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Status Report on Vertex Charge Reconstruction

Status Report on Vertex Charge Reconstruction. LCFI collaboration meeting, May 2004. efficiency with : # (jets) with L Dec > 300 m m and : # (jets) in event selection cuts. Purity for discerning b from b-bar: : # (jets) from b-quark with

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Status Report on Vertex Charge Reconstruction

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  1. Status Report on Vertex Charge Reconstruction LCFI collaboration meeting, May 2004

  2. efficiency with : # (jets) with LDec > 300 mm and : # (jets) in event selection cuts Purity for discerning b from b-bar: : # (jets) from b-quark with + # (jets) from b-bar quark with Reminder: LCWS result

  3. Conclusion from LCWS: • need to study and improve track attachment and flavour tagging algorithms • try tounderstand at the track level why purity stays < 90% using MC information • Procedure: • find all stable decay products from the B decay chain: • ideally, all these and only these should be assigned to the seed vertex • compare these ‘tracks truly from the B decay chain’ to those actually assigned • by ZVTOP and the track attachment code (‘assigned tracks’) • study ‘missed tracks’: truly from B, but not assigned, and • ‘added tracks’: assigned, but not from B

  4. with the LCWS track attachment, for wrong Qsum vertices: • about 40% of the tracks from the B decay chain are missed • about 5% of the assigned tracks ought not to be assigned (added) •  missed tracks are the main problem # added tracks/vtx # missed tracks/vtx Qsum right Qsum wrong • What are the sources of missed & added tracks? Why are they missed/added? • How can these misassignments be reduced?

  5. Missed Tracks • numbers of tracks not seen anywhere in detector or not in jet negligible 3D MC track origin for 0 VXD hit tracks missed tracks: number of VXD hits • 20% of missed tracks for Qsum wrong have 0 hits in VXD due to track origin • outside VXD fail track selection for ZVTOP; > 90% of these are pions • 0 VXD hit tracks mainly Ks and L decay products should not be reason for Qsum • being wrong, but blur the picture (consistent with 60% contribution to right Qsum)

  6. Added Tracks p • about 70% of added • tracks are pions • mainly comingfrom • decay of strange particles • (track history variable) S p X K D

  7. Masking Ks and L • both missed and added tracks hint at decay products of Ks and L being problematic • (directly or by blurring the real problem) •  remove them in track selection for ZVTOPand do not treat them as missed tracks • purity P(b) for discerning b, b-bar • reaches approx. 95% • purity for Qvtx reconstruction • increases by 2-6% depending • on efficiency

  8. A new approach: using Dnorm for track attachment Dnorm: d0/s(d0): 2D impact parameter significance in Rf Dnorm L/D tracks from B decay chain tracks from IP other • with the Gaussian impact parameter distribution assumed by SGV, • Dnorm allows IP tracks to be completely suppressed, • while for L/D thereis some contamination from IP tracks even at large values • actual non-Gaussian Dnorm-tails can be considered a 2nd order effect

  9. Dnorm for track attachment: result • cut on Dnorm less good than L/D • loose too many tracks close to PV • for lower Dnorm cut, effects from • added tracks increase • next step: find out, if and where there is a principle limitation of the method • by only feeding tracks from the B decay chain into ZVTOP: • How close to 100% can purity be pushed in such a clean environment?

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