manfred jeitler, hephy vienna trigger systems at lhc experiments instr-2008, novosibirsk, 4 march...
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Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 1
Trigger Systems at
LHC Experiments
Manfred Jeitler
Instr-2008, Novosibirsk, March 2008
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Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 2
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
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 3
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
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 4
Higgs -> 4 +30 MinBias
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 5
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
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 6
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 7
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 8
example: trigger logic of the L1-trigger of the CMS experiment
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 9
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 10
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– 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– luminosity quoted for ATLAS and CMS
» reduced luminosity for LHCb (b-physics experiment)» heavy-ion luminosity much smaller
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 11
the LHC experiments
4 major experiments 3 different main physics goals
– “general purpose”: Higgs, Susy, ..... : ATLAS+CMS
– b-physics: LHCb
– heavy ion physics: ALICE
different emphasis on trigger:– ATLAS+CMS: high rates, many different trigger channels
– LHCb: lower luminosity, need very good vertex resolution (b-decays)
– ALICE: much lower luminosity for heavy ions, lower event rates, very high event multiplicities
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 12
Event size (bytes)
trigger: first level high levelATLAS, CMS 40 MHz 100 kHz 100 Hz LHCb 40 MHz 1 MHz 2 kHzALICE 10 kHz 1 kHz 100 Hz
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 13
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 14
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
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 15
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 16
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++)– see Marta Felcini’s talk tomorrow
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 17
ATLAS+CMS:what’s the difference ?
similar task similar conditions similar technology
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 18
ATLAS+CMS:what’s the difference ? similar task
similar conditions similar technology
let’s hope both ATLAS and CMS will fly ....
.... and none will crash
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 19
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 20
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 21
ATLAS+CMS:which signals are 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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 22
how to find muon tracks ?(CMS: solenoidal field)
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 23
calorimeter trigger
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 24
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 25
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 Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 26
LHCb: the challenge
study b-meson decays need very good vertex resolution lower luminosity
– 2 1032
– 50 times lower than ATLAS and CMS
– aim at having only one proton-proton collision per bunch crossing» to correctly attribute primary and secondary vertices: “pile-up protection”
» important difference from ATLAS & CMS !
– achieved by deliberately defocusing the beam
detector looks rather like a fixed-target experiment– concentrate on forward direction
– single-arm forward spectrometer
can take useful data at initial low-luminosity LHC operation
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 27
LHCb: the approach
Level-0 trigger– implemented in hardware
– reduction from 40 MHz bunch-crossing rate down to 1 MHz» 10 times more than ATLAS or CMS !
– allowed latency: 4 μs
– reject pile-ups (several proton collisions in one bunch crossing)
– uses also vertex detector (“VELO”, VErtex LOcator)» different from other LHC experiments
» alongside muon and calorimeter information
High-Level trigger– computer farm, using full detector information
– reduce data rate to 2 kHz» 20 times more then ATLAS or CMS
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 28
ALICE: the challenge
study heavy-ion collisions– quark-gluon plasma– study also proton-proton interactions for comparison
lower bunch-crossing frequency and luminosity for heavy ions– bunches arrive every 125 ns – luminosity: 1027 cm-2 s-1
» factor of 10 millions below proton-proton luminosity » rate: 10 kHz for lead-lead collisions» 200 kHz for proton collisions
enormous complexity of events – tens of thousands of tracks per event – per pseudorapidity interval: dN / dη ~ 8000
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 29
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 30
ALICE: the approach
detectors can be slower, but must cope with high track multiplicity choice of Time Projection Chamber (TPC) as principal tracking
detector (drift time up to 100 μs !)
no spatial correlation of objects in first-level trigger– is done by High Level Trigger, which has much bigger time budget
“past-future protection”: protect against pile-up of events from different bunch crossings
some detectors are faster than the TPC for certain studies, read out only them– “detector clusters”
– only LHC detector which aims at analyzing partial events
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 31
ALICE: the approach
detectors can be slower, but must cope with high track multiplicity choice of Time Projection Chamber (TPC) as principal tracking
detector (drift time up to 100 μs !)
no spatial correlation of objects in first-level trigger– is done by High Level Trigger, which has much bigger time budget
“past-future protection”: protect against pile-up of events from different bunch crossings
some detectors are faster than the TPC for certain studies, read out only them– “detector clusters”
– only LHC detector which aims at analyzing partial events
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 32
ALICE: the trigger structure
no pipeline (as in the other LHC experiments) trigger dead-time
– only one or a few events can be buffered at low level before readout
several low-level triggers with different latencies for different subdetectors– Level-0: 1.2 μs
– Level-1: 6.5 μs
– Level-2: 88 μs
computer farm for High-Level Trigger
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 33
Where we are today ...
trigger is one of the most challenging and important tasks in major experiments at modern hadron colliders
at LHC, very similar setup for most experiments– only heavy-ion experiment ALICE differs significantly
when accelerator goes online, we will see which solutions are most appropriate– draw lessons for the future
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 34
... 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– remember Hans-Jürgen Simonis’ comments on the CMS tracker
probably, computers will get faster and more (all?) trigger processing will be done in software
stay tuned ... and help!
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 35
THANK YOU
Спасибо to the organizers of Instr-2008 for inviting me to give this presentation
Thanks to all members of the ATLAS, CMS, LHCb and ALICE collaborations who helped me to prepare it
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 36
backup
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 37
structure of the ATLAS Level-1 trigger
Manfred Jeitler, HEPHY Vienna Trigger Systems at LHC Experiments Instr-2008, Novosibirsk, 4 March 2008 38
structure of the CMS Level-1 trigger