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ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 1
The Vertex Detector in the LDC Concept
Sonja Hillert (Oxford)
on behalf of the LCFI collaboration
A Vertex Detector for the ILC
Workshop at Ringberg Castle ~ 28 – 31 May 2006
Vertex detector requirements and differences in the 3 concepts
Vertex charge reconstruction for performance evaluation
Comparison of different detector designs
Effects not yet included in simulation
The LCFI Vertex Package – Plans and Status
Summary
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 2
At the ILC, the most stringent requirements on the vertex detector are common
to all the concepts.
In part they are given by the machine:
• readout at least at effectively 50 s intervals
• excellent point resolution / small pixel size
• adequate radiation hardness
Other requirements result from physics needs:
• hermeticity / good angular coverage (to cos = 0.96)
• proximity to the IP / small radius of innermost detector layer
• minimal material amount, as far as possible evenly distributed over layer
• low power consumption to permit gas cooling
Requirements for the ILC vertex detector
337 ns
2820x
0.2 s
0.95 ms Beam bunch structure at ILC
Multiple collisions
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 3
Detector concepts differ in overall size and therefore in B-field and
inner radius of the vertex detector.
Other differences are not necessarily related to global detector design:
• SiD: short barrel and forward disks
• LDC: long barrel and tracking from additional silicon detectors in forward region
Differences between vertex detectors of the 3 concepts
250 mm
15 mm60 mm
100 mm
LDC vertex detector
SiD vertex detector
design concept
(Norman Graf)
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 4
Vertex charge reconstruction
in the 40% of cases where b quark hadronises to charged B-hadron quark sign can
be determined by vertex charge – good performance indicator for vertex detector
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
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
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 5
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
(jet cone is broader and
more low momentum tracks
multiple scattering more severe)
results obtained with SGV fast MC
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 6
Comparing detector with different inner layer radii250 mm
15 mm
60
mm
100 mm
8 mm25 mm
e+e- BEAM
importance of small beam pipe radius demonstrated
by recent study of probability 0 for reconstructing
neutral hadron as charged (Snowmass 2005)
can be translated into ‘luminosity factor’: factor
by which luminosity would have to be varied to
reach same statistical significance as for standard
detector (plot: processes requiring vtx chg for 2 jets)
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 7
Comparison with SiD & GLD vertex detectors: Results
material at the end of SiD short barrel staves compromises performance at large cos
GLD performance affected by larger beam pipe radius compared to LDC detector
50
(August 05) (August 05)
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 8
Sensitivity to other parameters
since vertex charge performance is
sensitive to multiple scattering need to
keep layer thickness small (target 0.1 % X0)
also strong dependence on momentum cut
(track selection) – this depends critically
on tracking performance:
• track finding capability
• background rates
• linking across subdetector boundaries
should push all these parameters to their limits, as all these effects will eventually
add up in the real detector
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 9
A future optimised full MC simulation and reconstruction will need to reliably model
• pair background
• cluster shapes (different for signal and pair background: also transverse component)
• time / position correlation (different for different
sensor technologies)
• dependence on any non-uniformities in
material distribution, effect of conical beam-pipe
• pattern recognition, leading to detailed understanding
of tracking precision, track finding inefficiency,
incorrect hit association, fake tracks
• non-Gaussian tails on track errors
• track linking between sub-detectors
• reconstruction of Ks and
ILC community will need to look at all these items in detail to arrive at reliable conclusions
regarding detector performance.
Effects not yet included
T Woolliscroft
(Liverpool U)
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 10
The LCFI Vertex Package
The LCFI collaboration is working hard to provide a C++ based Vertex Package comprising
the vertex finder ZVTOP, NN-based flavour tagging and vertex charge reconstruction.
To make our code available to the community and to profit from future improvements of
the ILC software frameworks, this package will be provided in the form of MARLIN processors
with input / output in LCIO format.
For part of the output, we have proposed the introduction of a new Vertex class into LCIO:
currently under discussion in the LCIO developers group, for more information see the
ILC forum at http://forum.linearcollider.org/index.php
Performance will be checked using SGV. Parts for which FORTRAN code exists will be
cross-checked against the FORTRAN version. Improvements of the code are guided by
comparison to MC information.
Coding / validation is making good progress – release envisaged this summer
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 11
interface SGV to
internal format
interface LCIO to
internal formatinput to LCFI Vertex Package
output of LCFI Vertex Packageinterface internal
format to SGV
interface internal
format to LCIO
ZVRES ZVKIN
ZVTOP:
vertex information
track attachment
assuming c jet
track attachment
assuming b jet
track attachment
for flavour tag
find vertex-
dependent
flavour tag
inputs
find vertex charge
find vertex-
independent
flavour tag
inputs
neural net flavour tag
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 12
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
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 13
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
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 14
tanh (MPt / 5 GeV)
joint probability
uds
c
b
Flavour tag Vertex package will provide flavour tag procedure developed by R. Hawkings et al
(LC-PHSM-2000-021) and recently used by K. Desch / Th. Kuhl as default
NN-input variables used:
• if secondary vertex found: MPt , momentum
of secondary vertex, and its decay length and
decay length significance
• if only primary vertex found: momentum and
impact parameter significance in R- and z for the
two most-significant tracks in the jet
• in both cases: joint probability in R- and z (estimator of
probability for all tracks to originate from primary vertex)
will be flexible enough to permit user further tuning of the input variables for the neural net,
and of the NN-architecture (number and type of nodes) and training algorithm
b
c
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 15
Status of C++ ZVTOP development ZVRES branch: coding completed, validation ongoing
left: comparison of decay length reconstructed by C++ to the FORTRAN value
right: comparison of C++ reconstructed to true track origin (iso = isolated tracks from ZVTOP)
Ben Jeffery (Oxford U) MC track origin
2 vertices 3 vertices
pri sec iso pri sec ter iso
Primary 98.6 0.4 1.1 98.1 1.1 0.0 0.8
B decay 8.8 75.6 15.6 2.3 90.1 3.7 3.9
D decay 2.1 80.5 17.4 0.5 17.8 77.4 4.4
Mark Grimes (Bristol U)
coding of ZVKIN branch ongoing, determination of ghost track direction complete
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 16
Towards completion of the Vertex Package
b
c
c (b bkgr)c
c (b bkgr)
b
Z peak ECM = 500 GeV
test of full chain of C++ ZVTOP with FORTRAN flavour tag and vertex charge imminent
pure FORTRAN results (from SGV) show below:
C++ code for calculation of inputs for flavour tag being written
Vertex charge reconstruction for c-jets under development
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 17
Summary and outlook
Stringent requirements from the accelerator on the one hand and from ILC physics on the
other hand make the design of an ILC vertex detector very demanding.
Careful modelling of all relevant aspects is necessary to arrive at reliable conclusions
regarding requirements and performance achieved.
Relaxing design targets prematurely could result in performance being severely compromised –
targets for inner layer radius, layer thickness and momentum cutoff should be maintained.
Realistic assessment of vertex detector performance and comparison of designs will be helped
by the LCFI vertex package, providing vertex finding, flavour tag and vertex charge
reconstruction – due to be released this summer.
Output of our code crucially relies on the quality of the input – precise reconstruction of
low momentum tracks particularly challenging, likely to influence design of FTD system.
ILC community still has a long way to go for many aspects of the reconstruction.
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 18
Additional Material
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 19
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
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 20
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
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 21
Vertex charge reconstruction
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 22
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
ILC VTX workshop at Ringberg, 29th May 2006 Sonja Hillert (Oxford) p. 23
Suggestion for a new Vertex class in LCIOa new vertex class could look like this:
+~Vertex()
+getMomentum() : const float*
+getMass() : float
+getCharge() : float
+getPosition() : const FloatVec&
+getCovMatrix() : const FloatVec&
+getChi2() : float
+getProbability() : float
+getDistanceToPreviousVertex() : float
+getErrorDistanceToPreviousVertex() : float
+getParameters() : const LCParameters&
+getTracks() : const TrackVec&
+addTrack() : void
+getPreviousVertex() : Vertex&