LCFI Simulation and Physics Studies

LCUK Meeting, 26th September 2006 Sonja Hillert (Oxford) p. 1

LCFI Simulation and Physics Studies

LCUK Meeting, Durham, 26th September 2006

Sonja Hillert (Oxford)

on behalf of the LCFI collaboration

Overview

Sensitivity to detector design parameters

The LCFI Vertex Package

Summary and Outlook


Simulation and Physics Studies - Overview aim: optimisation of vertex detector parameters and evaluation of its performance

two main areas of work:

Development of high-level reconstruction tools:

• current focus: development of C++ based Vertex Package, interfaced to LCIO/MarlinReco

comprising vertex finder ZVTOP, NN-based flavour tag and quark charge sign selection

• aim for first release at the time of ECFA 2006 (Valencia)

• release will be followed by work on upgrades

Study of Physics channels:

• goal is to realistically quantify performance following vertex detector contribution to

physics analysis through entire analysis chain

in parallel: performance comparison of different detector designs using fast MC SGV,

to be replaced by full simulation and reconstruction framework in the future


Sensitivity to detector design parameters

left: probability 0 to reconstruct neutral

B hadron as charged

since vertex charge performance is

sensitive to multiple scattering need to

keep layer thickness small (target 0.1 % X0)

also strong dependence on momentum cut

(track selection) – this depends critically

on tracking performance:

• track finding capability

• background rates

• linking across subdetector boundaries

should push all these parameters to their limits, as all these effects will eventually

add up in the real detector


The ZVTOP vertex finderD. Jackson,

NIM A 388 (1997) 247

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

The ZVRES algorithm: very general algorithm

that can cope with arbitrary multi-prong decay topologies

• ‘vertex function‘ calculated from Gaussian

´probability tubes´ representing tracks

• iteratively search 3D-space for maxima of this function

and minimise 2 of vertex fit

ZVKIN: more specialised algorithm to extend coverage to b-jets with

1-pronged vertices and / or a short-lived B-hadron

• additional kinematic information

(IP-, B-, D-decay vertex approximately

lie on a straight line) used to find

vertices

• should improve flavour tag


Flavour tag and quark charge sign selection aim of flavour tag: distinguish between b-jets, c- jets and light-quark / gluon jets

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

NN-approach to combine inputs; most sensitive: secondary vtx Pt-corrected mass & momentum

for charged B-hadrons (40% of b-jets): quark sign can be determined from vertex charge:

need to find all stable tracks from B-decay chain

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

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

Klaus Desch/ Thorsten Kuhl


interface SGV to

internal format

interface LCIO to

internal formatinput to LCFI Vertex Package

output of LCFI Vertex Packageinterface internal

format to SGV

interface internal

format to LCIO

ZVRES ZVKIN

ZVTOP:

vertex information

track attachment

assuming c jet

track attachment

assuming b jet

track attachment

for flavour tag

find vertex-

dependent

flavour tag

inputs

find vertex charge

find vertex-

independent

flavour tag

inputs

neural net flavour tag


Current Status

coding, validation and documentation far advanced;

remaining issues mainly to do with integration / system test

ZVTOP (Ben Jeffery): coding completed

• validation has focused on ZVRES branch:

performance at least as good as FORTRAN shown in detailed tests of increasing complexity;

• initial tests of ZVKIN look promising;

coding calculation of flavour-tag inputs (Erik Devetak)

and integration of NN-software into our package (Mark Grimes) progressing

internal working classes designed, largely implemented and tested:

• integration into MarlinReco beginning: test processors for ZVTOP & flavour-tag inputs exist

• interface to LCIO and to event-input from MarlinReco-based event reconstruction need

further work


Comparison of C++ & FORTRAN ZVRES

compare distributions of number of vertices and of tracks per vertex between

C++, FORTRAN and MC truth for 100 GeV b-jets (using identical input from fast MC SGV)

excellent agreement between the results from C++ and from FORTRAN

Ben Jeffery


Comparison of track-vertex association

tables show percentages of tracks from the primary, B- and D-hadron decay at MC level

that are assigned to primary, secondary (and tertiary) vertex or left isolated by ZVRES;

excellent agreement also seen in this detailed comparison of the two versions

results above were obtained for 100 GeV b-jets;

corresponding studies at 25 GeV and 250 GeV jet energy show equally good agreement

Mark Grimes


54% negative (C++ version better)

Ben Jeffery

Performance of decay length reconstructiondifference between the C++ and the FORTRAN deviation from MC in decay length

right: distribution of this difference, left: difference plotted vs MC decay length

fitter yields different

vertex position

differences in track-

vertex associations

C++ finds some vertices FORTRAN misses

and comes closer to MC truth than FORTRAN for entire range of decay lengths


LCIO persistency framework has been extended by dedicated vertex class to

accommodate the output of our software:

• each ReconstructedParticle points to one vertex from which it originated & to decay vertex

will provide MARLIN processors (modules) giving example code for

• running ZVTOP (one processor for each of the two branches ZVRES, ZVKIN)

• calculating neural net input variables from input to package & ZVTOP output

• training neural nets for flavour tag, obtaining NN outputs for pre-trained nets,

• vertex charge calculation

• combined processor: ZVRES + Hawkings flavour tag + vertex charge calculation

Interfacing the Vertex Package

Frank Gaede (DESY)


Summary and outlook

Development and validation of the LCFI Vertex Package are far advanced.

Excellent agreement is found between C++ and FORTRAN ZVRES branch of ZVTOP.

A new Vertex class has been introduced into LCIO. Integration of our package into

MarlinReco is beginning.

Interfacing to LCIO and to event-input from MarlinReco event reconstruction and

system tests need further work.

The first release of the code is planned for November.

Detailed comparisons with MarlinReco input and quantitative exploration of

improvements from the ghost track algorithm will be the next steps after the release,

in parallel to first studies of benchmarks processes sensitive to the detector design.


Additional Material


ZVTOP - Progress

Initial aim: replace FORTRAN ZVRES in SGV for testing- allows comparison of intermediate algorithm states when working on identical tracks- new version can be verified to be at least as good as FORTRAN

SGV FORTRAN ZVRES Flavour Tagtracks vertices

SGV FORTRAN ZVRES Flavour Tag

C++ ZVRES

Current Status

Add ZVKIN: SGV C++ ZVRES Flavour Tag

C++ ZVKIN

LCIO C++ ZVTOP LCIOFinally:


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


tanh (MPt / 5 GeV)

joint probability

uds

c

b

Flavour tag Vertex package will provide flavour tag procedure developed by R. Hawkings et al

(LC-PHSM-2000-021) and recently used by K. Desch / Th. Kuhl as default

NN-input variables used:

• if secondary vertex found: MPt , momentum

of secondary vertex, and its decay length and

decay length significance

• if only primary vertex found: momentum and

impact parameter significance in R- and z for the

two most-significant tracks in the jet

• in both cases: joint probability in R- and z (estimator of

probability for all tracks to originate from primary vertex)

will be flexible enough to permit user further tuning of the input variables for the neural net,

and of the NN-architecture (number and type of nodes) and training algorithm

b

c


Towards completion of the Vertex Package

b

c

c (b bkgr)c

c (b bkgr)

b

Z peak ECM = 500 GeV

test of full chain of C++ ZVTOP with FORTRAN flavour tag and vertex charge imminent

pure FORTRAN results (from SGV) show below:

C++ code for calculation of inputs for flavour tag being written

Vertex charge reconstruction for c-jets under development


ECM = 100 GeV

b

c (b-bkgr)

c

open: FORTRAN

full: C++

First full test of C++ ZVRES in FORTRAN framework compare FORTRAN to C++ ZVRES when fed with identical input events

unrealistically large fixed track errors used in both versions for diagnostic purposes

identical FORTRAN code for flavour tag (left) and vertex charge (right)

flavour tag: good agreement between C++ and FORTRAN, with C++ slightly better for b-jets

for vertex charge C++ in this first test performs considerably less well than FORTRAN –

but could be more sensitive to fixed huge track errors

Sonja Hillert


Future plans beyond the Vertex Package release

more detailed comparisons with performance obtained when using MarlinReco –

in particular, compare results from ‘cheaters’ to full track reconstruction, for which algorithms

independent of the old BRAHMS code, are now under development performance of our tools

with different choices for the input can provide useful feedback for tracking code development

important for us: step towards full framework, ensure we understand later physics results

explore and quantify what can be gained for flavour tag and vertex charge

when using the ghost track (ZVKIN) algorithm

should feed into future update, so the community can profit from our studies

begin study of benchmark physics processes:

e+e- b bbar with extra dimensions model as studied by Riemann, Hewett

SUSY physics

further physics processes


Vertex charge study for c-jets

Victoria Martin

vertex charge (ZVTOP & L/D) and #(c decay tracks) for various decay channels (ECM = 500 GeV)

first results look very promising; next look at corresponding leakage rates and optimise L/D


from: Steve Magill, Digitization simulation using DigiSim and Marlin, talk at Cambridge ILC workshop, April 2006

LCFI Simulation and Physics Studies

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