Vertex findingand

B-Taggingfor the

ATLAS Inner Detector

A.H. WildauerUniversität Innsbruck

CERN ATLAS Computing Group

on behalf of the ATLAS collaboration

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Introduction

The development of the vertexing and B-Tagging software followed these basic principles:

• Requirements:

- integrated in the ATLAS Athena framework

- compatible with the ATLAS Event Data Model

• Define and use standardized interfaces

• It has to be modular and expandable for future needs

e.g.: implementation of new vertex fitting methods

adding new tag algorithms to the B-Tagging

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

ATLAS Event Data Model

ESDEvent Summary Data

AODAnalysis Object Data

Analysis

raw data~1.6 MB

~500 kB

~100 kB

• amount of data per year: ~ 1PB

• cost of distribution and storage

• standardized Event Data Model

for online and offline is necessary

• e.g. common EDM for tracking(see talk by E. Moyse)

• Vertexing and B-Tagging should work on ESD and AOD

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Design of Vertexing Package

A client feeds a fit method with tracks andgets back a vertex candidate

It can choose the fit method and other parameters via a Python steering file

VertexFit package implements differentfitting methods through inheritance

The base class takes care of commonframework issues

Modularity: derived fit classes only haveto implement the fit method

tracks vertex

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

• Currently implemented are 2 fit methods: “full” and “fast”

• Mathematical basis: the “Billoir” method (Fast vertex fitting with a local parameterization of tracks

Nuclear Instruments and Methods in Physics Research A311 (1992) 139-150)

• Local parameterization of track parameters q at a fixed point, e.g. the perigee, as a function of V and p

• Minimize the 2:

• Result: vertex and track parameters

Vertex Fitting Methods

),( where2ipVFmeas

iqiqiqiWiTiq

ipiEViDoip

oVFipVF ),(),(

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Primary Vertex Finder

• applied standard track selection on pt, # of hits, aso.

• 2nd vertex fit after 2 track selection

Reconstruction of the primary vertex in WH(120)uu

Example application using the VertexFit package

preliminary

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Primary Vertex Finder with pile-up

• look for primary vertices in windows of z

• the primary vertex is defined as the one with the highest pt

Reconstruction of the primary vertex in

H(130) ZZ* 4l with pile-up

Pile-up: about 23 vertices will cover the primary vertex of an interesting physics event

another example

preliminary

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

B-TaggingUse Cases and Considerations

• Ingredients for B-Tagging are e.g.

1) Signed impact significance

2) Secondary vertex information (mass, multiplicity, …)

3) Particle identification (lepton tag)

• And are provided by the

1) Track/TrackParticle and primary vertex (ESD/AOD)

2) vertex finder using the VertexFit package

3) particle objects in the AOD

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

B-Tagging Package Design

The Algorithm retrieves relevant info from StoreGate and calls several tag Tools

The Tools perform the tagging and add their results to the BJet object

The Algorithm combines their information into a single btag likelihood and returns the resulting BJet

Tools, order, cuts aso. can be set via Python steering files

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Design of the BJet Object

The BJet object stores all information of the btag

It inherits from several classesto obtain the functionality necessary for an EDM object

In addition to the kinematics ofthe jet, it contains the overall likelihood of the btag

It has a vector of IBInfo like objects which are used tostore tag specific information

1… *

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Various discriminating variables used for B-Tagging:

• Signed impact significance

• 2D distance between 2nd and primary vertex

• vertex mass

B-Tagging: Results

b-Jetsu-Jets

preliminary

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Likelihood

Combined likelihood and efficiency with

• 2D impact parameter

• secondary vertex tools

b-Jetsu-Jets

preliminary

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

Conclusion

Vertexing and B-Tagging software is presented which:

• is implemented in the ATLAS Athena framework

• is compatible with the ATLAS Event Data Model

and therefore works on ESD and AOD

• is modular and expandable to suit future needs

• first prototype implementations have been done and look encouraging

• future work involves lots of tuning of parameters

28-Sep-2004 CHEP 2004 Interlaken Wildauer Andreas

SPARE

Secondary Vertex Finding

• Inside the secondary vertex tag tool, two strategies for finding secondary vertices in a jet are in use:

Build-Up

• fit all 2 track verticesin a jet

• take lowest 2 one• add tracks 1 by 1 and refit• take track if 2 < 4

Tear-Down

• use all tracks in a jet tofit a vertex

• remove tracks with 2 > 4• refit the vertex• take track if 2 < 4

• the vertex fitter Tool is called from within the SecVtxTag