the cdf online silicon vertex tracker i. fiori infn & university of padova 7th international...
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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
• 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
- various items …
• Conclusions
Outline
nsL = 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
Time Of Flight
The Tevatron and CDF Upgrades for Run II
CAL COT MUON SVX CES
XFT XCESMUONPRIM.
XTRP
SVTL2
CAL
L1CAL
GLOBALL1
L1MUON
L1TRACK
GLOBALLEVEL 2
TSI/CLK
detector elements New for CDF run II at Level 1:
• 2-D COT tracks available (XFT)• Fast SVXII readout (105 channels) : ~ 2.5 s
Track reconstruction with “offline” resolutionat Level 2
SVTSVT
• 2-D tracks (drop SVXII stereo info)• Pt > 2 GeV/c• parallelized design : 12 -slices (30°each)
which reflects SVX II geometry
Fast ! ~10 s (50 KHz L1 accept rate)
d = 35 m (at 2 GeV/c)d , Pt ,
CDF Trigger in run II
Why do we need the Silicon Vertex Tracker ?
The Tevatron produces a lot of B events
b ( ~ 10 events in Run II )
11bb
But hidden in 1,000 more background
mb pp
We need a Trigger
“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(Nonetheless, many results from CDF)
“The New way” - SVT: trigger on displaced tracks
Improved B physics capabilities: fully hadronic B decays
B
Primary Vertex
Secondary Vertex
<d>100 m
c45
0m
B
SVT: Silicon Vertex Tracker (Chicago-Geneva-Pisa-Roma-Trieste)
SVT receives : • COT tracks from Level 1 XFT ( ,Pt ) • Digitized pulse height in SVXII strips
Performs tracking in a two-stage process:
1. Pattern recogniton: Search “candidate” tracks (ROADS) @ low resolution
2. Track fitting: Reassociate full resolution hits to roads and fits 2-D track parameters (d, , Pt ) using a linearized algorithm
SVX II geometry :• 12 -slices (30°each) “wedges”• 6 modules in z (“semi-barrels”)
Reflected in SVT architecture
Road
The SVT Algorithm (Step I)
Fast pattern Recognition• Hardware Implemented via the
Associative Memory Chip (full custom - INFN Pisa) :
– Receives the list of hit coordinates– Compares each hit with all the
Candidate Roads in memory in parallel
– Selects Roads with at least 1 hit in each SuperStrip (found roads)
– Outputs the list of found roads
• FAST! : pattern rec. is complete as 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
Single Hit
SuperStrip (bin)
“XFT layer”
Si Layer1
Si Layer2
Si Layer3
Si Layer4
The SVT Algorithm (Step II)
Track Fitting When the track is confined to a road, fitting becomes easy ! :
Pi = Fi * Xi + Qi ( Pi = pt , , d )
P0i + Pi = Fi * ( X0i + Xi ) + Qi
P0i = Fi * X0i + Qi
Pi = Fi * Xi
• … then refer them to the ROAD boundary :
Road boundary
TRACK
P0i
X5XFT layer X05
SuperStrip 250 m
• the task reduces to compute the scalar products (FPGA) :
• Fi and P0 i coefficients are calculated in advance (using detector geometry) and stored in RAM
• Linear expansion of Parameters in the hit positions Xi :
SVX layer 4 X4 X04
SVX layer 3 X3 X03
SVX layer 2 X2 X02
SVX layer 1 X1 X01
The SVT Boards
Hit Finder
AM Sequencer AM Board
Hit Buffer
Track Fitter
Detector Data
Hits
SuperStrip
Matching
PatternsRoads
Roads + Corresponding
Hits
L2 CPU Tracks +
Corresponding Hits
SVT racks
Sinusoidal shape is the effect of beam displacement from origin of nominal coordinates
d = X0·sin () - Y0·cos ()
SVX only
(X0,Y0)
d
28 Aug 2001 data, 2<40 no Pt cut
Can find the beam consistently in all wedges even using only SVX
X0 = 0.0153 cmY0 = 0.3872 cm
track
SVT Performance (I)d - correlation
I.P. with respect to beam position (online) :
Independent fit on each SVXII z -barrel (6)
Can subtract beam offset online :
Run 128449 - October 6 2001
d = 69 mBeam is tilt is : dX/dZ 840 raddY/dZ -500 rad
Online fit of X-Y Beam position
d2 = B
2 + res2
• Present (online) 69• Correct relative wedge misalignment 63• Correct for d and non-linearity 57• Correct internal (detector layers) alignment 55• Correct offline for beam z misalignment 48• GOAL : SVXII + COT offline tracks (1wedge) 45
beam size i.p. resolution d ( m )
Can be implemented soon
Need to have beam aligned
Understanding the width of d distribution
Because of the linear approximation done by Track Fitters, SVT measures :• dSVT d /cos ()
SVT tan ()
Becomes important for large beam offset
Can correct it making the online beam position routine fit a straight line on each wedge
Effect of non-linearity :
Commissioning Run - Nov. 2000 : SVT simulation with SVX hits + COT Offline
From SVT TDR (’96) : SVT simulation using RunI data
~ 45 m
~ 45 m
Expected SVT performance
= 21m
= 0.33 ·10-4 cm
= 2 mrad
: SVT – COT
Curvature : SVT - COT
d : SVT - COT
SVT Performance (II)correlation with offline
tracks
<d1 · d2> = B2 · cos
Can extract B from the
correlation between impact parameters of track pairs
• If the beam spot is circular :
B = 40 m
• But at present the beam is tilt (beam spot is an ellipsis) :
short = 35 m
long = 42 m
Intrinsic transverse beam width
L2 : • N. SVT tracks > 2• | d | > 50 m
• 2 < 25• Pt > 2 GeV/c
• L1 prerequisite : 2 XFT tracksTrigger selection successfully implemented !
About 200 nb-1 of data collected with this trigger where we can start looking for B’s !
Cut Online on chi2, Pt, d, N.tracks
• 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 !
Conclusions
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