1. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 0 Vertex Detector Contribution to ILC Physics Analyses Sonja Hillert (Oxford) on behalf of the LCFI collaboration 3 rd ECFA workshop on Physics and Detectors at the Linear Collider Vienna, 14 – 17 November 2005 Introduction the ZVTOP vertex finder flavour tag vertex charge reconstruction the vertex package under development by LCFI

2. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 1 SM e+e+ e-e- t t W W+W+ q q q q b b e.g. c s e.g. s c Questions the vertex detector can help answer a typical e + e - t t event: without quark sign selection with perfect quark sign selection Which type of event is one looking at? identify quark flavours of all jets involved Are jets stemming from quarks or antiquarks? quark sign selection useful for e.g. studying top polarisation, unfolding cross sections, reducing combinatorial background S. Riemann LC-TH-2001-007

3. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 2 Vertex detector input to physics analyses In the event reconstruction, tracks are found from hits in the vertex detector, the intermediate and the outer (barrel + forward) tracker. Particle flow algorithms combine tracking and calorimeter information and yield the four-momenta that form the basis for jet-finding. Using track information on a jet by jet basis, the vertex detector provides: - vertex finding : identify subsets of tracks inside a jet, which are then fit, yielding vertex position, momentum, mass, fit- 2 - based on these and other quantities further obtain: flavour-tag : distinguish between b-, c- and light-flavour, gluon jets quark sign selection : distinguish between b and b, c and c

4. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 3 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 2 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 D. Jackson, NIM A 388 (1997) 247

5. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 4 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 ZVRES GHOST SLD VXD3 bb-MC

6. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 5 Flavour tag aim: distinguish between b-jets, c- jets and light-quark / gluon jets heavy flavour jets contain secondary decays, generally observed as secondary vertices if secondary vertex is found variables with highest separation power are Pt-corrected vertex massand vertex momentum Vertex Mass (GeV/c 2 ) uds cb SLD

7. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 6 Neural net based flavour tagging procedure To obtain even better purities of the resulting flavour-tagged jet-samples, further variables are added to the procedure and all inputs combined by using neural nets. Example: procedure developed by R. Hawkings et al. (LC-PHSM-2000-021) and recently used by K. Desch / Th. Kuhl (LC-note in preparation) NN-input variables used: if secondary vertex found: M Pt, momentum of secondary vertex, and its decay length and decay length significance additionally (also in case of only primary vertex found): momentum and impact parameter significance in R- and z for the two most-significant tracks in the jet joint probability in R- and z (estimator of probability for all tracks to originate from primary vertex)

8. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 7 Vertex charge reconstruction in the 40% of cases where b quark hadronises to charged B-hadron quark sign can be determined by vertex charge need to find all stable tracks from B decay chain: define seed axis cut on L/D (normalised distance between IP and projection of track POCA onto seed axis) tracks that form vertices other than IP are assigned regardless of their L/D probability of mis-reconstructing vertex charge is small for both charged and neutral cases neutral vertices require charge dipole procedure from SLD still to be developed for ILC

9. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 8 Energy and polar angle dependence impurity is given by probability 0 of reconstructing neutral B hadron as charged degradation at large cos : multiple scattering of oblique tracks in detector material, loss of tracks at detector edge effects more strongly seen at lower jet energy (more low momentum tracks multiple scattering more severe) results obtained with SGV fast MC

10. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 9 Comparing detectors with different beam pipe radius Compare 3 detectors with different inner layer radius: 25 mm 8 mm 250 mm 15 mm 60 mm 100 mm standard detector: large R bp detector:small R bp detector: R bp = 15 mm, thickness 0.4 mm innermost layer at 15.5 mm; layer thickness 0.1 % X 0 (same for all detectors) R bp = 25 mm, thickness 1 mm innermost layer removed new inner layer at 25.5mm has full length of 250 mm R bp = 8 mm, thickness 0.4 mm innermost layer moved inwards to 8.5 mm, positions of other layers retained Note that the beam pipe has to be made thicker if its radius is increased

11. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 10 Results obtained for different beam pipe radii detectors with different radii of the innermost layer differ in performance over entire jet energy range differences most pronounced at lower jet energies

12. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 11 Translation into effective luminosity consider multi-jet final state channel, in which vertex charge needs to be found for two jets of similar energy (e.g. 50 GeV each for decay products of 2 Higgses in ZHH) red curve shows factor by which integrated luminosity would have to change to obtain result of same statistical significance, if the inner layer radius were changed from its baseline value of 15 mm note: plot to be updated using more realistic procedure based on jet-weighting according to their 0 value for E Jet = 25 GeV, 2-jet luminosity factor of 2.14 (shown in plot) is reduced to 1.65 – 1.85, depending on assumed background

13. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 12 Vertex-package under development by LCFI release planned in mid 2006 charge dipole use of additional information (e.g. particle ID) later extensions track & jet parameters (LCIO format) vertex quantities, flavour tag, vertex charge (LCIO format) neural net ZVTOP (ZVRES & ZVKIN) NN-based flavour tag quark-sign selection for charged vertices first release

14. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 13 Dependence on quality of the input Vertex tools are only as good as the track / jet input fed into them! current studies / code development based on perfect track finding (using ´MC truth´) and realistic track fitting parameters as provided by fast MC ´Simulation a Grande Vitesse´ (SGV) SGV studies indicate sensitivity to lower limit for reconstruction of track momentum worth pushing track reconstruction down to |p| > 0.1 GeV if possible

15. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 14 Towards inclusion of vertex package in full MC and reconstruction framework initial tests to be performed using SGV: C++ version of ZVTOP code to be tested against expected performance (ongoing) and compared to results obtained from SLD version in parallel test Hawkings flavour tag within SGV then interface vertex package to LCIO; perform further tests, continuing beyond time of code release: comparison of performance using full MC and reconstruction to SGV results in 2 steps: 1) full MC, but use ´cheaters´ for reconstruction 2) replace cheaters with full track reconstruction input to interpret results of step 2) would be good if developers of track reconstruction code could provide study of track reconstruction over full momentum and full polar angle range, including tracks originating outside the vertex detector

16. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 15 Areas for later upgrades I Components of the vertex package are likely to need further optimisation and extension after release, e.g. choice of input variables for flavour tag neural net quark sign selection: reassess reconstruction procedure once c-jets are included, extend to neutral vertices (charge dipole) where available from particle-ID and ECAL, take additional information (e.g. from Kaons, leptons) into account

17. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 16 Areas for later upgrades II also interplay between components and between vertex package and global event reconstruction will need to be explored and could lead to further improvement, e.g. improvement of flavour tag using information from ghost track algorithm use of vertex information to improve track-jet-assignment to some extent, optimal procedure will depend on the physics channel studied : vertex package will provide enough flexibility to allow user to tune e.g. which inputs are used for flavour tagging; this flexibility should be used, in a similar way as one would tune a jet-finder (dont consider the software as a black-box!) LCFI to study e + e - bb and Higgs self-coupling; code should be useful for many other channels – the more widely the package will be used the better

18. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 17 Summary and outlook The vertex detector will provide crucial tools for physics at the ILC. Vertex detector high-level reconstruction tools (vertex finding, flavour tag, vertex charge reconstruction) are essential input to optimisation of vertex detector, global detectors, full MC and event reconstruction frameworks. LCFI is working on a vertex package, aiming to release mid 2006. New RA´s to join LCFI, with equivalent of 100% of a person´s time dedicated to simulation and physics studies, will allow us to continue work on upgrades of the package and on extensions of its capabilities.

19. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 18 Additional Material

20. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 19 Typical event processing at the ILC reconstruction of tracks, CAL-cells energy flow objects first order jet finding b-jets c-jets uds-jets gluon-jets tune track-jet association for tracks from SV or TV contained in neighbouring jet associate with parent jet in some cases; tag some as c-cbar or b-bbar classify B as charged or neutral classify D as charged or neutral charged B charged D neutral B neutral D charge dipole, protons, charged kaons or leptons from SV, TV charged kaons or leptons b bbar b c cbar c bbar flavour identification

21. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 20 Track description by Gaussian probability tubes D. Jackson, Seminar on ZVTOP, Vienna 2004

22. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 21 Straightness of B decay chain (SLD) D. Jackson, Seminar on ZVTOP, Vienna 2004

23. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 22 Ghost Track Algorithm ZVRES Algorithm (for SLD vertex detector) Efficiency improvement obtained from ZVKIN algorithm N. deGroot From Pixels to Physics, Contribution to Vertex 2001 workshop

24. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 23 quark b hadron B - B 0 vtx charge X -- X - X 0 X - X 0 X + Interpretation b 0.4(1- pm )+0.3 0 +0.3 0 0 1- 0 pm 1-2 pm 0 b-bar B 0 -bar B + X - X 0 X + X 0 X + X ++ b-bar 0.4(1- pm )+0.3 0 +0.3 0 b quark sign selection

25. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 24 Varying the centre of mass energy At low energy: lower average track momentum more strongly affected by multiple scattering seed vertex on average closer to IP track assignment more challenging, although on average more hits / track available expect performance to become worse at lower energy

26. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 25 Position of vertices wrt detector layers: percentages CM energy (GeV) 50 100 200 500 inside beam pipe99.99 % 99.49 % 94.69 % 74.49 % between layer 1 & layer 2 0.01 % 0.48 % 4.48 % 14.95 % between layer 2 & layer 3 -- 0.02 % 0.67 % 5.78 % between layer 3 & layer 4 -- -- 0.11 % 2.49 % between layer 4 & layer 5 -- -- 0.04 % 1.11 % outside vertex detector -- -- 0.01 % 1.19 % CM energy (GeV) 50 100 200 500 inside beam pipe95.39 % 93.97 % 85.89 % 57.64 % between layer 1 & layer 2 0.53 % 1.37 % 7.82 % 20.83 % between layer 2 & layer 3 0.42 % 0.34 % 1.46 % 9.03 % between layer 3 & layer 4 0.33 % 0.27 % 0.43 % 4.32 % between layer 4 & layer 5 0.32 % 0.23 % 0.21 % 2.10 % outside vertex detector 3.00 % 3.82 % 4.18 % 6.08 % MC-level secondary vertex MC-level tertiary vertex

27. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 26 Vertex charge reconstruction

28. Vertex Detector Contribution to ILC Physics Analyses, 16 th November 2005Sonja Hillert (Oxford)p. 27 Increasing beam pipe thickness for standard detector difference in layer thickness between standard detector and 4-layer detector only small effect main difference between the detectors due to difference in inner layer radius