pushing the limits: triggering and forward physics at the lhc
DESCRIPTION
Pushing the limits: Triggering and forward physics at the LHC. Monika Grothe U Wisconsin/ U Turin. Boundary conditions for triggering at the LHC Trigger architecture at the LHC/CMS Level-1 trigger algorithm at the LHC/CMS High level trigger algorithms at the LHC/CMS - PowerPoint PPT PresentationTRANSCRIPT
Monika Grothe, Triggering and forward physics at the LHC, July 2007 1
Pushing the limits: Triggering and forward physics
at the LHC
Monika GrotheU Wisconsin/ U Turin
• Boundary conditions for triggering at the LHC
• Trigger architecture at the LHC/CMS
• Level-1 trigger algorithm at the LHC/CMS
• High level trigger algorithms at the LHC/CMS
• Pushing the limits: Triggering on forward physics
• Diffractive and forward physics with CMS + Totem (+ FP420)
• Outlook: A Level-1 track trigger for the SLHC
Monika Grothe, Triggering and forward physics at the LHC, July 2007 2
Boundary conditions for triggering
at the LHC
Monika Grothe, Triggering and forward physics at the LHC, July 2007 3
Simple example of a trigger
trigger decision
some logicoperation and synchronizationof signals:“trigger logic”
data is storeddepending on trigger decision
Record pulse height spectrumof cosmic rays in a 30 degree slice
An interesting event:Pulses with height above a threshold are seen in coincidence with Z12 in 30o slice around Z12
(U Bonn)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 4
Boundary conditions
First try: A trigger at the LHC
Simple-minded approach: At the LHC, each crossing of bunches that leads to one (or more) pp collisions is an interesting event. Why don’t we simply trigger oneach of them and record all associated subdetector data ?
At the LHC, per year 1016 eventsFor each event total size of data to eventually store is about 1 MByte
1016 x 1 MHz = 107 PetaByte
Job of a trigger:Indicate that something of interest was seen in the detectorIf an interesting “event” occured, record all its data for later detailed analysis
Attention: An event not flagged as interesting by the trigger is lost forever
! ?Need to compromise between physics and affordabilityAffordable ? CMS Tier-1 centers will provide about O(1) PByte of storage in 2008
Reduction of 7 orders of magnitude !! What does this mean for physics ?
Monika Grothe, Triggering and forward physics at the LHC, July 2007 5
Boundary conditions
Event rates at the LHC
• Inelastic pp reactions: 109 / s • bb pairs 5 106 / s • tt pairs 8 / s
• W e 150 / s• Z e e 15 / s
• Higgs (150 GeV) 0.2 / s• Gluino, Squarks (1 TeV) 0.03 / s
Rates for L = 1034 cm-2 s-1: (LHC)
For cost reasons:First level trigger output limited to
~100 kHzHigher level triggers output limited to
~100 Hz
Monika Grothe, Triggering and forward physics at the LHC, July 2007 6
s = 14 TeV search for new massive particles up to m ~ 5 TeV7 x higher than Tevatron
Ldesign = 1034 cm-2 s-1 search for rare processes with small (N = L )102 x higher than Tevatron
s = 14 TeV search for new massive particles up to m ~ 5 TeV7 x higher than Tevatron
Ldesign = 1034 cm-2 s-1 search for rare processes with small (N = L )102 x higher than Tevatron
Boundary conditions
The LHC and its experiments
ATLAS and CMS :pp, general purpose
pp
ALICE : heavy ions LHCb :
pp, B-physics
27 km ring used fore+e- LEP machine in 1989-2000 + TOTEM at the CMS IP
+ LHCf at the ATLAS IP
Monika Grothe, Triggering and forward physics at the LHC, July 2007 7
Most interactions are due to interactions at large distance between incoming protons→ small momentum transfer, particles in the final state have large longitudinal, but small transverse momentum
Boundary conditions
Physics at a proton-proton collider
p
pT
pT = p sin
plane perpendicular to the beam
“Interesting events”: Processes with large transverse momentum pT
sx1p x2p
Proton beam can be seen as beam of quarks and gluons with a wide band of energiesHard scatter between these proton constituents
p pqq
H
WW
Monika Grothe, Triggering and forward physics at the LHC, July 2007 8
Boundary conditions
Event pile-up
Because of very high particle density in LHC proton bunches:
Event pile-up per bunch crossing
Consequence: High particle density in detector
H ZZ, Z cleanest "golden" signature
But at L = 1034 cm-2 s-1
overlapped by O(25) non-elastic events
And this (not the Higgs though) repeats every 25 ns
Monika Grothe, Triggering and forward physics at the LHC, July 2007 9
Boundary conditions for triggering at the LHC:
Affordability limits read-out bandwidth and storage capacity
Huge cross section at LHC energies
Rate reduction of overall 7 orders of magnitude needed
High particle density in detector because of event pile-up
Discovery physics with relatively high ET/pT
Base trigger on selection of high ET/pT objects
Recap: Boundary conditions
Monika Grothe, Triggering and forward physics at the LHC, July 2007 10
Trigger architectureat the LHC/CMS
Monika Grothe, Triggering and forward physics at the LHC, July 2007 11
Trigger architecture Multi level trigger
Monika Grothe, Triggering and forward physics at the LHC, July 2007 12
L1: trigger decision algorithms implemented in fast, custom-made electronics only very much reduced information on event at its disposalHLT: implemented as software algorithms run on a processor farm in principle full event information at it disposal
107 channels
1000 units
103 x 103 switch fabric
4x106 MIPS
Trigger architecture Multi level trigger (II)
L1 In: 1 GHzL1 Out: 100 kHz
HLT In: 100 kHzHLT Out: 100 Hz
Decision within a few s
Decision within a few 100 ms
Monika Grothe, Triggering and forward physics at the LHC, July 2007 13
Trigger architecture Pipelines on L1
Speed of light in air: 0.3m/nsOuter diameter of CMS detector ~7m, hence particle needs up to 23ns to cause signal in CMS muon chamber
At LHC 25ns btw bunch crossings
It is impossible to form a trigger decision within 25 ns of each bunch crossing
Since in principle every bunch crossing could result in an interesting event, need way to store them till trigger came to a decision: pipelines
L1 trigger latency = depth of pipeline = maximum time available for L1 trigger decision
g
Monika Grothe, Triggering and forward physics at the LHC, July 2007 14
General trigger architecture at the LHC/CMS:
Pipelined, multi-level trigger
L1: latency a few s, coarse granularity information, custom-made electronics
HLT: latency a few 100 ms, full event info, computer farm
Recap: Trigger architecture
Monika Grothe, Triggering and forward physics at the LHC, July 2007 15
Trigger algorithms for Level-1 and Higher Levels Trigger
at the LHC/CMS
Monika Grothe, Triggering and forward physics at the LHC, July 2007 16
L1 trigger algorithms
Information used for L1 algorithms
Monika Grothe, Triggering and forward physics at the LHC, July 2007 17
L1 trigger algorithms
Why no track information on L1 ?
Monika Grothe, Triggering and forward physics at the LHC, July 2007 18
L1 trigger algorithms
Example: L1 calorimeter trigger
Trigger towers:smallest unit trigger electronics looks at
L1 calorimeter trigger searchesfor trigger tower clusters with maximum ET
Small cluster: L1 electron or photonBig cluster: L1 jet
Monika Grothe, Triggering and forward physics at the LHC, July 2007 19
L1 trigger algorithms
L1 electrons/photons and jets
Sliding 3x3 region windowRegion = 4x4 trigger towers
L1 electron or photon:
L1 jet:
Monika Grothe, Triggering and forward physics at the LHC, July 2007 20
Algorithm implemented in asics
isolated L1 electron or photon
L1 trigger algorithms
Algo example: Isolated electron/photon
Monika Grothe, Triggering and forward physics at the LHC, July 2007 21
3 safety factor 50 kHz (expected start-up DAQ bandwidth)
L1 trigger algorithms
Example L1 trigger table (L = 2x1033 cm-2 s-1)
Background to e/ are mainly in em-rich jetsBackground to jets are jets - huge rate of QCD jets
Purity is not the issue, but 100%efficiency for “interesting events”while respecting bandwidth limits
Monika Grothe, Triggering and forward physics at the LHC, July 2007 22
HLT trigger algorithms
HLT electron algorithm“Level 2” step:Starting from L1 em object information as seed, reconstruct cluster of ECAL crystals in which electron has deposited its energy reconstruct electron energy and position from cluster
“Level 2.5” step:Match cluster with hits in the pixel detector
“Level 3” step:Starting from pixel detector seed, reconstruct full track information
e efficiency vs jet rejectionfor L2.5 pixel matching:
CMS pixel detector coverageECAl barrel
ECALendcap
1 trigger tower= 5x5 crystals
Monika Grothe, Triggering and forward physics at the LHC, July 2007 23
Trigger algorithms on L1 and HLT at LHC/CMS:
Algorithms on L1 based on calorimeter and muon system information, tracking only on HLT
Main objective of L1 trigger: Keep output rate under control, i.e. purity is not the issue, but maximum signal efficiency is
Example L1 calo trigger: Looks for local maxima in ET, large cluster = L1 jet , small cluster: L1 electron/photon
Example HLT electron trigger: Reduction of jet background by factor 10 by requiring matching pixel track stub
Allocated trigger bandwidth per trigger condition summarized in L1 and HLT trigger menus
Trigger menus determine physics reach of experiment
Recap: Trigger algorithms
Monika Grothe, Triggering and forward physics at the LHC, July 2007 24
Pushing the limits: Triggering on
forward physics
Monika Grothe, Triggering and forward physics at the LHC, July 2007 25
Triggering on forward physics
Forward physics
Experimental definition:All processes in which particles are produced at small polar angles.
= 90o = 0
= 10o 2.4 = 170o -2.4
= 1o 5.0edge of coverage of centralCMS/ATLAS detectors
Monika Grothe, Triggering and forward physics at the LHC, July 2007 26
Triggering on forward physics
Example of forward physics:Diffraction
Monika Grothe, Triggering and forward physics at the LHC, July 2007 27
Diffraction as tool for discovery physics ! ?
Great. And why bother at the LHC ?
Monika Grothe, Triggering and forward physics at the LHC, July 2007 28
Triggering on forward physics
Suppose you want to detect a light SM Higgs (say MH=120 GeV) at the LHC...
SM Higgs with ~120 GeV:gg H, H b bbar highest BRBut signal swamped by gg jet jetBest bet with CMS: H
Vacuum quantum numbers“Double Pomeron exchange”
shields color charge ofother two gluons
Central exclusive productionpp pXpSuppression of gg jet jetbecause of selection rules forcingcentral system to be (to good approx) JPC = 0++
Monika Grothe, Triggering and forward physics at the LHC, July 2007 29
Triggering on forward physics
Diffraction as tool for discovery physics:
CEP pp pXp with X = H(~120 GeV) b bbar
In non-diffractive production hopeless, signal swamped by QCD di-jet background
Selection rules: central system is JPC = 0++ (to good approx) I.e. a particle produced with proton tags has known quantum #
For light (~120 GeV) Higgs: Proton tagging improves S/B for SM Higgs dramatically CEP may be discovery channel in certain regions in MSSM
CP quantum numbers and CP violation in Higgs sector directly measurable from azimuthal asymmetry of the protons
beam
p’
p’roman potsroman pots
dipoledipole
Needed: Proton spectrometer using the LHC beam magnetsDetect diffractively scattered protons inside of beam pipe
Monika Grothe, Triggering and forward physics at the LHC, July 2007 30
CMS IP T1/T2, Castor ZDC RPs@150m RPs@220m
possibly detectors@420m
Triggering on forward physics
CMS + TOTEM (+ FP420)
Possible addition FP420: Silicon tracking detectors and fast timing Cherenkov detectors, integrated into a LHC cryostat at 420m from IP
TOTEM: An approved experiment at LHC for measuring tot and elastic
uses same IP as CMS
TOTEM’s trigger and DAQ system will be integrated with those of CMS , i.e. common data taking CMS + TOTEM possible
TOTEM aims at start-up at the same timescale as CMS (2008)
Triggering on forward physics
The difficulty of triggering on a 120GeV Higgs
120 GeV Higgs decays preferably into 2 b-jets with ~60 GeV each
At 2x 1033 cm-1 s-1 without any additional condition on fwd detectors:
L1 1-jet trigger threshold O(150 GeV)
L1 2-jet trigger threshold O(100 GeV)
2 x 1033 cm-2 s-1 L1 jet trigger rates
L1 outputbandwidth:100 kHz
Is it possible to lower the CMS jet triggerthresholds significantly by combiningcentral CMS jet trigger condition withcondition on forward detectors ?
Attention: Cumulative rate shownTotal number of events with ET abovethreshold and function of threshold
L1 ET threshold (GeV)
Rat
e (k
Hz)
10
1
100
60
Note: Usable in L1 trigger only 220m proton taggers, 420m too far away from IP for signal to arrive within L1 latency of 3.2 s
Monika Grothe, Triggering and forward physics at the LHC, July 2007 32
→ CMS trigger thresholds for nominal LHC running too high for diffractive events
→ Use information of forward detectors to lower in particular CMS jet trigger thresholds
→ The CMS trigger menus now foresee a dedicated forward detectors trigger stream with 1% of the total bandwidth on L1 and HLT (1 kHz and 1 Hz)
single-sided 220m conditionwithout and withcut on
Achievable total reduction: 10 (single-sided 220m) x 2 (jet iso) x 2 (2 jets same hemisphere as p) = 40
Triggering on forward physics
A dedicated forward detectors L1 trigger stream
Demonstrated that for luminosities up to 2x 1033 cm-1 s-1 including 220m detectors into the L1 trigger provides a rate reduction sufficient to lower the 2-jet threshold substantially to 40GeV while still meeting the CMS L1 bandwidth limits
!
Monika Grothe, Triggering and forward physics at the LHC, July 2007 33
H(120 GeV) → b bbar
L1 trigger threshold [GeV]
Eff
icie
ncy 420m
220m
420+420m
420+220m
Triggering on forward physics
Efficiency of forward detectors L1 stream for diffractive events
Central exclusive production pp pHp with H (120GeV) bb:
Can gain another 10% from trigger
pp p jj X2-jet trigger
Attention: Gap survival probability not taken into account; normalized to number of events with 0.001 < < 0.2 and with jets with pT>10GeV
Eff
icie
ncy
L1 trigger threshold [GeV]
no fwd detectorscondition
single-arm 220m
single-arm 420m
Eve
nts
per
pb
-1
Monika Grothe, Triggering and forward physics at the LHC, July 2007 34
Triggering on forward physics
Experimental challenge:Pile-up background !
Monika Grothe, Triggering and forward physics at the LHC, July 2007 35
TOTEM
xL=P’/Pbeam=
det@420
d(
ep
eXp
)/d
x L [n
b]
Number of PU events with protons within acceptance of near-beamdetectors on either side:
~2 % with p @ 420m
~6 % with p @ 220m
Coincidence of non-diffractive event with protons from pile-up events in the near-beamdetectors: fake double-Pomeron exchange signature
Triggering on forward physics
Pile-up background (II)
Non-diffractive event with signature in the central CMS detector identical to some DPE signal event: At 2x 1033 cm-2s-1 10% of these non-diffractive events will be mis-identified as DPE event. This is independent of the specific signal.
Diff events characterized by low fractional proton momentum loss
diffractivepeak
Monika Grothe, Triggering and forward physics at the LHC, July 2007 36
Can be reduced on the High Level trigger:
Requiring correlation between ξ, M measured in the central detector andξ, M measured by the near-beam detectors
Fast timing detectors that can determine whether the protons seen in the near-beam detector came from the same vertex as the hard scatter within 3mm
Further offline cuts possible:
Condition that no second vertex befound within 3mm vertex windowleft open by fast timing detectors
Exploiting difference inmultiplicity between diff signal and non-diff background
Triggering on forward physics
Handles against pile-up background
; 1 2 s = M2
(p tagger)(
jets
)
CEP H(120) bb incl QCD di-jets + PU
M(2-jets)/M(p’s)
CEP of H(120 GeV) → b bbar andH(140 GeV) → WW:S/B of unity for a SM Higgs
Monika Grothe, Triggering and forward physics at the LHC, July 2007 37
CMS trigger menus now foresee a dedicated forward detectors trigger stream:
Trigger at LHC designed for high pT physics, trigger thresholds generally too high for forward physics which has by definition lower pT
Combining central CMS detector trigger conditions with condition on Totem 220m proton tagger allows to lower in particular L1 jet trigger thresholds substantially while respecting trigger output rate limits
HLT forward detectors trigger algorithms reduce background from pile-up substantially and could use 420m proton tagger information
Central exclusive production of a low mass Higgs boson is physics channel profits substantially from new stream
Recap: Triggering on forward physics
Monika Grothe, Triggering and forward physics at the LHC, July 2007 38
Diffractive and forward physics with CMS + Totem *
at nominal LHC optics
* possibly also including FP420
The thus designed dedicated forward detectors trigger stream forms an essential part of an extension of its baseline program that CMS wishes to implement:
Monika Grothe, Triggering and forward physics at the LHC, July 2007 39
The CMS + Totem (+ FP420) program
CERN/LHC 2006-039/G-124
Objective:Carry out a program of diffractive and forward physics as integral part of the routinedata taking at CMS, i.e. at nominal beam optics and up to the highest available luminosities.This program spans the full lifetime of the LHC.
M. Grothe, J. Mnich primaryeditors from CMS side
Areas covered, in addition to diffraction as tool for discovery physics in central exclusive production:
• Diffraction in the presence of a hard scale: “Looking at the proton through a lense that filters out anything but the vacuum quantum numbers• Diffractive structure functions• Soft rescattering effects/underlying event and rapidity gap survival factor
• Low xBJ structure of the proton
• Saturation, color glass condensates
• Rich program of and p physics
• Validation of cosmic ray air shower MC
Monika Grothe, Triggering and forward physics at the LHC, July 2007 40
TOTEM
xL=P’/Pbeam=
det@420
d(
ep
eXp
)/d
x L [n
b]
CMS + TOTEM (+ FP420) Unprecedented kinematic coverage
TOTEM T2:GEM tracking detector
CMS Castor thungsten/quartzCherenkov calorimeter
CMS ZDC thungsten/quartzCherenkov calorimeterTOTEM Silicon tracking
det. housed in Roman pots
Castor Castor
ZDC ZDC
Monika Grothe, Triggering and forward physics at the LHC, July 2007 41
CMS + Totem intend to carry out a joint program on diffractive and forward physics
Unprecedented kinematic coverage
LoI submitted to LHCC Dec 2006
LoI identifies and addresses for the first time at the LHC central experimental
questions for carrying out a program that spans the full lifetime of the LHC
FP420 as possible extension of program currently under review in CMS (and ATLAS), document on results of extensive R&D effort over the past several years in preparation, decision by end of this year
Recap: CMS + Totem (+ FP420) program
Monika Grothe, Triggering and forward physics at the LHC, July 2007 42
Outlook:A Level-1 track trigger for the SLHC
Monika Grothe, Triggering and forward physics at the LHC, July 2007 43
Outlook: L1 track trigger
Tracking in the L1 trigger: CMS ideas Not done at the LHC because of difficulty of high density of low pT tracks in tracker At the SLHC, the increase of luminosity x10, to 1035 cm-2 s-1, will degrade efficiency of LHC algorithms in keeping the L1 output rate under control
Tracking trigger on L1 the solution ?
Example electron trigger: On HLT, matching of Calo electron candidate with pixel detector hits reduces background by a factor 10
Tracking trigger challenge: Find only high pT track stubs and match them with L1 electron and muon objects Because of high occupancy at the SLHC, need to do so while keeping read-out data from detector at a minimum On-detector hit correlator needed to reduce combinatorics
Suggested solution: Stacked pixel layers
rB
rLSearchWindow
A track like this wouldn’t trigger:
<5mm
w=1cm ; l=2cm
y
x
-- C. Foudas &
J. Jones
Monika Grothe, Triggering and forward physics at the LHC, July 2007 44
CorrelatorASIC
CoolingSystem
ThermalEpoxyOptical fibre to
OptoTX card
Kevlar-Carbon FibreLaminate
Support Structure
OpticalTransceiver Flip bonded
sensors
Outlook: L1 track trigger Tracking in the L1 trigger: CMS ideas (II)
• Use closely spaced stacked pixel layers• Angle of track bisecting sensor layers defines pT
• Track stub: Combination of hits in the 2 layers which are at most 1 pixel apart
• Pipelined column-parallel readout architecture where each pixel in a column forms a single cell in the pipeline
• Self-timed, asynchronous system with self-triggering pixels
J. Jones et al, A pixel detector for L1 triggering at SLHC, LECC2005J. Jones et al, Stacked tracking for CMS at SLHC, LECC 2006
• One module of 2 stacked layers a few millimeters apart would allow track stub reco• Two modules. e.g. at 10cm and 20cm from the beam line, would allow full track reco
Monika Grothe, Triggering and forward physics at the LHC, July 2007 45
Grand summary
• Triggering at the LHC is not an easy job
• Pipelined multi-level trigger architecture with algorithms that identify high ET/pT objects
• Extension of the CMS trigger menus:
A dedicated forward detectors trigger stream
• Extension of the CMS baseline physics program:
A joint CMS + Totem (possibly + FP420) program on diffractive and forward physics
• Motivation - Capitalize on diffraction as tool for discovery physics:
Central exclusive Higgs production
• A possible upgrade of the CMS L1 trigger for the SLHC:
Conceptual design for a Level-1 track trigger at the SLHC
Monika Grothe, Triggering and forward physics at the LHC, July 2007 46
Backup
Monika Grothe, Triggering and forward physics at the LHC, July 2007 47
HLT trigger algorithms
High level triggers strategy
In CMS all trigger decisions beyond Level-1 are performed in a Filter Farm running ~normal CMS reconstruction software on “PCs”
The filter algorithms are setup in several steps
HLT does partial event reconstruction “on demand” seeded by the L1 objects found, using full detector resolution
Algorithms are essentially offline quality but optimized for fast performance
Monika Grothe, Triggering and forward physics at the LHC, July 2007 48
30-40 GeV for or e20 GeV each for
250 GeV jets80 GeV
Trigger cuts determine physics reach!•Efficiency for H and H4 leptons = >90% (in fiducial volume of
detector)•Efficiency for WH and ttH production with Wl = ~85%•Efficiency for qqH with H (1/3 prong hadronic) = ~75%•Efficiency for qqH with Hinvisible or Hbb = ~40-50%
L1 trigger rates
Monika Grothe, Triggering and forward physics at the LHC, July 2007 49
Trigger Mapping onto Cal surface
Area coveredby 1 jet
Monika Grothe, Triggering and forward physics at the LHC, July 2007 50
How does it look in real life ?
Monika Grothe, Triggering and forward physics at the LHC, July 2007 51
How does it look in real life ?
•Underground Counting Room–Central rows of racks fortrigger
–Connections via high-speed copper links to adjacent rows of ECAL & HCAL readout racks with trigger primitive circuitry
–Connections via opticalfiber to muon trigger primitive generatorson the detector
–Optical fibersconnected via“tunnels” to detector(~90m fiber lengths)
Rows of Racks containing trigger & readout
electronics
7m thickshielding
wall
USC55
Monika Grothe, Triggering and forward physics at the LHC, July 2007 52
RCT HCAL HTR
Location:USC55 - S2
ECAL TCC
GCT Source Cards
To GCT Source Cards
How does it look in real life ?
Monika Grothe, Triggering and forward physics at the LHC, July 2007 53
1 0 1 1 0 0 1 1 0 0 1 0 0 1 1 0 0 1 0 0 1 0 0
Step 1
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Step 1
Step 2
Step 3
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Step 1
Step 2
Step 3
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0 10 0
128-bit L1 Word
HLT Bits…HL
T A
lgor
ithm
Ste
psP
ath
stop
s w
hen
a se
lect
ion
step
fai
ls e/
2 e/
ME
T
1-Je
t2-
Jet
e+Je
t
HT
HLT trigger algorithms
High level triggers strategy (II)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 54
Forward physics at the LHC
Forward physics with special LHC beam optics:Elastic scattering: High precision absolute luminosity measurement at ATLAS
Forward physics at nominal LHC beam optics:The CMS + TOTEM (+ FP420) joint program - diffraction - low-x proton structure - Cosmics MC validation
Monika Grothe, Triggering and forward physics at the LHC, July 2007 55
Forward physics:Elastic scattering
Monika Grothe, Triggering and forward physics at the LHC, July 2007 56
Forward physics:Absolute luminosity determination
At ATLAS IP: ALFA near-beam detectors and LUCID luminosity monitor
AT CMS IP: TOTEM
Monika Grothe, Triggering and forward physics at the LHC, July 2007 57
The rate of produced events for a given physics process is given by:
N = L σ
dimensions: s-1 = cm-2 s-1 · cm2
L = Luminosity = cross section
In order to achieve acceptable production rates for the interesting physicsprocesses, the luminosity must be high !
L = 1034 cm-2 s-1 LHC design luminosity, very large !! (1000 x larger than LEP-2, 50 x Tevatron Run II design)
Luminosity depends on the machine:important parameters: number of protons stored, beam focus at interaction region,….
Luminosity
Need to know the luminosity with precision in order to detect new physics effects that may manifest themselves in deviations of the measured from the expected one
Dedicated ATLAS luminosity system aims at precision of 2-3%Means: Elastic scattering in the Coulomb-nuclear interference region
Monika Grothe, Triggering and forward physics at the LHC, July 2007 58
PQCD: 1/t8
“structure”
BS
W -
20
03
Nuclear slope: ebt
Coulomb region: 1/t2
C–N interferenceSensitivity to
Elastic scattering in the CNI region
( )2
2
0
2
4
2 tb
eit
affL
dt
dN EMNC
t
−
≈
++−≈+= ñó
L tot
πππ
Using the optical theorem, the measured elastic rate at small t values can be expressed as
Fit measured t distribution to obtain tot, b and L
p
p
p
p
t: 4-momentum transfersquared between 2 p’s
Monika Grothe, Triggering and forward physics at the LHC, July 2007 59
radGeVa
ffttTOT
EMNC ϑ
5.32468
|)||(| minmin ≤→−×≈≈=−≤
y*
y*
parallel-to-point focusingydet
IP Leff
*,
**det yyeffy Ly ϑϑββ ==
Elastic scattering in the CNI region II
In order to reach CNI region at the LHC
Need special beam opticswith minimal intrinsic beamangular spread at IP:
Displacement ydet is independent of the vertex position:
Need to approach beam with detectorsas closely as possible: “Roman pots”
tracking detectorsRoman Pot
Proton beam line
z-y view x-y view
Monika Grothe, Triggering and forward physics at the LHC, July 2007 60
ALFA and LUCIDALFA: Absolute Luminosity for ATLAS
2 stations at 240mfrom ATLAS IPapproaching the beam to within 1.2mm
10+10 planes ofscintillating fibredetectors spatial resolution 30m edge <100m
Installation of detectors during firstlong LHC shutdown (2009 ?)
LUCID: Luminosity measurement with a Cherenkov Integrating Detector
Aluminium tubes filled with isobutane incylinder (length 1.5m, diameter 13.7cm)around beam pipe 17 m from ATLAS IP
Absolute lumi measurement at ~ 10-27 cm-2 s-1
Extrapolation from there to luminosity at nominal LHC running via track counting in LUCID
Monika Grothe, Triggering and forward physics at the LHC, July 2007 61
Forward physics:Diffraction
Monika Grothe, Triggering and forward physics at the LHC, July 2007 62
In diffractive events look at the proton constituents through a lens that filters out all parton combinations except those with the vacuum quantum numbers
X
Double Pomeron exchange (DPE):
X
Single diffraction (SD):
central CMSapparatus
central CMSapparatus
Near-beam detectors Near-beam
detectors
Near-beam detectors
IP
IP
IP
rap gap
A new way to probe the proton
2-gluon exchange:LO realisation of vacuum quantum numbers in QCD
pp
pp
IP
If X = anything: Measure fundamental quantities of soft QC
If X includes jets, W’s, Z’s, Higgs (!): Hard processes, calculable in perturbative QCD. Measure proton structure, QCD at high parton densities, discovery physics
Monika Grothe, Triggering and forward physics at the LHC, July 2007 63
TOTEM
xL=P’/Pbeam=
FP420
CMS + TOTEM (+ FP420): Coverage in
Note: Totem RP’s optimized for special optics runs at high β*β* is measure for transverse beam size at vertexTOTEM coverage in improves with increasing β*
At nominal LHC optics, β*=0.5m
Points are ZEUS data
diffractivepeak
Monika Grothe, Triggering and forward physics at the LHC, July 2007 64
Cosmic ray physics
Tune cosmic ray shower models with forward particle flows measured at the LHC
Study of the underlying event at the LHC:
→ Multiple parton-parton interactions and rescattering effects accompanying a hard scatter
→ Closely related to gap survival and factorization breaking in hard diffraction
Heavy-ion and high parton density physics:
Proton structure at low xBj → saturation → Color glass condensates
Photon-photon and photon-proton physics:
Also there protons emerge from collision intact and with very low momentum loss
Multiple connection points to other areas in High-Energy-Physics !
Prospects for diffractive and forward physics
with CMS + TOTEM (+ FP420)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 65
known
Photon-mediated processes:Exclusive μμ production
Calibration process both for luminosity and energy scales of near-beam detectors
Striking signature: acoplanarity angle between leptonsAllows reco of proton values with resolution of 10-4, i.e. smaller than beam dispersion
Expect ~300 events/100 pb-1 after CMS muon trigger
(di-muons) - (true)
rms ~ 10-4
Monika Grothe, Triggering and forward physics at the LHC, July 2007 66
pQCD
No
n p
ertu
rbat
ive
reg
ion
Saturation(Colour glass condensate)
Qs2(x)
1/x
Q2 [GeV2]
Low-x QCD - Saturation
• Steep rise in the gluon density at small x observed at HERA
• Growth cannot continue indefinitely, would eventually violate unitarity
• Growth tamed by gluon fusion: saturation of parton densities
• So far not observed in pp interactions
Monika Grothe, Triggering and forward physics at the LHC, July 2007 67
Low-x QCD: Forward Drell-Yan
Gives access to low-xBJ quarks in proton in case of large imbalance of fractionalmomenta x1,2 of leptons, which are then boosted to large rapidities
CASTOR with 5.3 ≤ || ≤ 6.6 gives access to xBJ~10-7
Measure angle of electrons with T2
Pdf’s known at large xBJ, hence can extract pdf’s at low xBJ
DY pairs suppressed in saturated PDF
saturated PDF
Sensitivity to saturation:
Monika Grothe, Triggering and forward physics at the LHC, July 2007 68
→ Models for showers caused by primary cosmic rays (PeV = 1015 eV range) differ substantially
→ Fixed target collision in air with 100 PeV center-of-mass E corresponds to pp interaction at LHC
→ Hence can tune shower models by comparing to measurements with T1/T2, CASTOR, ZDC
Validation of hadronic shower models in cosmic ray physics
Monika Grothe, Triggering and forward physics at the LHC, July 2007 69
Forward physics covers about 50% of the pp cross section at the LHC
Elastic scattering at special LHC optics used as tool for high precision luminositydetermination (ALFA/LUCID at ATLAS IP, TOTEM at CMS IP)
Joint CMS + TOTEM (+FP420) program foresees rich physics program(hard diffraction, Higgs discovery, low-x proton structure, forward particle flow) at nominal LHC optics and up to the highest luminosities
Possible upgrade of ALFA with Silicon tracking detectors: Capitalize on experience gained with operating detectors near powerful LHC beam Diffractive and forward physics program competitive with the one at CMS + TOTEM Possible Si option: edgeless 3-D Silicon Actively pursued in ATLAS as option for SLHC ATLAS tracking system upgrade
Possible addition FP420: Radiation hard Silicon and fast Cherenkov timing detectors at 420m from the IP R&D complete, discussion in ATLAS and CMS for inclusion as subdetector started
Monika Grothe, Triggering and forward physics at the LHC, July 2007 70
Summary and Outlook (II)
possible upgrade RP220 with Si detectors
possibleaddition
SLHC
Still an opportunity for an original contribution to the LHC detectors !
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Experimental situation at the CMS IP Additional option: The FP420 R&D project
The aim of FP420 is to install high precision silicon tracking and fast timing detectors close to the beams at 420m from ATLAS and / or CMS
FP420 R&D fully funded (~1000K CHF) Proposal to ATLAS and CMS in 2007 Detector installation could take place during first long LHC break (~2009)
Technological challenge:420m is in the cryogenic region of the LHC
Monika Grothe, Triggering and forward physics at the LHC, July 2007 72
FP420 project:
How to integrate detectors into the cold section of the LHC
420m from the IP is in the cold section of the LHC !Need to modify LHC cryostat: Use Arc Termination Modulesfor cold-to-warm transition such that detectors can be operated at ~ room temperature
Scattered protonsemerge here
Movable beam-pipe (pipelets)with detector stations attachedMove detectors toward beam envelopeonce beam is stable
Monika Grothe, Triggering and forward physics at the LHC, July 2007 73
FP420 project:
Which technology for the detectors ?3D edgeless Silicon detectors: Edgeless, i.e. distance to beam envelope can be minimized Radiation hard, can withstand 5 years at 1035 cm-2 s-1
Protons
PMT
Lens? (focusing)
MirrorCerenkov medium (ethane)
~ 15 cm~ 5 cm
(Flat or Spherical?)
Aluminium pump
Injection of gas (~ atmospheric pressure)
Ejection of gas
~ 10 cm
Time-of-Flight detectors:Time resolution of ~10ps would translate in z-vertex resolution of better than 3mm
GASTOF (UC Louvain)Cherenkov medium is a gas
QUARTIC (U Texas Arlington)Cherenkov medium is fused Silica
Monika Grothe, Triggering and forward physics at the LHC, July 2007 74
CRCLouvain-la-NeuveCRCLouvain-la-Neuve
Integration of the moving beampipe and detectors
Benoît Florins, Krzysztof Piotrzkowski, Guido Ryckewaert
ATM
Vacuum Space
BPM
Pockets
ATM
Line X
Bus Bar Cryostat
BPM
Vacuum Space
Transport side
QRLFixed Beampipe
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CMS ideas for readout of SLHC track trigger(very preliminary)
Column-wisereadout
Bias generator Timing (DLL)
Diode+’Amp’
Comparator
Local Address Pipe cell
Reset/TransferLogic
Data passesthrough cellin each pixelin column
•At end of column, column address is added to each data element•Data concatenated into column-ordered list, time-stamp attached at front
Inner Sensor Outer Sensor
Column compare
c2c1
• If c2 > c1 + 1, discard c1
• If c2 < c1 – 1, discard c2
• Else copy c2 & c1 into L1 pipeline
This determines your search windowIn this case, nearest-neighbour
L1A Pipeline
L1T Pipeline
•Use sorted-list comparison (lowest column first)
•All hits stored for readout
Monika Grothe, Triggering and forward physics at the LHC, July 2007 76
• Rates calculated using CMS software (ORCA) at 80 MHz double them for 40 MHz.
• No charge sharing has been included. Hence, another factor of at least two to make them realistic. 10 Gb/s/cm2 100 Tb/s
C.Foudas, A. Rose, J. Jones, G. Hall, LECC2005, Heidelberg
CMS ideas for SLHC track trigger: Occupancy(very preliminary)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 77
• Large Factors to be gained also for muons.• Outer tracker subs may be important here.
CMS ideas for SLHC track trigger: Muons(very preliminary)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 78
• Have a stacked pixel detector where the two layers have a radius difference of few mm.
• Require coincidences between the pixels of the two layers to select high Pt stubs
Momentum cut.• Removes all the low pt particles.• See: J. Jones et al. , A Pixel Detector for L1
Triggering at SLHC, LECC 2005
α
SLHC track trigger: Stacked pixel layers (very preliminary)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 79
ComponentOutput
bandwidth per unit
Rate Reduction
Number of units per upstream
component
Total number of
units
Aggregateoutput
bandwidth
Sensor (20cm2)10Gb/s cm2
N/A ~0.4 ~1600 ~140Tb/s
Hit Correlator(5 per 20cm2
stack)~1.6Gb/s X50 ~2 ~2000 ~3Tb/s
Opto TX and SERDES
~3.2Gb/s N/A 12 ~1000 ~3Tb/s
12xSFP to SNAP12 Cable
~40Gb/s N/A 5 ~90 ~3Tb/s
Regional Track Generator
~50Gb/s X4 ~3 ~18 ~1Tb/s
Global Track Generator
~4Gb/s X40 ~6 ~6 ~25Gb/s
Global Track Sorter
~10-20Gb/s
X2? N/A 1 ~10-20Gb/s
SLHC track trigger: Bandwidth requirement(very preliminary)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 80
• By introducing a second double stacked detector one can measure angles and transverse energy of tracks at reasonable resolution.
• See: J. Jones, A. Rose et al., Stacked Tracking with CMS at SLHC• LECC 2006, Valencia.
SLHC track trigger: Two stacked detectors(very preliminary)
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SLHC track trigger: Architecture(very preliminary)
Monika Grothe, Triggering and forward physics at the LHC, July 2007 82
= 10-3 – 10-4
PT / PT = 10-2
SLHC track trigger: Two stacked detectors(very preliminary)