, c and c


The ZVTOP vertex finder

two branches: ZVRES and ZVKIN (also known as ghost track algorithm)

The ZVRES algorithm:

tracks approximated as Gaussian ´probability tubes´

from these, a ´vertex function´ is obtained:

3D-space searched for maxima in the vertex function that satisfy

resolubility criterion; track can be contained in > 1 candidate vertex

iterative cuts on 2 of vertex fit and maximisation of vertex

function results in unambiguous assignment of tracks to vertices

has been shown to work in various environments differing in

energy range, detectors used and physics extracted

very general algorithm that can cope with arbitrary multi-prong decay topologies

D. Jackson,

NIM A 388 (1997) 247


The ZVKIN (ghost track) algorithm

more specialised algorithm to extend coverage to b-jets in which one or both

secondary and tertiary vertex are 1-pronged and / or in which the B is very

short-lived;

algorithm relies on the fact that IP, B- and D-decay vertex lie on an approximately

straight line due to the boost of the B hadron

should improve flavour tagging capabilities

ZVRES

GHOST

SLD VXD3 bb-MC


Flavour tag aim: distinguish between b-jets, c- jets and light-quark / gluon jets

heavy flavour jets contain secondary decays, generally observed as secondary vertices

if secondary vertex is found variables with highest separation power are

Pt-corrected vertex mass and vertex momentum

Vertex Mass (GeV/c2)

uds c b

SLD


Neural net based flavour tagging procedureTo obtain even better purities of the resulting flavour-tagged jet-samples, further

variables are added to the procedure and all inputs combined by using neural nets.

Example: procedure developed by R. Hawkings et al. (LC-PHSM-2000-021)

and recently used by K. Desch / Th. Kuhl (LC-note in preparation)

NN-input variables used:

• if secondary vertex found: MPt , momentum

of secondary vertex, and its decay length and

decay length significance

additionally (also in case of only primary vertex found):

• momentum and impact parameter significance in

R- and z for the two most-significant tracks in the jet

• joint probability in R- and z (estimator of

probability for all tracks to originate from primary vertex)


Vertex charge reconstruction

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

probability of mis-reconstructing vertex charge is small for both charged and neutral cases

neutral vertices require ‘charge dipole’ procedure from SLD still to be developed for ILC


Energy and polar angle dependence

impurity is given by probability 0

of reconstructing neutral B hadron

as charged

degradation at large cos :

multiple scattering of oblique

tracks in detector material,

loss of tracks at detector edge

effects more strongly seen at

lower jet energy

(more low momentum tracks

multiple scattering more severe)

results obtained with SGV fast MC


Comparing detectors with different beam pipe radius

Compare 3 detectors with different inner layer radius:

25 mm8 mm

250 mm

15 mm60 mm

100 mm

standard detector: large Rbp detector: small Rbp detector:

Rbp = 15 mm, thickness 0.4 mm

innermost layer at 15.5 mm;

layer thickness 0.1 % X0

(same for all detectors)

Rbp = 25 mm, thickness 1 mm

innermost layer removed

new inner layer at 25.5mm

has full length of 250 mm

Rbp = 8 mm, thickness 0.4 mm

innermost layer moved inwards

to 8.5 mm, positions of other

layers retained

Note that the beam pipe has to be made thicker if its radius is increased


Results obtained for different beam pipe radii

detectors with different radii

of the innermost layer differ

in performance over entire

jet energy range

differences most pronounced

at lower jet energies


Translation into effective luminosity

consider multi-jet final state channel, in which

vertex charge needs to be found for two jets

of similar energy (e.g. 50 GeV each for decay

products of 2 Higgses in ZHH)

red curve shows factor by which integrated

luminosity would have to change to obtain

result of same statistical significance, if the

inner layer radius were changed from its

baseline value of 15 mm

note: plot to be updated using more realistic

procedure based on jet-weighting according to their 0 value

for EJet = 25 GeV, 2-jet luminosity factor of 2.14 (shown in plot) is reduced to 1.65 – 1.85,

depending on assumed background


Vertex-package under development by LCFI

release planned in mid 2006

charge dipole

use of additional

information

(e.g. particle ID)

later extensions

track & jet parameters (LCIO format)

vertex quantities, flavour tag,

vertex charge (LCIO format)

neural net ZVTOP

(ZVRES & ZVKIN)

NN-based

flavour tag

quark-sign selection

for charged vertices

first release


Dependence on quality of the input Vertex tools are only as good as the track / jet input fed into them!

current studies / code development based on perfect track finding (using ´MC truth´) and

realistic track fitting parameters as provided by fast MC ´Simulation a Grande Vitesse´ (SGV)

SGV studies indicate sensitivity to lower

limit for reconstruction of track momentum

worth pushing track reconstruction

down to |p| > 0.1 GeV if possible


Towards inclusion of vertex package

in full MC and reconstruction framework

initial tests to be performed using SGV:

• C++ version of ZVTOP code to be tested against expected performance (ongoing)

and compared to results obtained from SLD version

• in parallel test Hawkings flavour tag within SGV

then interface vertex package to LCIO; perform further tests, continuing beyond

time of code release:

• comparison of performance using full MC and reconstruction to SGV results in 2 steps:

1) full MC, but use ´cheaters´ for reconstruction

2) replace cheaters with full track reconstruction input

• to interpret results of step 2) would be good if developers of track reconstruction code

could provide study of track reconstruction over full momentum and full polar angle range,

including tracks originating outside the vertex detector


Areas for later upgrades I

Components of the vertex package are likely to need further optimisation

and extension after release, e.g.

• choice of input variables for flavour tag neural net

• quark sign selection: reassess reconstruction procedure once c-jets

are included,

extend to neutral vertices (charge dipole)

• where available from particle-ID and ECAL, take additional information

(e.g. from Kaons, leptons) into account


Areas for later upgrades II

also interplay between components and between vertex package and global

event reconstruction will need to be explored and could lead to further improvement, e.g.

• improvement of flavour tag using information from ghost track algorithm

• use of vertex information to improve track-jet-assignment

to some extent, optimal procedure will depend on the physics channel studied:

• vertex package will provide enough flexibility to allow user to tune e.g. which inputs are

used for flavour tagging;

• this flexibility should be used, in a similar way as one would tune a jet-finder

(don‘t consider the software as a ‘black-box‘!)

• LCFI to study e+e- bb and Higgs self-coupling;

code should be useful for many other channels – the more widely the package will be

used the better


Summary and outlook

The vertex detector will provide crucial tools for physics at the ILC.

Vertex detector high-level reconstruction tools (vertex finding, flavour tag,

vertex charge reconstruction) are essential input to optimisation of vertex

detector, global detectors, full MC and event reconstruction frameworks.

LCFI is working on a vertex package, aiming to release mid 2006.

New RA´s to join LCFI, with equivalent of 100% of a person´s time dedicated to

simulation and physics studies, will allow us to continue work on upgrades of the

package and on extensions of its capabilities.


Additional Material


Typical event processing at the ILC

reconstruction

of tracks, CAL-cells

energy flow objects

first order

jet finding

b-jets

c-jets

uds-jets

gluon-jets

tune track-jet

association

for tracks

from SV or TV

contained in

neighbouring

jet

associate with parent jet in some cases;

tag some as c-cbar or b-bbar

classify B

as charged

or neutral

classify D

as charged

or neutral

charged B

charged D

neutral B

neutral D

charge dipole,

protons, charged

kaons or leptons

from SV, TV

charged kaons

or leptons

b

bbar

b

c

cbar

c

cbar

bbar

flavour

identification


Track description by Gaussian probability tubes

D. Jackson, Seminar on ZVTOP, Vienna 2004


‘Straightness’ of B decay chain (SLD)

D. Jackson, Seminar on ZVTOP, Vienna 2004


Ghost Track Algorithm

ZVRES Algorithm

(for SLD vertex detector)

Efficiency improvement obtained from ZVKIN algorithm

N. deGroot ‘From Pixels to Physics’, Contribution to Vertex 2001 workshop


quark b

hadron

B- B0

vtx charge

X-- X- X0 X- X0 X+

Interpretation

b

0.4(1-pm)+0.30+0.30

01-0pm pm1-2pm

0

b-bar

B0-bar B+

X- X0 X+ X0 X+ X++

b-bar

0.4(1-pm)+0.30+0.30

b quark sign selection


Varying the centre of mass energy

At low energy:

lower average track momentum

more strongly affected by

multiple scattering

seed vertex on average closer to IP

track assignment more

challenging, although on average

more hits / track available

expect performance to become

worse at lower energy


Position of vertices wrt detector layers: percentages

CM energy (GeV) 50 100 200 500

inside beam pipe 99.99 % 99.49 % 94.69 % 74.49 %

between layer 1 & layer 2 0.01 % 0.48 % 4.48 % 14.95 %

between layer 2 & layer 3 -- 0.02 % 0.67 % 5.78 %

between layer 3 & layer 4 -- -- 0.11 % 2.49 %

between layer 4 & layer 5 -- -- 0.04 % 1.11 %

outside vertex detector -- -- 0.01 % 1.19 %

CM energy (GeV) 50 100 200 500

inside beam pipe 95.39 % 93.97 % 85.89 % 57.64 %





outside vertex detector 3.00 % 3.82 % 4.18 % 6.08 %

MC-level

secondary

vertex

MC-level

tertiary

vertex


Vertex charge reconstruction


Increasing beam pipe thickness for standard detector

difference in layer thickness

between standard detector

and 4-layer detector

only small effect

main difference between the

detectors due to difference

in inner layer radius

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Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 0 Vertex Detector Contribution to ILC Physics Analyses