Trigger Philosophy
Triggering job is like scooping out drops of
water in a fast moving stream:
- You better be fast (if you want to
scoop enough to feed 350 physicists)
However, we don’t want just any drop of water,
we want THE drop of water:
- Trigger needs to be fast and right
(once that precious Higgs event is
missed, it’s gone forever)
Introduction to CDF L1/L2 Trigger
Cheng-Ju Lin
Fermilab
Nov 18, 2004
CDF Detector Lecture Series
Basic Concept (Buffering and Filtering)
• Need sufficient buffer while filtering
(overflow
deadtime)
Reservoir
• Volume is reduced at each stage which
allows more refined filtering at
subsequent stages
• If volume after filtering still exceeds the
capacity of the subsequent stage:
Filters
(1) Add filters (tighter trigger cuts)
(2) tighten the faucet (prescale)
(3) buy better filters (upgrade)
• Bottomline: store as many golden drops
as possible
CDF Implementation
• CDF has implemented a 3-tier trigger
• Level-1 is a synchronous hardware trigger
- Can process one event every 132ns
- L1 decision always occurs at a fixed
time (~5µs after beam collision)
- Input rate = 1.7MHz (396ns 36x36 bunches)
L1A rate ~ 20KHz (limited by L2)
• Level-2 is a combination of hardware and
software trigger (asynchronous)
- Average Level-2 processing time is ~30µs
- L2A rate ~350Hz (limited by event-builder)
• Level-3 is purely a software trigger
- Massive PC farm
- L3A rate ~ 75Hz (limited by tape writing)
• Data reduction rate (L1+L2+L3)
1 : 23000
RunIIa L1, L2 Trigger System
• Various trigger subsystem generates
primitives that we can “cut” on
• Available trigger primitives are:
At L1:
- XFT track (phi, pT),
- Electron (EM+HAD/EM+XFT),
- Photon (EM+HAD/EM),
- Jet (EM+HAD)
- Muon (Muon tower + XFT),
- Missing Et, SumEt,
At L2:
- SVT (displaced track, d0)
- Jet cluster
- Isolated cluster
- EM ShowerMax
Note: CLC and TOF triggers are not shown here
Closer Look at XFT
• XFT uses COT hits from four axial SLs to reconstruct tracks
• Track finding in done in stages:
(1) Digitize raw COT hits (TDC+XTC mezzanine cards)
(2) Form track segments form the digitized hits (XFT finder boards)
(3) Link segments into track (XFT linker boards)
XFT segments
XFT Hit Digitization and Segment Finding
• Each raw COT hit is classified as either prompt or delay hits by the XTC card
Prompt and delay hits provide crude (two bin) timing information on the hits
• The digitized hits are used by the finder board to find segment in a given SL.
Segments are found by comparing hits patterns to a list of patterns (masks)
• When a segment is found, Finderboard
reports the position of the segment at the
center of the 12-wire cell (for the inner
two axial SLs)
• For the outer two layers, the Finderboard
also reports the slope of the pixel
(low pT+, low pT-, high pT)
Track Linking
• Similar to segment finding where the Finderboard compares hit pattern
to a list of masks, Linker board compares the pixels in all
four layers to a list of valid pixel patterns (or “roads”) to find tracks.
• A valid track is required to match pixels locations as well as the
slope of the segments at the outer two SLs.
Big Trigger Issue
• Constant battle to keep the trigger alive
• Luminosity is both our friend and foe:
Trigger rate can be parameterized as:
Rate = σ + A + B + C
Physics
Cross section
2
+…
Background
Growth terms
• A deadtime of 50% is equivalent to
a prescale of 2 across the board !!!
• Trigger has been kept alive by a group
of dedicated people with clever ideas
Some Clever Ideas (Prescale)
• What’s so clever about prescale?
Nothing really, if you just impose a fixed prescale
• Fixed prescales are generally imposed to keep the system
alive (low deadtime) at high luminosity running
• But as luminosity falls during a run, trigger resources are freed up.
If prescales are kept the same
under-utilize the trigger bandwidth
• Clever idea # 1: Dynamic prescale (DPS)!!!
Based on the availability of the trigger
bandwidth, automatically reduce the prescales
as the luminosity falls.
Dynamic prescales up and running in late 2002.
More on Prescales
• But it has one un-desirable
feature. Could take hours
between prescale changes.
The issue is that we are dealing
with integer prescale values
L1Accept Rate (Hz)
• DPS worked as advertised J
DPS Prescale change
~1hr
• Clever idea # 2: Fractional prescale (FPS)!!!
As the name suggests, with FPS, the prescale
value is not limited to integer values
FPS implemented using XTRP (2-track board) with an 8-bit cycling counter
It made it into the default trigger table shortly before 2004 shutdown
Some More Prescale Stuff
• FPS does a great job leveling the trigger rate. But the fun does not stop here
• What about clever idea # 3: Uber Prescale (UPS) !!!
No numerical prescale value in UPS
Effectively adjusting L1 prescale
at micro-sec scale based on
feedback from the buffer usage
• Clever idea # 4:
Any more clever ideas?
L1Accept Rate (Hz)
A event is allowed to pass L1 if there are three or more
L2 buffers empty. Squeezing events through
when the system is least busy.
UPS kicks in
FPS
FPS=1
Looking Ahead
• Several significant trigger upgrades are in the pipeline:
- XFT stereo upgrade
- L2 global decision crate upgrade (Pulsar)
- SVT upgrade
• XFT Stereo upgrade:
- XFT fake rate has been a major problem for the XFT system
- Exacerbated by the higher than expected COT occupancy
- Was designed for 132nsec operation for RunIIb
With 396
~10 interactions during high luminosity running (current XFT
cannot handle this environment)
Min Bias with Luminosity ~ 3E31
Min Bias with Luminosity ~ 4E32
XFT Upgrade Configuration
• Adding 2 or 3 outer stereo layers information at L1 and L2
,
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Level-2 Upgrade
• Recall that the ceiling of L1A rate is limited by how fast we can
clear events out of L2 (L2 processing time)
1
Setup
5
Digitize
10
15
Time Since L1A (µsec)
20
25
30
Silicon
SVX R/O
r-phi r-z
ISL R/O
R/O
L00 R/O
R/O (16us Read-All)
SVT
SVT Processing
Load in α
CF, µ , track…Process and Load
Process in α
35
40
45
Unpack, Algorithms, TS Handshake
(Note: the timing diagram is old but good enough for illustration)
• SVT processing and the L2 algorithm are the two biggest
chuncks of L2 processing time
• Will need to improve both to get significant performance increase
at L2. Hence, the motivation for L2 global decision crate and
SVT upgrades (see Roberto’s talk for SVT upgrade).
Level-2 Decision Crate Upgrade
• Various L2 interface boards (for bringing
XFT, L1 muon, L1 CAL, ShowerMax,
SVT, etc… data into L2) will be
replaced by Pulsar boards
• Common interface board means easier
to maintain and debug the system.
(Pulsar boards will also be used for the
SVT upgrade and for bringing the
stereo XFT data to L2)
• Each Pulsar board has three powerful
FPGA onboard that can be used to
preprocess data (eg. Zero-suppression)
• L2 algorithm will be run in a commodity
linux box. Can easily upgrade the PC
when faster processor is available.
RunIIb Level-2 Global Decision Crate Configuration
L1 muon
Muon
Merger
L1 XTRP
L1 trigger
L2 CAL
Cluster
TS
L2A/R
SLINK
(CLIST/Iso)
Linux
PC
SVT
L2toTS
PreFred
SVT
ShowMax
(RECES)
Electron
Merger
• Merger Pulsar merges all non-SVT
data and forward the packet to the PC
• Due to the long SVT latency,
SVT has its own link to the PC
• Expect to integrate the system into
CDF operation by March 2005
Summary
• Trigger is a key element in a ppbar collider experiment.
It’s also a very challenging aspect of the experiment.
• It’s a never ending crusade to keep the trigger running (a.k.a. keeping
the experiment alive).
• We have made tremendous progress in the past few years to meet the
the challenges. Many thanks to people who are dedicated to the
trigger effort. We will need more brain power and clever ideas to face
the impending high luminosity period.
• Given the time constraint, we will only be able to selectively cover a
few trigger topics in this lecture. If you want to learn more, get involved.
• “Ask not what the trigger can do for you, ask what you can do for
the trigger system !!!”