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Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 1
Vertex Detector Contribution
to ILC Physics Analyses
Sonja Hillert (Oxford)
on behalf of the LCFI collaboration
3rd 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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 2
SM
e+ e-
t
t
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 3
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 4
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 5
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 6
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 mass and vertex momentum
Vertex Mass (GeV/c2)
uds c b
SLD
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 7
Neural net based flavour tagging procedureTo 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: MPt , 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)
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 8
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 9
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 10
Comparing detectors with different beam pipe radius
Compare 3 detectors with different inner layer radius:
25 mm8 mm
250 mm
15 mm60 mm
100 mm
standard detector: large Rbp detector: small Rbp detector:
Rbp = 15 mm, thickness 0.4 mm
innermost layer at 15.5 mm;
layer thickness 0.1 % X0
(same for all detectors)
Rbp = 25 mm, thickness 1 mm
innermost layer removed
new inner layer at 25.5mm
has full length of 250 mm
Rbp = 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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 11
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 12
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 EJet = 25 GeV, 2-jet luminosity factor of 2.14 (shown in plot) is reduced to 1.65 – 1.85,
depending on assumed background
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 13
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 14
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 15
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 16
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 17
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
(don‘t 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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 18
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.
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 19
Additional Material
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 20
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
cbar
bbar
flavour
identification
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 21
Track description by Gaussian probability tubes
D. Jackson, Seminar on ZVTOP, Vienna 2004
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 22
‘Straightness’ of B decay chain (SLD)
D. Jackson, Seminar on ZVTOP, Vienna 2004
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 23
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 24
quark b
hadron
B- B0
vtx charge
X-- X- X0 X- X0 X+
Interpretation
b
0.4(1-pm)+0.30+0.30
01-0pm pm1-2pm
0
b-bar
B0-bar B+
X- X0 X+ X0 X+ X++
b-bar
0.4(1-pm)+0.30+0.30
b quark sign selection
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 25
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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 26
Position of vertices wrt detector layers: percentages
CM energy (GeV) 50 100 200 500
inside beam pipe 99.99 % 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 pipe 95.39 % 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
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 27
Vertex charge reconstruction
Vertex Detector Contribution to ILC Physics Analyses, 16th November 2005 Sonja Hillert (Oxford) p. 28
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
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