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Ghost Trackers

Ghost Trackers. Dave Jackson Oxford University / RAL. If there’s something strange (or charm or bottom) in your neighbourhood…. LCFI Collaboration 28 th March 2006. ZVRES. ZVKIN.

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Ghost Trackers

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  1. Ghost Trackers Dave Jackson Oxford University / RAL If there’s something strange (or charm or bottom) in your neighbourhood… LCFI Collaboration 28th March 2006

  2. ZVRES ZVKIN In the conventional ZVTOP algorithm secondary vertices are found first. The linearity of the B->D decay chain is then used for ‘L/D’ track attachment In the Ghost Track algorithm the ‘straight’ IP->B->D topology is exploited first, to estimate the B/D flight direction before vertex finding

  3. The straight ghost track is anchored at the IP, initially given a 25μm width then moved in θ and φ to minimise ∑ χ2 (sum over jet tracks). Final ‘fitted’ width of ghost track (minimum 25μm) calculated for ‘compatibility’ with jet tracks: each has χ2 ≤ 1.0 with fitted ghost track SLD

  4. The SLD Ghost Track used straight Gaussian tubes for fast analytic track fits – this required reparametrising the track near a vertex location. This should not be so much of an issue for the LCFI C++ code in which the approximation is not made

  5. Care with errors and χ2s important since vertex finding relies on probability of vertex fit calculation SLD PROB(∑χ2,2N-3) For N tracks in jet from same MC B vertex (spike at zero due to non-Gaussian tails) PROB(∑χ2,2N-4) Where N now includes the Ghost Track in the fit – so less ‘free’ than the jet tracks alone. Probability is now a measure of a good secondary vertex fit AND compatibility with the B direction (ghost track).

  6. SLD The algorithm proceeds to build vertices according to the highest Probability; while Prob>1%. So this distribution needs to be fairly flat for genuine vertices; and to be flat for a range of track multiplicity (shown here), decay length, etc. The pre-requisite for this is a fitter that has the right properties for jet tracks alone. Probability of ghost + B tracks fit

  7. At SLD the B’s came in back-to-back pairs. The EVENT would be tagged with ZVRES before running ZVKIN for analysis of each jet For LCFI generally would like to consider each jet independently; design flavour/charge tagger for each jet with ZVTOP3 in C++ SLD

  8. BACKUP SLIDES…

  9. VXD2 b-jets Monte Carlo Track Origin D Decay B Decay IP L/D for non-Seed tracks passing T < 1mm Cut at L/D > 0.3 to attach tracks from B decay chain to Seed

  10. Highly boosted B kinematics: IP→B→D straight to ~1% (for Z0) • B,D vertex locations are not independent in 3D space • ‘Ghost Track Algorithm’ builds in this information from the beginning

  11. Angle between true B flight and jet axis Angle between true B flight and ghost track Radians • Stage 1: • Pivot straight ghost track at IP, initially along jet axis direction • Give ghost track a 25μm width and calculateχ2 of ghost to each track in jet • Swivel ghost track in θ and φ to minimise ∑ χ2 (sum over jet tracks)

  12. Stage 2: • PROB(χ2,ndof) for jet track(s) to be consistent with each other and with the ghost track or IP ellipsoid is constructed • For jet with N tracks, initially N+1 candidate vertices: 1 2 N+1 Ghost • Calculate fit probability for all pairs of objects (if IP is not included, then ghost track is added) • If maximim PROB > PCUT (typically 1%) then: combine the two objects and iterate Else: vertex reconstruction is complete: IP 3 N PRI SEC TER Allows reconstruction of ‘1-prong’ vertices

  13. The Topologies For a B decay to a single cascade charm D lD B lB IP •••• Measured lB Measured lD True MC lB cm True MC lD cm

  14. Compare ‘ZVTOP’ with ‘Ghost Track Algorithm’ ZVTOP GHOST Number of Found Vertices ZVTOP GHOST GeV/c2 ‘B Decay’ Invariant Mass

  15. Options • The ‘Tidy’ cuts: • For SLD ~20% of jets contained ≥ 1 high impact parameter track from Ks, Λ, detector interaction etc. • Tidy cuts are applied first to prevent the Ghost Track direction being distorted (~half background tracks removed at SLD) • For each algorithm ~4 tunable parameters that effect efficiency vrs purity of vertex reconstruction also: • ZVTOP – can guide vertex finding with V(r) weighting • Ghost Track – can force the topology to find fixed number of vertices – Momentum factor

  16. World BS mixing sensitivity B0 : b→c : D+ or D0 July 2003

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