Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 1
The Level-1 Trigger of the CMS Experiment
at LHCDesign, challenges and performance
Manfred Jeitler
MGU, 11 July 2011Московский государственный университетIn
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Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 2
The Large Hadron Collider
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 3
LHC p-p CollisionsOperations 2010-2011
2010: √s = 7 TeV – Peak Instantaneous
Luminosity:> 2×1032 cm-2s-1
2011: √s = 7 TeV– Peak Instantaneous
Luminosity:> 1.25 ×1033 cm-2s-1
– And increasing!
– Current Maximum of ~1400 bunches/beam in LHC
– Possible to gain another factor of 4
2808
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 4
CMS Detector
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 5
first particle physics experiments needed no trigger
were looking for most frequent events
people observed all events and then saw which of them occurred at which frequency
what is a trigger?
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 6
later physicists started to look for rare events
– “frequent” events were known already
searching “good” events among thousands of “background” events was partly done by auxiliary staff
– “scanning girls” for bubble chamber photographs
what is a trigger?
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 7
Получше — в горшочек, похуже — в зобочек!
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 8
Higgs -> 4 +30 MinBias
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 9
find the needle!
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 10
due to the extremely small cross sections of processes now under investigation it is impossible to check all events “by hand”
– ~ 1013 background events to one signal event
it would not even be possible to record all data in computer memories
we need a fast, automated decision (“trigger”) if an event is to be recorded or not
X-s
ecti
on
X
-secti
on
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 11
detectors yielding electrical output signals allow to select events to be recorded by electronic devices
– thresholds (discriminators)
– logical combinations (AND, OR, NOT)
– delays
– available in commercial “modules”
– connections by cables (“LEMO” cables)
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 12
because of the enormous amounts of data at major modern experiments electronic processing by such individual modules is impractical
– too big
– too expensive
– too error-prone
– too long signal propagation times
use custom-made highly integrated electronic components (“chips”)
400 x 1 x
~ 10 logical operations / module ~ 40000 logical operations in one chip
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 13
example: trigger logic of the L1-trigger of the CMS experiment
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 14
When do we trigger ?
„bunch” structure of the LHC collider– „bunches” of particles– 40 MHz
» a bunch arrives every 25 ns » bunches are spaced at 7.5 meters from each other» bunch spacing of 125 ns for heavy-ion operation
at nominal luminosity of the LHC collider (1034 cm-2 s-1) one expects about 20 proton-proton interactions for each collision of two bunches for ATLAS and CMS– only a small fraction of these “bunch crossings” contains at least one collision
event which is potentially interesting for searching for “new physics”– in this case all information for this bunch crossing is recorded for subsequent
data analysis and background suppression
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 15
Event size (bytes)
trigger: first level high levelATLAS, CMS 40 MHz 100 kHz 200 Hz LHCb 40 MHz 1 MHz 2 kHzALICE 10 kHz 1 kHz 100 Hz
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 16
How do we trigger ?
use as much information about the event as possible – allows for the best separation of signal and background
– ideal case: “complete analysis” using all the data supplied by the detector
problem: at a rate of 40 MHz it is impossible to read out all detector data
– (at sensible cost)
have to take preliminary decision based on part of the event data only be quick
– in case of positive trigger decision all detector data must still be available
– the data are stored temporarily in a “pipeline” in the detector electronics» “short term memory” of the detector
» “ring buffer”
» in hardware, can only afford a few μs
how to reconcile these contradictory requirements ?write
read
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 17
multi-level trigger
first stage takes preliminary decision based on part of the data
– rate is already strongly reduced at this stage– ~1 GHz of events (= 40 MHz bunch crossings) ~100 kHz– only for these bunch crossings are all the detector data read out of the
pipelines– still it would not be possible (with reasonable effort and cost) to write
all these data to tape for subsequent analysis and permanent storage
the second stage can use all detector data and perform a “complete analysis” of events
– further reduction of rate: ~100 kHz ~100 Hz– only the events thus selected (twice filtered) are permanently recorded
40 MHz
100 kHz
300 Hz
to tape
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 18
How does the trigger actually select events ?
the first trigger stage has to process a limited amount of data within a very short time
– relatively simple algorithms– special electronic components
» ASICs (Application Specific Integrated Circuits)» FPGAs (Field Programmable Gate Arrays)
– something in between “hardware” and “software”: “firmware”» written in programming language (“VHDL”) and compiled » fast (uses always same number of clock cycles)» can be modified at any time when using FPGAs
the second stage (“High-Level Trigger”) has to use complex algorithms
– not time-critical any more (all detector data have already been retrieved)– uses a “computer farm” (large number of PCs)– programmed in high-level language (C++)
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 19
How does the trigger actually select events ?
the first trigger stage has to process a limited amount of data within a very short time
– relatively simple algorithms– special electronic components
» ASICs (Application Specific Integrated Circuits)» FPGAs (Field Programmable Gate Arrays)
– something in between “hardware” and “software”: “firmware”» written in programming language (“VHDL”) and compiled » fast (uses always same number of clock cycles)» can be modified at any time when using FPGAs
the second stage (“High-Level Trigger”) has to use complex algorithms
– not time-critical any more (all detector data have already been retrieved)– uses a “computer farm” (large number of PCs)– programmed in high-level language (C++)
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 20
signals used by the first-level trigger
muons– tracks– several types of detectors (different requirements for barrel and endcaps):– in ATLAS:
» RPC (Resistive Plate Chambers): barrel» TGC (“Thin Gap Chambers”): endcaps» not in trigger: MDT (“Monitored Drift Tubes”)
– in CMS:» DT (Drift Tubes): barrel» CSC (Cathode Strip Chambers): endcaps» RPC (Resistive Plate Chambers): barrel + endcaps
calorimeters– clusters– electrons, jets, transverse energy, missing transverse energy– electromagnetic calorimeter– hadron calorimeter
only in high-level trigger: tracker detectors – silicon strip and pixel detectors, in ATLAS also straw tubes – cannot be read out quickly enough
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 21
how to find muon tracks in the CMS solenoidal field?(Drift Tubes)
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 22
RPC ComparatorRPC Comparator
Pattern matching Pattern matching
barrel: 3 layers/4 or 4/6barrel: 3 layers/4 or 4/6
endcaps: 3/3endcaps: 3/3
how to find muon tracks in the CMS solenoidal field?(Resistive Plate Chambers)
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 23
Muon Trigger
Muon Trigger•3 muon detectors to ||<2.4
• Drift Tubes
• Track Segment ID and Track Finder
• Cathode Strip Chambers
• Track Segment ID and Track Finder
• Resistive Plate Chambers
• Pattern Matching
•4 candidates per subsystem to Global Muon Trigger
•Global Muon Trigger sorts, removes duplicates, 4 top candidates to Global Trigger
•Track building at 40 MHz
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 24
Global Muon Trigger
match & merge barrel:
DT-RPC endcap:
CSC-RPC cancel duplicates
overlap region:DT-CSC
sort by momentum and quality
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 25
calorimeter system
Hadronic calorimeter (HCAL)Hadronic calorimeter (HCAL)
Electromagnetic (ECAL)Electromagnetic (ECAL)
HFHF
Hadron Forward CalorimeterHadron Forward Calorimeter
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 26
calorimeter trigger
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 27
Photon and electron trigger objects
L1 e/gam L1 e/gam HLT e/gam HLT e/gam
L2 Step
Spatial matching of ECAL clusters with
e/γ candidates at L1
Superclusters are formed
ET cut applied
Calorimetric (ECAL+HCAL) isolation
L3 Photons
Tight track isolation
L3 Electrons
Electron track reconstruction
Spatial matching of ECAL cluster Spatial matching of ECAL cluster and pixel trackand pixel track
Loose track isolation in a “hollow” cone
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 28
e/and Jet Algorithms
4x4 Tower sums from RCT to GCTJet or ET
• 12x12 trig. tower ET sliding in 4x4 steps w/central 4x4 ET > others
: isolated narrow energy deposits• Energy spread outside veto pattern
sets veto• Jet if all 9 4x4 region vetoes off
GCT uses Tower sums used for ET,MET jets for HT, MHT
Electron (Hit Tower + Max)• 2-tower ET + Hit tower H/E • Hit tower 2x5-crystal strips >90% of ET in 5x5 (Fine Grain)
Isolated Electron (3x3 Tower)• Quiet neighbors: all towers
pass Fine Grain & H/E• One “L” of 5 EM ET < Thr.
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 29
Global Trigger
Receives Trigger Objects:• 4 forward and 4 central jets, 4 -jets, 4 isolated and 4 non-isolated e/, total ET,
missing ET, HT and position information from GCT• 4 with position, sign, and quality information from GMT
128 different conditions (thresholds, topological cuts) can be combined to make 128 physics triggers
Forwards Level-1 Accept Trigger, Timing and Control (TTC) system for read-out of the detector front-end electronics
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 30
Global Trigger and Trigger Control System
Every bunch crossing for which a potential Level-1 accept is inhibited is counted as “dead”.– needed to calculate recorded
integrated luminosity
CMS
regionaltriggers
GCTGMT
40 MHz pipeline
calo & muonobjects
GLOBALTRIGGER
LOGIC1. Logic OR2. Prescale
3. Rate Counters
128xalgos
technical trigger signals(beam monitoring systems etc.)
64xtechnical
GLOBAL TRIGGER
FINAL DECISIONLOGIC
physics
calib
random
rand calibTRIGGERCONTROLSYSTEM
L1A candidate
Level-1 Accept (via TTC system to detector)
backpressure,trigger rules
deadtimecounters
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 31
L1 Trigger Custom Hardware
– Hundreds of boards
– Thousands of:» ASICs
» FPGAs
» Copper Cables
» Optical Fibers
» (Wo)man hours
Global Muon Trigger&
Global Trigger(Vienna)
RCT(Wisconsin)
CSCTF(Florida)DTTF
(Vienna)
GCT(Imperial)
RPC PaC(Warsaw)
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 32
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 33
The CMS Level-1 Trigger Only calorimeter and muon systems participate in CMS L1
0.9<||<2.4
4 4 4 4 4+4 4+4
4 4
MIP+ISO bitsMIP+
ISO bits
e, J, ET, HT, ETmisse, J, ET, HT, ET
miss
L1A L1A
40 M
Hz
pipe
line
40 M
Hz
pipe
line
Calorimeter TriggerCalorimeter Trigger
ECALTrigger
Primitives
ECALTrigger
Primitives
HCAL/HFTrigger
Primitives
HCAL/HFTrigger
Primitives
RegionalCalorimeter
Trigger
RegionalCalorimeter
Trigger
GlobalCalorimeter
Trigger
GlobalCalorimeter
Trigger
Muon TriggerMuon Trigger
RPC hitsRPC hits CSC hitsCSC hits DT hitsDT hits
Segment finder
Segment finder
Track finderTrack finder
Pattern Comparator
Pattern Comparator
Segment finder
Segment finder
Track finderTrack finder
Global Muon TriggerGlobal Muon Trigger
Global TriggerGlobal Trigger
TTC systemTTC system TTS systemTTS system
Detector FrontendDetector Frontend
StatusStatus
Link systemLink
system
32 partitions32 partitions
CMS experiment
0<||<5||<3 ||<1.2||<3
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 34
Global Trigger interface
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 35
ATLAS+CMS:what’s the difference ? similar task
similar conditions similar technology
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 36
ATLAS+CMS:what’s the difference ? similar task
similar conditions similar technology
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 37
ATLAS+CMS:what is common ?
same physics objectives same input rate
– 40 MHz bunch crossing frequency similar rate after Level-1 trigger
– 50 .. 100 kHz similar final event rate
– 100 .. 200 Hz to tape similar allowed latency
– pipeline length– within this time, Level-1 trigger decision must be taken and detectors must be read
out– ~ 3 μs
» 2.5 μs for ATLAS, 3.2 μs for CMS
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 38
ATLAS+CMS:what is different ?
different magnetic field– toroidal field in ATLAS (plus central solenoid)
» get track momentum from η (pseudorapidity)
– solenoidal field only in CMS» get track momentum from φ (azimuth)
number of trigger stages– two stages (“Level-1” and “High-Level Trigger”) in CMS– ATLAS has intermediate stage between the two: “Level-2”
» Level-2 receives “Regions of Interest (RoI)” information from Level-1» takes a closer look at these regions» reduces rate to 3.5 kHz» allows to reduce data traffic
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 39
data are stored in “pipelines” for a few microseconds– e.g. in CMS: 3.2 s = 128 clock cycles at 40 MHz
– question of cost
– decision must never take longer! (must be synchronous!) no iterative algorithms
decision based on “look-up tables”– all possible cases are provided for
– OR, AND, NOT can also be represented in this way
ATLAS+CMS:principle of the first-level trigger
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 40
principle of event selection: ATLAS vs. CMSCMS:
no “cuts” at individual low-level trigger systems
– only look for muons, electrons, jets, choose the “best” of these candidate objects and forward to Level-1 Global trigger
so, all possible kinds of events may be combined
selection is only made by “Level-1 Global Trigger”
– trigger information from all muon detectors and calorimeter systems available: completely flexible
High-Level trigger analyzes complete detector data for the whole detector
ATLAS:
Level-1 trigger delivers numbers of candidate objects
– muon tracks, electron candidates, jets over appropriate threshold
– no topological information used (no angular correlation between different objects)
Level-2 trigger carries out detailed analysis for “Regions of Interest”
– using complete detector information for these regions
High-Level trigger analyzes complete detector data for the whole detector
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 41
First Collisions Tuesday March 30, 2010 12:58 L~1027cm-2s-1 ~ 60 Hz Collision Rate
Time to optimize the trigger synchronization!
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 42
Minimum Bias trigger detectors
HF – Hadron Forward Calorimeter
BSC – Beam Scintillator Counters (hit and coincidence rate, MIP detection eff >96%)
BPTX – Beam Pick-up Timing for the eXperiments monitors
(precise info about bunch structure and timing of the incoming beam, res. 0.2 ns)
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 43
2010 Commissioning: Synchronizing the Trigger Collision products take longer to get
to outer part of detector– e.g. to reach CSC takes longer than
ECAL Take into account many different
cable lengths– Even within a sub-detector
First runs with only a Minimum-Bias Trigger– Take data with a number of delays
– Find the best alignment between subsystem (CSC shown) and Minimum-Bias Trigger
– Repeated for each Trigger subsystem
– Cross-checks available in DQM
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 44
LHC is design to deliver one bunch crossing (BX) every 25 ns
Trigger has to provide a correct BX assignment
Optimization of the trigger synchronization allows for an overall efficiency increase
ExampleExample:: L1 RPC trigger synchronization is in a very L1 RPC trigger synchronization is in a very
good shape good shape
no pre-no pre-
triggerstriggers
TRIGGERSTRIGGERSRPC HitsRPC Hits
BX
=0 :
99.9
99%
BX
=0 :
99.9
99%
2010 Commissioning: Synchronizing the Trigger
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 45
2010 Operational Challenge: Rates Final month of 2010 p-p running, max inst. lumi
increased by > O(4)!– No longer relying on Minimum Bias but a full
menu of physics object triggers– L1 works with the HLT to maintain a sustainable
HLT stored data rate» Up to 65 kHz in, ~400 Hz out
– New L1 & HLT configurations and menus:» Higher thresh. L1 Triggers for HLT seeds» Prescaling at L1 and HLT to reduce rates» Several prescale “sets” to allow as much data
to tape as possible• Lumi decreases -> change set of prescales,
keep rate about the same
– Possible to predict output rates with MC– Must take into account other factors
» Cosmic backgrounds, pile-up effects, detector effects, etc.
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 46
Threshold efficiency – probability that for reconstructed muon with momentum PT, track finder assigns momentum greater or equal to the threshold value
F it f unct ion:
p 0 E r f ( p1 x p 2 ) p 3
Eta region 1.2 <|η|
<2.1
CSC trigger performanceCSC trigger performance
DT trigger performanceDT trigger performance
DTTF PT > 10 GeV/c from J/PsiDTTF PT > 10 GeV/c from J/Psi
“Turn-on” expected to reach >“Turn-on” expected to reach >
90% efficiency at the nominal 90% efficiency at the nominal
cutcut
Eta region |η|<
1.1
2010 Trigger Efficiencies
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 47
2011: Operations: Controlling the Rates
LHC has been steadily increasing instantaneous luminosity delivered to the experiments in 2011– Well beyond what was delivered in 2010
– How do we best control rates while keeping as much physics as possible? Just a few examples of what the L1 Trigger can do:
– e/ triggers
» Energy corrections to improve resolution
» ECAL Spike Killing
» Isolation and H/E for e/ candidates
– Jet triggers
» Improve the resolution with jet energy corrections
– Muon triggers
» Improve ghost busting and pattern recognition
– Continue to develop the L1-HLT menus with prescale sets
» Must anticipate physics groups’ needs as conditions change
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 48
2011: Operations: Avoiding pre-triggers
Problem: – Triggers are pre-firing at about ~5% of the
time
– Forward hadron calorimeter Can we avoid this source of inefficiency?
– CMS BPTX_AND Trigger
» AND of two precision electrostatic beam pick-ups
» Provides colliding bunch structure in CMS for every fill
– Copy, reduce delay by one bunch-crossing unit (25 ns) and shorten to ~20ns
– Use it as veto on pre-triggers
– But it vetoes the slow particle (HSCP) trigger (BX+1) when the bunch spacing is 50 ns!
How to fix this? (Next Slide)
before
after
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 49
2011 Operations: Trigger on HSCPs
Heavy Stable Charged Particles (HSCPs) take as much as 2 BX the detector
– Look for late signal in muontriggers
– But preBPTX veto during 50 ns bunch spacing vetoes these!
RPC Trigger can extend detector signal to 2 BX
– Reduce RPC delay into GMT by 1 BX
– preBPTX will veto early in-time muons
– In-time muons still captured with collision BX
– HSCPs aligned with collision BX
– Create special HLT path to keep slow particles (the BXid will be +1 wrt to Tracker, for example).
HSCPs Triggering!
layer 6layer 5layer 4
Chamber hits extended hits
BX
Muon candidate
In-time muon
layer 3layer 2layer 1
BPTX
layer 6layer 5layer 4
Chamber hits Extended hits
BX
Muon candidate
HSCP
layer 3layer 2layer 1
BPTX
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 50
Halfway through 2011
Middle of 2011 p-p run:
323 pb-1 data taken Maximum sustained L1 rate
of ~65 kHZ– 912b – 874 colliding at CMS, 50ns
spacing
– XX pb-1 recorded in a single fill
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 51
CMS Trigger & DAQ Systems
LHC beam crossing rate is 40 MHz & at full Luminosity of 1034 cm-2s-1 yields 109 collisions/s
Reduce to 100 kHz output to High Level Trigger and keep high-PT physics
Pipelined at 40 MHz for dead time free operationLatency of only 3.2 sec for collection, decision, propagation
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 52
... and where do we go from here?
upgrading the LHC to Super-LHC makes sense only if trigger systems are upgraded at the same time
ATLAS and CMS will have to use their trackers in the first-level trigger
but this is easier said than done probably, computers will get faster and more (all?) trigger
processing will be done in software
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 53
Conclusions
CMS L1 Trigger is performing well– 2010 and 2011 not without its challenges
– Complex system, in Hardware and Software
» Excellent pool of experts
» Problems addressed in a timely manner
LHC still increasing the luminosity – Will still stress system, both detectors and triggers
» Must remain diligent, even if luminosity stabilizes
– Expect L1 Menus to continue evolving
» Physics results may dictate changes instead of lumi
– Challenge to balance physics needs with rate limitations,
» L1T must continue to work with HLT
Looking forward to lots of interesting physics results!
Manfred Jeitler, HEPHY Vienna CMS Level-1 Trigger MGU, 11 July 2011 54
Спасибо за приглашение!
[email protected] http://wwwhephy.oeaw.ac.at/u3w/j/jeitler/www