Vertex finding and B-Tagging for the ATLAS Inner Detector A.H. Wildauer Universität Innsbruck CERN ATLAS Computing Group on behalf of the ATLAS collaboration

28-Sep-2004CHEP 2004 InterlakenWildauer 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-2004CHEP 2004 InterlakenWildauer Andreas ATLAS Event Data Model ESD Event Summary Data AOD Analysis 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-2004CHEP 2004 InterlakenWildauer Andreas Design of Vertexing Package A client feeds a fit method with tracks and gets back a vertex candidate It can choose the fit method and other parameters via a Python steering file VertexFit package implements different fitting methods through inheritance The base class takes care of common framework issues Modularity: derived fit classes only have to implement the fit method tracks vertex

28-Sep-2004CHEP 2004 InterlakenWildauer 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) ) 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

28-Sep-2004CHEP 2004 InterlakenWildauer Andreas Primary Vertex Finder applied standard track selection on p t, # of hits, aso. 2 nd vertex fit after  2 track selection Reconstruction of the primary vertex in WH(120)  uu Example application using the VertexFit package preliminary

28-Sep-2004CHEP 2004 InterlakenWildauer 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 p t 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-2004CHEP 2004 InterlakenWildauer Andreas B-Tagging Use 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-2004CHEP 2004 InterlakenWildauer 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-2004CHEP 2004 InterlakenWildauer Andreas Design of the BJet Object The BJet object stores all information of the btag It inherits from several classes to obtain the functionality necessary for an EDM object In addition to the kinematics of the jet, it contains the overall likelihood of the btag It has a vector of IBInfo like objects which are used to store tag specific information 1… *

28-Sep-2004CHEP 2004 InterlakenWildauer Andreas Various discriminating variables used for B-Tagging: Signed impact significance 2D distance between 2 nd and primary vertex vertex mass B-Tagging: Results b-Jets u-Jets preliminary

28-Sep-2004CHEP 2004 InterlakenWildauer Andreas Likelihood Combined likelihood and efficiency with 2D impact parameter secondary vertex tools b-Jets u-Jets preliminary

28-Sep-2004CHEP 2004 InterlakenWildauer 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-2004CHEP 2004 InterlakenWildauer 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 vertices in 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 to fit 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