LCFI Simulation and Physics Studies

1. LCUK Meeting, Durham, 26th September 2006 LCFI Simulation and Physics Studies • Overview • Sensitivity to detector design parameters • The LCFI Vertex Package • Summary and Outlook Sonja Hillert (Oxford) on behalf of the LCFI collaboration

2. 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

3. Sensitivity to detector design parameters • left: probability l0 to reconstruct neutral • B hadron as charged • since vertex charge performance is • sensitive to multiple scatteringneed 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

4. D. Jackson, NIM A 388 (1997) 247 The ZVTOP vertex finder • 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 c2 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

5. Klaus Desch/ Thorsten Kuhl 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

6. interface internal format to SGV interface SGV to internal format interface LCIO to internal format interface internal format to LCIO output of LCFI Vertex Package input to LCFI Vertex Package ZVTOP: ZVRES ZVKIN vertex information track attachment for flavour tag track attachment assuming b jet track attachment assuming c jet find vertex- independent flavour tag inputs find vertex- dependent flavour tag inputs neural net flavour tag find vertex charge

7. 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 ZVRESbranch: 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

8. 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) Ben Jeffery • excellent agreement between the results from C++ and from FORTRAN

9. 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; Mark Grimes • 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

10. fitter yields different vertex position differences in track- vertex associations Performance of decay length reconstruction difference between the C++ and the FORTRAN deviation from MC in decay length right: distribution of this difference, left: difference plotted vs MC decay length Ben Jeffery 54% negative (C++ version better) C++ finds some vertices FORTRAN misses and comes closer to MC truth than FORTRAN for entire range of decay lengths

11. Frank Gaede (DESY) Interfacing the Vertex Package • 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

12. 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.

13. Additional Material

14. vertices tracks SGV FORTRAN ZVRES Flavour Tag FORTRAN ZVRES SGV Flavour Tag Current Status C++ ZVRES C++ ZVRES Add ZVKIN: SGV Flavour Tag C++ ZVKIN Finally: LCIO LCIO C++ ZVTOP 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

15. D. Jackson, NIM A 388 (1997) 247 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 c2 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

16. ZVRES GHOST SLD VXD3 bb-MC 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

17. tanh (MPt / 5 GeV) b c joint probability uds b c 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-f and z for the two most-significant tracks in the jet • in both cases: joint probability in R-f 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

18. b c (b bkgr) c (b bkgr) c c b Towards completion of the Vertex Package • test of full chain of C++ ZVTOP with FORTRAN flavour tag and vertex charge imminent • pure FORTRAN results (from SGV) show below: Z peak ECM = 500 GeV • C++ code for calculation of inputs for flavour tag being written • Vertex charge reconstruction for c-jets under development

19. ECM = 100 GeV c (b-bkgr) open: FORTRAN full: C++ b 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) Sonja Hillert • 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

20. 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

21. Vertex charge study for c-jets • vertex charge (ZVTOP & L/D) and #(c decay tracks) for various decay channels (ECM = 500 GeV) Victoria Martin • first results look very promising; next look at corresponding leakage rates and optimise L/D

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