Transcript 2nd ECFA workshop on Physics and Detectors at the Linear Collider Durham, 2nd September 2004 Recent results on vertex charge reconstruction Sonja Hillert (Oxford) on.

2nd ECFA workshop on Physics and Detectors at the Linear Collider
Durham, 2nd September 2004
Recent results on
vertex charge reconstruction
Sonja Hillert (Oxford)
on behalf of the LCFI collaboration
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 0
Introduction: aim of the studies presented
 physics studies performed in the context of R&D work of the LCFI collaboration
 aim at providing a guideline for vertex detector design, e.g.
• How close to the interaction point does the inner layer need to be?
• Which layer thickness should be aimed at? (multiple scattering)
• How many layers are needed?
 to answer these questions study e.g.
• impact parameter resolution
• vertex charge reconstruction
• specific physics channels expected to be sensitive (future)
 need to be sure to use all available information that might depend on
detector design  develop existing flavour tagging tools further
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 1
Software tools
 Simulation a Grande Vitesse (SGV) version 2.31 by M. Berggren
http://berggren.home.cern.ch/berggren/sgv.html
• flexible, well-tested fast simulation, originated from DELPHI
• interfaced to PYTHIA version 6.1.52
• JADE algorithm (y-cut 0.04) for jet finding
• vertex finding: ZVTOP by D. Jackson
 future: Java analysis studio, version 3 (JAS3), by T. Johnson
http://jas.freehep.org/jas3/index.html
• object oriented software being developed at SLAC
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 2
Motivation and selection cuts
 discerning charged b jets (
) from charged b-bar jets (
)
can aid background suppression in multi-jet events, e.g. Higgs-decay,
allow measuring parity of Higgs boson, CP asymmetries in SUSY processes
 study monoenergetic jets from
at
with
• exactly 2 jets found
• jets sufficiently back-to-back:
• jets well contained in detector:
thrust angle within
• B hadron initiating jet can be
unambiguously identified:
 at generator level, 40% of jets stem
from charged hadrons
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 3
Vertex finding and track attachment
goal: find all stable B decay chain tracks -- procedure:
 ‘mask’ all KS, L decay products: removed at track selection step using MC information;
expected to be reliably identified in the reconstruction and recovered at a later stage
 run ZVTOP to find vertex candidates, require tracks to have d0 < 1.0 cm
 seed vertex (candidate furthest from IP) used to
define the vertex axis
 consider all tracks initially passed to ZVTOP and
assign those to B decay chain, which at point of
closest approach to the vertex axis have
• T < 1 mm: cleaning cut, only small effect
• (L/D)min < L/D < 2.5: main cut,
where (L/D)min is optimised for each
detector configuration independently
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 4
Vertex charge and Pt-corrected mass
vertex charge Qvtx and MPt determined from tracks assigned to B decay chain:
 sum of charges of these tracks: Qsum
 reconstructed vertex charge
 from sum of four-momenta: Pvtx, Mvtx
 apply kinematic correction (partly
corrects for missing neutral particles):
 MPt used as ‘b tag’ parameter
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 5
Improvement since LCWS – 1
comparison of reconstructed Qsum distributions for the different generator level charges
_
LCWS
MC: B
new
MC: neutral
B hadrons
MC: B
Sonja Hillert, University of Oxford
+
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 6
Definition of efficiency and purity
efficiency
with
: # (jets) with LDec > 300 mm and
MPt,cut varied between 0 and 4.5 GeV
:
# (jets) in event selection cuts
Purity for discerning b from b-bar:
: # (jets) from b-quark with
+ # (jets) from b-bar quark with
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 7
Improvement since LCWS – 2
 differences in the procedure:
in new result
• KS, L decay products
are masked
• L/D cut for track assignment
has been optimised
 large increase in purity and
efficiency, especially at large
efficiencies:
at eb ~ 70% (MPt > 2.0 GeV):
Deb = 5%, DP(b) = 4%
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 8
Detector configurations considered
Standard detector characterised by:
 good angular coverage (cos q = 0.96)
 proximity to IP, large lever arm:
5 layers, radii from 15 mm to 60 mm
 minimal layer thickness ( 0.064 % X0 )
to minimise multiple scattering
 excellent point resolution (3.5 mm)
standard detector is compared to
 degraded detector: beam pipe radius 25 mm, inner layer removed
factor 2 worse point resolution
 improved detector: factor 4 less material ( factor 2 less multiple scattering)
factor 2 better point resolution
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 9
Optimisation of L/D cut
 maximise P(b) and efficiency
cut value chosen
note: fragmentation tracks (MC level) assigned to B decay chain may result in
wrong Qvtx and MPt > 5.5 GeV  reason for drop of P(b) and eb near 0
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 10
Purity and efficiency for optimised L/D procedure
conclusions:
 purity flat out to efficiency of ~ 70%
for standard detector
 significant detector dependence:
• at lower eb detectors differ
mainly in efficiency:
at eb = 20%: Deb = 5%
• at higher eb they differ also
significantly in purity:
at eb = 70% (MPt > 2.0 GeV):
Deb = 6%, DP(b) = 2%
 result underlines the need for a small beam pipe radius
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 11
Future plans
 replace one-dimensional L/D cut by a more powerful neural net approach
similar to what was formerly used at SLD ( T. Wright’s thesis, SLAC-Report-602 ):
• five input variables: T, L, L/D, angle of track to vertex axis,
normalised 3D impact parameter of track to IP
• NN configuration: 5 inputs, 6 nodes in hidden layer, 1 output node
• neural network package has already been developed (D. Bailey)
 develop improved NN-based flavour-tagging procedure
 extend study to other jet energies
 study impact of vertex charge reconstruction on
analysis of benchmark physics processes,
such as Higgs physics, e.g. Higgs parity, and CP asymmetries in SUSY
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
p. 12
Additional Material ~ Additional Material ~ Additional Material
 main track attachment cut on L/D needs to be optimised for each detector
separately to obtain fair comparison
 otherwise difference in performance between detectors may decrease or increase:
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
Additional Material ~ Additional Material ~ Additional Material
Optimisation of track attachment cut
p. 13
Additional Material ~ Additional Material ~ Additional Material
 track selection cuts currently fixed; too little known
about global detector to relax them at present
 consider tracks from B decay chain that are not assigned
to B decay chain in the reconstruction (“missed tracks”)
 upper plot:
fraction of wrong charge vertices with missed tracks
vs efficiency
 lower plot: fraction of missed tracks, which are missed
because of the track selection:
for standard detector below 20%
 conclusion: track selection is not a limiting factor;
there is still much room for improvement
Sonja Hillert, University of Oxford
2nd ECFA LC workshop, Durham, 2nd September 2004
Additional Material ~ Additional Material ~ Additional Material
Effect of track selection cuts
p. 14