Recent results on vertex charge reconstruction

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

2. Introduction: aim of the studies presented • 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

3. Software tools • 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

4. 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 unambiguouslyidentified: • at generator level, 40% of jets stem • from charged hadrons Motivation and selection cuts

5. Vertex finding and track attachment • goal: find all stable B decay chain tracks -- procedure: • ‘mask’ all KS, L 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 d0 < 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. vertex charge Qvtx and MPtdetermined from tracks assigned to B decay chain: • sum of charges of these tracks: Qsum • reconstructed vertex charge • from sum of four-momenta: Pvtx, Mvtx • apply kinematic correction (partly • corrects for missing neutral particles): • MPtused as ‘b tag’ parameter Vertex charge and Pt-corrected mass

7. _ MC: B MC: neutral B hadrons + MC: B Improvement since LCWS – 1 comparison of reconstructed Qsum distributions for the different generator level charges new LCWS

8. Purity for discerning b from b-bar: : # (jets) from b-quark with + # (jets) from b-bar quark with Definition of efficiency and purity efficiency with : # (jets) with LDec > 300 mm and MPt,cut varied between 0 and 4.5 GeV : # (jets) in event selection cuts

9. Improvement since LCWS – 2 • differences in the procedure: • in new result • KS, L decay products are masked • L/D cut for track assignment has been optimised • large increase in purity and • efficiency, especially at large • efficiencies: • at eb ~ 70% (MPt > 2.0 GeV): • Deb = 5%, DP(b) = 4%

10. Detector configurations considered • Standard detectorcharacterised by: • good angular coverage (cos q = 0.96) • proximity to IP, large lever arm: • 5 layers, radii from 15 mm to 60 mm • minimal layer thickness ( 0.064 % X0) • to minimise multiple scattering • excellent point resolution (3.5 mm) • 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. cut value chosen Optimisation of L/D cut • maximise P(b) and efficiency note: fragmentation tracks (MC level) assigned to B decay chain may result in wrong Qvtx and MPt > 5.5 GeV  reason for drop of P(b) and eb near 0

12. Purity and efficiency for optimised L/D procedure • conclusions: • purity flat out to efficiency of ~ 70% • for standard detector • significant detector dependence: • at lower eb detectors differ • mainly in efficiency: at eb =20%: Deb = 5% • at higher eb they differ also • significantly in purity: at eb = 70% (MPt > 2.0 GeV): • Deb = 6%, DP(b) = 2% • result underlines the need for a small beam pipe radius

13. 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. 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 Additional Material ~ Additional Material ~ Additional Material

15. 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 Additional Material ~ Additional Material ~ Additional Material