Ghost Trackers

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Ghost Trackers
Dave Jackson Oxford University / RAL

• If theres something strange (or charm or
  bottom) in your neighbourhood

LCFI Collaboration 28th March 2006
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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
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The straight ghost track is anchored at the IP,
initially given a 25µm width then moved in ? and
f to minimise ? ?2 (sum over jet tracks). Final
fitted width of ghost track (minimum 25µm)
calculated for compatibility with jet tracks
each has ?2 1.0 with fitted ghost track
SLD
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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
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Care with errors and ?2s important since vertex
finding relies on probability of vertex fit
calculation
SLD
PROB(??2,2N-3) For N tracks in jet from same MC B
vertex (spike at zero due to non-Gaussian tails)
PROB(??2,2N-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).
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SLD
The algorithm proceeds to build vertices
according to the highest Probability while
Prob1. 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
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At SLD the Bs 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
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BACKUP SLIDES
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L/D for non-Seed tracks passing T Cut at L/D 0.3 to attach tracks from B decay
chain to Seed
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• 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

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• Stage 1
• Pivot straight ghost track at IP, initially
  along jet axis direction
• Give ghost track a 25µm width and calculate ?2
  of ghost to each track in jet
• Swivel ghost track in ? and f to minimise ? ?2
  (sum over jet tracks)

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• Stage 2
• PROB(?2,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 N1 candidate
  vertices

1
2
N1
Ghost
IP
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• 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

N
PRI
SEC
TER
Allows reconstruction of 1-prong vertices
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The Topologies
For a B decay to a single cascade charm
D
lD
B
lB
IP

Measured lB
Measured lD
True MC lB
True MC lD
cm
cm
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Compare ZVTOP with Ghost Track Algorithm
ZVTOP
GHOST
Number of Found Vertices
ZVTOP
GHOST
GeV/c2
B Decay Invariant Mass
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Options

• The Tidy cuts
• For SLD 20 of jets contained 1 high impact
  parameter track from Ks, ?, 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

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World BS mixing sensitivity
B0 b?c D or D0
July 2003