Transcript The Fermilab Accelerator Complex

The CDF Online
Silicon Vertex Tracker
I. Fiori
INFN & University of Padova
7th International Conference on Advanced Technologies
and Particle Physics
Villa Olmo (Como) - October 15-19 2001
Outline
• Tevatron and CDF upgrades for Run II
• The SiliconVertexTracker :
- Physics motivation
- Working principle
- Architecture
- Track finding and fitting technique
• SVT performance during summer 2001 Tevatron run
• Conclusions
- various items …
The Tevatron and CDF Upgrades for Run II
Time Of Flight
TIB = 396 (3636) - 132 (104 104) ns
L = 1 - 2 * 1032 cm-2 s-1
ECM = 2.0 TeV
• New Tracking System
• COT
• L00 +SVXII + ISL
• 3-D info
• Extended tracking to forward region
• New Trigger
CDF Trigger in run II
New for CDF run II at Level 1:
detector elements
CAL
COT
MUON
XFT
MUON
PRIM.
SVX
CES
• 2-D COT tracks available (XFT)
• Fast SVXII readout (105 channels) : ~ 2.5 s
XCES
XTRP
L1
CAL
L1
TRACK
SVT
L1
MUON
d , Pt ,f
GLOBAL
L1
Track reconstruction
with “offline” resolution
at Level 2
d = 35 m (at 2 GeV/c)
Fast ! ~10 s (50 KHz L1 accept rate)
SVT
L2
CAL
GLOBAL
LEVEL 2
TSI/CLK
• 2-D tracks (drop SVXII stereo info)
• Pt > 2 GeV/c
• parallelized design : 12 -slices (30°each)
which reflects SVX II geometry
Why do we need the Silicon Vertex Tracker ?
The Tevatron produces a lot of B events
 bb =
11
50 b ( ~ 10 events in Run II )
But hidden in 1,000 more background
 pp =
We need a Trigger
50 mb
“The Old Way”
- Trigger on leptonic B decays
- SVX tracks used Offline to
reconstruct Secondary Vertices
Problems
- Low B.R. for leptonic modes
- Detector acceptance limited
“The New way”
- SVT: trigger on displaced tracks
Primary
Vertex
B
B
<d>100 m
Secondary
Vertex
(Nonetheless, many results from CDF)
Improved B physics capabilities: fully hadronic B decays
SVT: Silicon Vertex Tracker
(Chicago-Geneva-Pisa-Roma-Trieste)
SVT receives :
• COT tracks from Level 1 XFT (f ,Pt )
• Digitized pulse height in SVXII strips
Performs tracking in a
two-stage process:
1. Pattern recogniton:
SVX II geometry :
• 12 -slices (30°each) “wedges”
• 6 modules in z (“semi-barrels”)
Reflected in SVT architecture
Search “candidate” tracks (ROADS)
@ low resolution
2. Track fitting:
Reassociate full resolution hits to
roads and fits 2-D track parameters
(d, f, Pt ) using a linearized
algorithm
The SVT Algorithm (Step I)
Fast pattern Recognition
• Hardware Implemented via the
Associative Memory Chip
Single Hit
“XFT layer”
Road
(full custom - INFN Pisa) :
– Receives the list of hit coordinates
Si Layer4 – Compares each hit with all the
Candidate Roads in memory in
parallel
Si Layer3
– Selects Roads with at least 1 hit in
each SuperStrip (found roads)
Si Layer2
– Outputs the list of found roads
• FAST! : pattern rec. is complete as
Si Layer1
SuperStrip (bin)
soon as the last hit of the event is read
• 32.000 roads for each 30° slice
• ~250 micron SuperStrips
• > 95% coverage for Pt >2 GeV
The SVT Algorithm (Step II)
Track Fitting
When the track is confined to a road, fitting becomes easy ! :
• Linear expansion of Parameters in the hit positions Xi :
Pi = Fi * Xi + Qi ( Pi = pt , f , d , c1 , c2 , c3 )
• … then refer them to the ROAD boundary :
P0i + dPi = Fi * ( X0i + dXi ) + Qi
P0i = Fi * X0i + Qi
Road boundary
P0i
XFT layer
dX5 X05
SVX layer 4
dX4
X04
SVX layer 3
dX3
X03
dX2
X02
SVX layer 2
• Fi and P0 i coefficients are calculated in
advance (using detector geometry) and
SVX layer 1
stored in RAM
• the task reduces to compute the scalar products (FPGA) :
dPi = Fi * dXi
TRACK
dX1 X01
SuperStrip 250 m
The SVT Boards
AM Sequencer
Super
Strip
AM Board
Hit Finder
Hits
Detector
Data
Roads
Matching
Patterns
Roads +
Corresponding
Hits
L2 CPU
Hit Buffer
Tracks +
Corresponding Hits
Track Fitter
SVT racks
SVT Performance (I)
d -  correlation
 Sinusoidal shape is the
effect of beam displacement
from origin of nominal coordinates
28 Aug 2001 data, c2<40 no Pt cut
SVX only
d = X0·sin () - Y0·cos ()
track
(X0,Y0)

d
 Can find the beam consistently
in all wedges even using only SVX
X0 = 0.0153 cm
Y0 = 0.3872 cm
Online fit of X-Y Beam position
Run 128449 - October 6 2001
Can subtract beam offset
online :
I.P. with respect to
beam position
(online) :
Independent fit on each
SVXII z -barrel (6)
Beam is tilt is :
dX/dZ  840 rad
dY/dZ  -500 rad
d = 69 m
Understanding the width of d distribution
d2 = B2 + res2
beam size
i.p. resolution
• Present (online)
• Correct relative wedge misalignment
• Correct for d and  non-linearity
• Correct internal (detector layers) alignment
• Correct offline for beam z misalignment
• GOAL : SVXII + COT offline tracks (1wedge)
d
69
63
57
55
48
45
( m )
Need to have
beam aligned
Effect of non-linearity :
Because of the linear
approximation done by Track
Fitters, SVT measures :
• dSVT  d /cos (f)
 fSVT  tan (f)
 Becomes important for large
beam offset
 Can correct it making the online
beam position routine fit a straight
line on each wedge
Expected SVT performance
From SVT TDR (’96) :
SVT simulation using RunI data
Commissioning Run - Nov. 2000 :
SVT simulation with SVX hits +
COT Offline
 ~ 45 m
 ~ 45 m
SVT Performance (II)
correlation with offline tracks
:
SVT – COT
Curvature : SVT - COT
 = 2 mrad
 = 0.33
·10-4
cm-1
d : SVT - COT
 = 21m
Intrinsic transverse beam width
Can extract B from the
correlation between impact
parameters of track pairs
• If the beam spot is circular :
<d1 · d2> = B2 · cos 
B = 40 m
• But at present the beam is tilt
(beam spot is an ellipsis) :
short = 35 m
long = 42 m
Cut Online on chi2, Pt, d, N.tracks
L2 :
• N. SVT tracks > 2
• | d | > 50 m
• c2 < 25
• Pt > 2 GeV/c
• L1 prerequisite :
2 XFT tracks
Trigger selection
successfully
implemented !
 About 200 nb-1 of
data collected with this
trigger where we can
start looking for B’s !
Conclusions
• SVT performs fast and accurate 2-D track reconstruction
(is part of L2 trigger of CDF II)
Tracking is performed in two stages:
- pattern recognition
- high precision track fitting
• Installation completed Autumn 2000
• Thoroughly tested on real Tevatron data April – September 2001
• Lots of tests and fine tuning done during the summer
• Many things understood, will be implemented after the October November Tevatron shutdown
• Performance already close to design !