1. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 0 Recent results on vertex charge reconstruction Sonja Hillert (Oxford) on behalf of the LCFI collaboration 2 nd ECFA workshop on Physics and Detectors at the Linear Collider Durham, 2 nd September 2004

2. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 1  physics studies performed in the context of R&D work of the LCFI collaboration  aim at providing a guideline for vertex detector design, e.g. How close to the interaction point does the inner layer need to be? Which layer thickness should be aimed at? (multiple scattering) How many layers are needed?  to answer these questions study e.g. impact parameter resolution vertex charge reconstruction specific physics channels expected to be sensitive (future)  need to be sure to use all available information that might depend on detector design  develop existing flavour tagging tools further Introduction: aim of the studies presented

3. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 2  Simulation a Grande Vitesse (SGV) version 2.31 by M. Berggren http://berggren.home.cern.ch/berggren/sgv.html flexible, well-tested fast simulation, originated from DELPHI interfaced to PYTHIA version 6.1.52 JADE algorithm (y-cut 0.04) for jet finding vertex finding: ZVTOP by D. Jackson  future: Java analysis studio, version 3 (JAS3), by T. Johnson http://jas.freehep.org/jas3/index.html object oriented software being developed at SLAC Software tools

4. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 3 Motivation and selection cuts  discerning charged b jets ( ) from charged b-bar jets ( ) can aid background suppression in multi-jet events, e.g. Higgs-decay, allow measuring parity of Higgs boson, CP asymmetries in SUSY processes  study monoenergetic jets from at with exactly 2 jets found jets sufficiently back-to-back: jets well contained in detector: thrust angle within B hadron initiating jet can be unambiguously identified:  at generator level, 40% of jets stem from charged hadrons

5. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 4 Vertex finding and track attachment goal: find all stable B decay chain tracks -- procedure:  ‘mask’ all K S,  decay products: removed at track selection step using MC information; expected to be reliably identified in the reconstruction and recovered at a later stage  run ZVTOP to find vertex candidates, require tracks to have d 0 < 1.0 cm  seed 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, where (L/D) min is optimised for each detector configuration independently

6. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 5 Vertex charge and Pt-corrected mass vertex charge Q vtx and M Pt determined from tracks assigned to B decay chain:  sum of charges of these tracks: Q sum  reconstructed vertex charge  from sum of four-momenta: P vtx, M vtx  apply kinematic correction (partly corrects for missing neutral particles):  M Pt used as ‘b tag’ parameter

7. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 6 Improvement since LCWS – 1 MC: B _ + MC: neutral B hadrons comparison of reconstructed Qsum distributions for the different generator level charges LCWS new

8. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 7 Definition of efficiency and purity efficiency with : # (jets) with L Dec > 300  m and M Pt,cut varied between 0 and 4.5 GeV : # (jets) in event selection cuts Purity for discerning b from b-bar: : # (jets) from b-quark with + # (jets) from b-bar quark with

9. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 8 Improvement since LCWS – 2  differences in the procedure: in new result K S,  decay products are masked L/D cut for track assignment has been optimised  large increase in purity and efficiency, especially at large efficiencies: at  b ~ 70% (M Pt > 2.0 GeV):  b = 5%,  (b) = 4%

10. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 9 Detector configurations considered 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 % X 0 ) to minimise multiple scattering  excellent point resolution (3.5  m) standard detector is compared to  degraded detector: beam pipe radius 25 mm, inner layer removed factor 2 worse point resolution  improved detector: factor 4 less material (  factor 2 less multiple scattering) factor 2 better point resolution

11. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 10 Optimisation of L/D cut  maximise  (b) and efficiency note: fragmentation tracks (MC level) assigned to B decay chain may result in wrong Q vtx and M Pt > 5.5 GeV  reason for drop of  (b) and  b near 0 cut value chosen

12. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 11 Purity and efficiency for optimised L/D procedure conclusions:  purity flat out to efficiency of ~ 70% for standard detector  significant detector dependence: at lower  b detectors differ mainly in efficiency: at  b = 20%:  b = 5% at higher  b they differ also significantly in purity: at  b = 70% (M Pt > 2.0 GeV):  b = 6%,  (b) = 2%  result underlines the need for a small beam pipe radius

13. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 12 Future plans  replace one-dimensional L/D cut by a more powerful neural net approach similar to what was formerly used at SLD (  T. Wright’s thesis, SLAC-Report-602 ): five input variables: T, L, L/D, angle of track to vertex axis, normalised 3D impact parameter of track to IP NN configuration: 5 inputs, 6 nodes in hidden layer, 1 output node neural network package has already been developed (D. Bailey)  develop improved NN-based flavour-tagging procedure  extend study to other jet energies  study impact of vertex charge reconstruction on analysis of benchmark physics processes, such as Higgs physics, e.g. Higgs parity, and CP asymmetries in SUSY

14. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 13 Optimisation of track attachment cut  main track attachment cut on L/D needs to be optimised for each detector separately to obtain fair comparison  otherwise difference in performance between detectors may decrease or increase: Additional Material ~ Additional Material ~ Additional Material

15. Sonja Hillert, University of Oxford 2 nd ECFA LC workshop, Durham, 2 nd September 2004 p. 14 Effect of track selection cuts  track selection cuts currently fixed; too little known about global detector to relax them at present  consider tracks from B decay chain that are not assigned to B decay chain in the reconstruction (“ missed tracks ”)  upper plot : fraction of wrong charge vertices with missed tracks vs efficiency  lower plot : fraction of missed tracks, which are missed because of the track selection: for standard detector below 20%  conclusion: track selection is not a limiting factor; there is still much room for improvement Additional Material ~ Additional Material ~ Additional Material