Post on 28-Mar-2015
Cosners House Forum on LHC startup April 07
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Minimum bias studies etc….
Craig Buttar
University of Glasgow
Cosners House Forum on LHC startup April 07
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Outline
• LHC start up• Minimum bias events
– Why study them?– What are they?
• Triggering minimum bias– Minimum bias trigger scintilators (MBTS)– Random trigger and track trigger
• Tracking– Reconstructing low pT tracks– Vertex reconstruction– Using pixel tracklets
• Underlying event– What is it and why measure it– Measurement with ATLAS
• Summary
Cosners House Forum on LHC startup April 07
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1.6 10136.2 10121.7 10128.6 1011intensity per beam
1.1.45.12.06beam energy (MJ)
65102.80.7event rate 1 1 5 (kHz)
701130.8W rate 2 1 5 (per 24h)
71.10.30.08Z rate 3 1 5 (per 24h)
1.6 10302.6 10297 10282 1028Luminosity 1 5
11111111* 1 5 (m)
10442ib (1010)
1561564343Kb
1.6 10136.2 10121.7 10128.6 1011intensity per beam
1.1.45.12.06beam energy (MJ)
65102.80.7event rate 1 1 5 (kHz)
701130.8W rate 2 1 5 (per 24h)
71.10.30.08Z rate 3 1 5 (per 24h)
1.6 10302.6 10297 10282 1028Luminosity 1 5
11111111* 1 5 (m)
10442ib (1010)
1561564343Kb
Installation
Hardware CommissioningShutdown
Engineering Run
450GeV
Hardware Commissioning
450GeV
Installation
Hardware CommissioningShutdown
Engineering Run
450GeV
Hardware Commissioning
450GeV
Collisions 450GeV
Ramp commissioning
Beam commissioningBeam commissioning
450GeV450GeV
Machine checkoutMachine checkout
450GeV450GeV
Collisions 450GeV
Ramp commissioning
Beam commissioningBeam commissioning
450GeV450GeV
Machine checkoutMachine checkout
450GeV450GeV
Events/xing<<1
LHC Pilot run: 2007
S. MyersAt L=1029, can get ~10M MB events in 1 day with 30% eff
Cosners House Forum on LHC startup April 07
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0.240.241.2 101.2 1032322.12.11.1 101.1 1033331571571.4 101.4 101414115 105 10101028082808
0.150.157.9 107.9 1031310.720.723.8 103.8 1032321261261.1 101.1 101414224 104 10101028082808
0.240.241.2 101.2 1032321.11.15.9 105.9 1032321571571.4 101.4 1014142255 1010101028082808
0.150.152.6 102.6 103131<< 1<< 12.4 102.4 10313142423.7 103.7 10131311114 104 101010936936
0.150.152.6 102.6 1031310.730.731.3 101.3 10323242423.7 103.7 101313224 104 101010936936
0.150.154.4 104.4 1030300.760.762.2 102.2 103131776.2 106.2 101212224 104 101010156156
0.150.151.2 101.2 103030<< 1<< 11.1 101.1 103030221.7 101.7 10121211114 104 1010104343
Rates in 2 (and 8)Rates in 2 (and 8)Rates in 1 and 5Rates in 1 and 5Beam levelsBeam levelsParametersParameters
0.150.15
Events/Events/
crossingcrossing
1.2 101.2 1030300.760.766.1 106.1 103030221.7 101.7 101212224 104 1010104343
LuminosityLuminosity
(cm(cm--22ss--11))
Events/Events/
crossingcrossing
LuminosityLuminosity
(cm(cm--22ss--11))
EEbeambeam
(MJ)(MJ)
IIbeambeam
protonproton* 1,5* 1,5
(m)(m)
NNkkbb
0.240.241.2 101.2 1032322.12.11.1 101.1 1033331571571.4 101.4 101414115 105 10101028082808
0.150.157.9 107.9 1031310.720.723.8 103.8 1032321261261.1 101.1 101414224 104 10101028082808
0.240.241.2 101.2 1032321.11.15.9 105.9 1032321571571.4 101.4 1014142255 1010101028082808
0.150.152.6 102.6 103131<< 1<< 12.4 102.4 10313142423.7 103.7 10131311114 104 101010936936
0.150.152.6 102.6 1031310.730.731.3 101.3 10323242423.7 103.7 101313224 104 101010936936
0.150.154.4 104.4 1030300.760.762.2 102.2 103131776.2 106.2 101212224 104 101010156156
0.150.151.2 101.2 103030<< 1<< 11.1 101.1 103030221.7 101.7 10121211114 104 1010104343
Rates in 2 (and 8)Rates in 2 (and 8)Rates in 1 and 5Rates in 1 and 5Beam levelsBeam levelsParametersParameters
0.150.15
Events/Events/
crossingcrossing
1.2 101.2 1030300.760.766.1 106.1 103030221.7 101.7 101212224 104 1010104343
LuminosityLuminosity
(cm(cm--22ss--11))
Events/Events/
crossingcrossing
LuminosityLuminosity
(cm(cm--22ss--11))
EEbeambeam
(MJ)(MJ)
IIbeambeam
protonproton* 1,5* 1,5
(m)(m)
NNkkbb
Possible Beam Parameter Evolution at 7TeV/beam
S. Myers
Cosners House Forum on LHC startup April 07
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Why measure min bias?
Not exactly what the LHC was built for!But….. • Physics: measure dN/d|=0
– Compare to NSD data from SppS and Tevatron
• MB samples for pile-up studies– Calorimeter– Physics analyses– Benchmark for sLHC
• Overlap with underlying events– analyses eg VBF, Jets…
• Demonstrate that ATLAS is operational• Inter-calibration of detector elements
– Uniform events• Alignment • Baseline for heavy ions
Cosners House Forum on LHC startup April 07
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What are minimum bias events?
pp collisions at
√s = 14TeV
PYTHIA6.323 PHOJET1.12
σtot
σelas
σsingle dif (AB→XB)
σ single dif (AB→AX)
σdouble dif
σnon-dif
101.5 mb
22.2 mb
7.2 mb
7.2 mb
10.3 mb
54.7 mb
119.1 mb
34.5 mb
5.5 mb
5.5 mb
4.1 mb
69.5 mbine
last
ic e
ven
ts
Trigger bias:Typically select non-single diffractive
(NSD) events
Cosners House Forum on LHC startup April 07
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What are minimum bias events?
• Minimum bias are inelastic collisions of two protons– Includes very rare high-pt scatters and very common low-pt scatters
• Minimum bias is an experimental definition– Defined by experimental trigger and analysis
• Relation between experiment and physics is:
inelasticndinelasticndddddsdsdmeasuredfff
fi are acceptances for different physics processes determined by the trigger
Cosners House Forum on LHC startup April 07
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Event characteristics
• Event characteristics– Non-single diffractive~non-diffractive
inelastic
– Soft tracks pTpeak~250MeV
– Approx flat distribution in to ||~3 and in
– Nch~30; ||<2.5
• Trigger rates ~70mb (NSD!)
– R~700kHz @ L=1031cm-2s-1
M. Leyton
Cosners House Forum on LHC startup April 07
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Triggering minimum bias
• Use forward scintillators (MBTS)• Random trigger
– inefficient where there are <<1 events/xing– Validate at L2/EF to reduce inefficiency
• Other triggers: jet, lepton triggers– High pt thresholds bias
Cosners House Forum on LHC startup April 07
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MBTS
• Trigger scintillation counters mounted on end of LAr calorimeter covering same radii as ID
ηη=2.0=2.0
ηη=3.8=3.8interactiointeraction pointn point
Beam-pipeBeam-pipe
++ηη
pan
nel
pan
nel
MBTMBTSS
UA5• To compare to UA5 and CDF data
need to understand composition of the sample trigger bias
• Currently generate inelastic and diffractive parts using PYTHIA
– Need to investigate other simulations-PHOJET
Cosners House Forum on LHC startup April 07
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• Noise spectrum fits well to a Gaussian with σ=2.52pC
• Beyond 9pC (almost 4σ), non-Gaussian behavior is possible
Accidental rate must be suppressed to ~Hz, limited by EF output-rate of 100Hz
suppression required of 10-6-10-7 at 900GeV and 14TeV
Noise and Accidental Suppression – Lab testsEric Feng
Cosners House Forum on LHC startup April 07
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Trigger Efficiencies and Bias
• Shown below for each coincidence logic:- Required counter threshold to suppress accidental rate to 1Hz- Corresponding trigger efficiency for each type of event
Coincidence Logic Counter Threshold [pC]
Trigger Efficiency
900 GeV 14 TeV
Non-Diff Single/
Double Diff.
Non-Diff Single/
Double Diff.
1+1 10.75 98% 35% 99% 42%
2+2 7.75 97% 27% 98% 36%
3+3 6.25 94% 20% 97% 30%
4+4 5.25 90% 15% 95% 24%
5+5 4.50 85% 10% 92% 19%
6+6 4.00 79% 7% 89% 14%
Eric Feng
Cosners House Forum on LHC startup April 07
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Track Trigger
L2 track trigger to validate random trigger when <<1 events/xing
LVL 1
LVL 2
Event-Filter
1 kHz eventrate
100 Hz eventrate
13 MHz eventrate
reduction by a factor 10
storage
pass-through
random trigger
R. Kwee
Cosners House Forum on LHC startup April 07
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Selecting MinBias Events• At L2 can use
– no. of spacepoints from pixel and SCT little bias
– no. of reconstructed tracks, needed to reject beamgas, halos pt-track > ~1GeV biased
• Rejection of empty events, compare to cosmic runs
<sct-spacepoints>=3 <sct-spacepoints>=0
noise spacepoints from ¼ sct-barrel from cosmics data simulation of noise in whole SCT
R. Kwee
Cosners House Forum on LHC startup April 07
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Rejecting beam-gas
To reject beam gas need track reconstruction
SCT-cut
at 20 sp
pixel-cut
at 900 sp
minbias 99.2 86.3
noise 0 0.4
beamgas 77.3 31.6
Diffractive (SD+DD)
50.2 15.0 R. Kwee
Beamgas Diffractive
Cosners House Forum on LHC startup April 07
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CMS MB trigger: Trigger Towers
• Use Forward HCAL – Towers: N vs P
• Strongly correlated
• Trigger should cut on nTowers hit on “N” side and “P” side– e.g. N>10 and P>10
ORCA Simulation
Cosners House Forum on LHC startup April 07
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Tracking in MB events
• Acceptance limited in rapidity and pt
• Rapidity coverage– Tracking covers ||<2.5
• pT problem
– Need to extrapolate by ~x2 Need to understand low pt
charge track reconstruction
1000 events1000 events
dNch/d
dNch/dpT
Black = Generated (Pythia6.2)
Blue = TrkTrack: iPatRec
Red = TrkTrack: xKalman
Reconstruct tracks with:Reconstruct tracks with: 1) pT>500MeV1) pT>500MeV 2) |d2) |d00| < 1mm| < 1mm 3) # B-layer hits >= 13) # B-layer hits >= 1 4) # precision hits >= 84) # precision hits >= 8
ppTT (MeV) (MeV)
Cosners House Forum on LHC startup April 07
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Where is the Momentum Limit ?
• Tracker is in principle sensitive to soft tracks– Pt = 400 MeV - tracks reach end of TRT
– Pt = 150 MeV - tracks reach last SCT layer
– Pt = 50 MeV - tracks reach all Pixel layers
150MeV
50MeV
400MeV
A.Salzburger
Cosners House Forum on LHC startup April 07
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Track reconstruction efficiencies
• MC/Track cuts: • η < 2.5, z0 < 150 mm, d0 < 1 mm• Efficiency: fraction of MC tracks with > 50%
matched hits
Pt=200MeV
Tracking efficiency >20-50% for pt>200MeV
M.Leyton
pT (MeV)
Cosners House Forum on LHC startup April 07
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Dan ClemensTrack Reconstruction efficiencies by track insertion
efficiency by track insertion
reconstruction with New Tracking
Plot by M.Leyton for comparison : truth matched
•Insert a track into a minimum bias event by overlapping a simulated single track event.
•Reconstruct merged event and look for inserted single track to determine efficiency.
Cosners House Forum on LHC startup April 07
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Track multiplicity
X V
erte
x R
esid
uasl
Minbias Vertex Reconstruction
I. Dawson, K. Prokofiev
• How well can we reconstruct primary vertex position of a minbias event?
– A good vertex reconstruction efficiency in multi-pp bunch crossings may allow measurement of σNSD.
– Knowledge of primary vertex position can improve signal to noise, eg, H →γγ (UE to Higgs similar to MB)
• Use VxBilloirTools package to fit vertex to minbias tracks.
• Shown here are residuals (difference between reconstructed and truth)
Typically ~250μm precision
Cosners House Forum on LHC startup April 07
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Tracklet Method
Primary vertex(standard tracking)
Hits in B-layer
Hits in Layer 1
ηΦ
Each hit assigned η,Φ relative to primary vertex
Calculate Δη,ΔΦ for each pair
“Tracklet” defined (now) as Δη<0.01,ΔΦ<0.15
Peter Steinberg, BNL
Exploring methods with sensitivity at lower pTwith even fewer pixel-hits
Cosners House Forum on LHC startup April 07
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Single Event
Φ
η
Hits in B-layer and L1. Black circles are Truth-primary charged hadrons.Clustered pairs of hits visible near truth hadrons.
Peter Steinberg, BNL
Cosners House Forum on LHC startup April 07
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“Momentum Reconstruction”
pT = 0.011/ΔΦ → Poor person’s tracking code. spatial resolution kicks in above ~2 GeV
Peter Steinberg, BNL
Cosners House Forum on LHC startup April 07
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Charged primary hadronsReconstructed
Peter Steinberg, BNL
at low pT (down to 100 MeV) the recontructed and truth pT match well, at higher pT reconstruction gets distorted
Momentum reconstruction
Cosners House Forum on LHC startup April 07
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Inclusive dNch/dη
Current parameters good for η~0, large backgrounds at |η|~2
Peter Steinberg, BNL
Charged primary hadrons
Reconstructed
Focus on |η|<1, look at multiplicity distributionslinear response,good resolution
Cosners House Forum on LHC startup April 07
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CMS Tracking
Similar to ATLAS pixel tracklets
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What is the underlying event
High PT scatter
Beam remnants
ISR
Hard and soft physics• ISR• beam remnants• central particle production• colour flow/string drawing
Cosners House Forum on LHC startup April 07
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VBF Signal (HWWll)
•forward tagging jets
•correlated isolated leptons
• low hadronic activity in central region
•central Higgs production
Tagging jet
Tagging jet
H
W
WZ/WZ/W
Important discovery channelFor Higgs in mass range120-200GeV
Cosners House Forum on LHC startup April 07
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Model Parameter
Simple MSTP(82)=1
PARP(81)=1.9
Complex MSTP(82)=4
PARP(82)=1.9
Tuned MSTP(82)=4
PARP(82)=1.8
PARP(84)=0.5
Model CJV (eff) LEPACC (eff) All Vetoes (eff)
Simple 0.943 ± 0.003 0.610 ±0.001 0.084 ±0.001
Complex 0.885 ± 0.003 0.575 ±0.001 0.076 ±0.001
Tuned 0.915 ± 0.003 0.589 ±0.001 0.080 ±0.001
Uncertainty at the level of ~6% on CJV Pt>20GeV and ~3% on leptonGiving a total uncertaintly in the range ~8%
Cosners House Forum on LHC startup April 07
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Average multiplicity of charged particles in the underlying event associated to a leading jet with Pt
ljet (GeV).
Average pTsum (GeV) of charged
particles in the underlying event associated to a leading jet with Pt
ljet (GeV).
Describing the region transverse to the leading jet
Cosners House Forum on LHC startup April 07
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Min-Max analysis
Jet #1 Direction
“Toward”
“TransMAX” “TransMIN”
“Away”
Jet #1 Direction
“Toward”
“TransMAX” “TransMIN”
Jet #2 Direction
“Away”
“Leading Jet”
“Back-to-Back”
•TransMAX and transMIN sensitive to radiation and soft UE respectively •Back-to-back sample suppresses radiation• difference between tranMAX region and transMIN in leading jet and b-2-b jet sample
Cosners House Forum on LHC startup April 07
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EETT jet #1jet #1 = 1.63TeV = 1.63TeV
EETT jet #2jet #2 = 1.16TeV = 1.16TeV
Measuring the underlying event at ATLAS
Cosners House Forum on LHC startup April 07
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First physics studies with the underlying event
~ 15 days of data taking in 2007 enough to cover up to pT(leading jet) ~ 40 GeV
~ 15 days of data taking in 2007 enough to cover up to pT(leading jet) ~ 40 GeV
ATLAS - √s=900 GeVATLAS - √s=900 GeVATLAS - √s=14 TeVATLAS - √s=14 TeV
√√s=14TeVs=14TeV
√√s=900GeVs=900GeV
Reconstructed tracksReconstructed tracks
Multiplicity of charged particles with pT > 0.5 GeV and ||< 1 in region transverse to leading jet
Cosners House Forum on LHC startup April 07
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Selecting the underlying event: |ηtrack | < 2.5, pT
track > 1.0 GeV/c
Njets > 1, |ηjet| < 2.5, ET
jet >10 GeV,
J1J1 J2J2 J3J3
J4J4J5J5
J6J6
charged particles (generator level)
J1J1 J2J2 J3J3
J4J4
J5J5
J6J6charged particles (generator level)
Measuring the underlying event in TeV jets
Cosners House Forum on LHC startup April 07
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Ra
tio
<N
Tra
ckR
eco>
/<N
Tra
ckM
C>
Ra
tio
<N
Tra
ckR
eco>
/<N
Tra
ckM
C>
Leading jet ET (GeV)Leading jet ET (GeV)
Selecting the underlying event: |ηtrack | < 2.5, pT
track > 1.0 GeV/c
Njets > 1, |ηjet| < 2.5, ET
jet >10 GeV,
Cosners House Forum on LHC startup April 07
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summary
• Minimum bias and underlying event are uncertain at LHC energies– Energy extrapolation for SppS and CDF not known– dN/d|=0 measurement can be done in early running
• Triggering– Both ATLAS and CMS are developing minimum bias triggers– Need to understand trigger bias to compare to UA5,CDF….
• Tracking– Low pt tracking is challenging but progress by both
ATLAS+CMS
• Underlying event can be measured with early data– Need to ensure overlap between jet and mb samples
• Waiting for data
Cosners House Forum on LHC startup April 07
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Extra slides
Cosners House Forum on LHC startup April 07
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We are taking REAL DATA !
• SR1 cosmics test stand– Final TRT barrel
– Final SCT barrel
– 1 sector cabled for both
Day zero start-up looks promising!
Cosners House Forum on LHC startup April 07
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• Offline reconstruction worked
with little problems within hours– Debug cabling…
• First results are very positive– See residuals and pulls
– No alignment or detailed
calibration yet !
– Reflects excellent mechanical
precision…
• More for Ringberg Workshop
in 2 weeks
• I am definitely very optimistic for first data taking having seen this !– Detector is on a good way !
• Pixels are recovering from cooling problem delay– Finish 3 layer system in time
First Results
Cosners House Forum on LHC startup April 07
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Tracking: Startup-Initial Alignment
• Very first alignment will be based on:– Mechanical precision
– Detailed survey data
– Cosmics data (SR1/Pit)
– Minimum bias events and inclusive bb
• Studies indicate good
efficiencies after initial alignment– Example taken from T.Golling
– Precision will need Zs and
resonances to fix energy scales,
constrain twists, etc…
M.Elsing Talk in SM meeting
Cosners House Forum on LHC startup April 07
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Non-diffractive+SD+DD
Non-diffractive only
Diffractive events
Coincidences in MBTSPYTHIA charged-particle level
Cosners House Forum on LHC startup April 07
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MBTS response
Non-diffractive inelastic events (sample 005001)
Non-diffractive Non-diffractive inelastic events: inelastic events: energy is deposited energy is deposited on both sides on both sides (typically)(typically)
Single and double diffractive inelastic events (sample 005002)
Single diffractive Single diffractive events: energy is events: energy is deposited on deposited on either sideeither side
Double diffractive Double diffractive eventsevents
A.Moraes
Cosners House Forum on LHC startup April 07
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How many events?
• For dN/d require ~10k• For UE need ~20M MB events to get some with leading jets
PT~30GeV• Need to overlap with jet triggers
– Prescaled PT>25GeV• ~40M events gives ~10 hits/strip in barrel SCT• CMS quote ~18M to intercalibrate their calorimeter with MB
Cosners House Forum on LHC startup April 07
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Performance goals
Cosners House Forum on LHC startup April 07
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Machine considerations
Cosners House Forum on LHC startup April 07
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LHC operational cycle
0
14000
-3000 -2000 -1000 0 1000 2000 3000
Time [s]
MB
cu
rre
nt
0
1
2
3
4
5
6
7
8
9
B [
T]
RAMP DOWNSTART RAMP
PHYSICS
PREPAREPHYSICS
BEAM DUMP
PREINJECTIONPLATEAU
INJECTION
T0 Tinj
SQUEEZE
PHYSICS
Ramp down 18 Mins
Pre-I njection Plateau 15 Mins
I njection 15 Mins
Ramp 28 MinsSqueeze 5 Mins
Prepare Physics 10 MinsPhysics 10 - 20 Hrs
Cosners House Forum on LHC startup April 07
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Stage I physics runStage I physics run
Start as simple as possibleStart as simple as possible
Change 1 parameter Change 1 parameter (k(kbb N N **1 , 51 , 5)) at a timeat a time
All values for All values for nominal nominal emittanceemittance
7TeV7TeV
10m 10m * in point 2 (luminosity looks fine)* in point 2 (luminosity looks fine)
Rates in 2Rates in 2Rates in 1 and 5Rates in 1 and 5Beam levelsBeam levelsParametersParameters
0.770.77
0.150.15
0.150.15
0.150.15
<< 1<< 1
<< 1<< 1
Events/Events/
crossingcrossing
2.2 102.2 1031313.93.91.1 101.1 10323216161.4 101.4 1013132299 10101010156156
4.4 104.4 1030300.760.762.2 102.2 103131776.2 106.2 101212224 104 101010156156
1.2 101.2 1030300.760.766.1 106.1 103030221.7 101.7 101212224 104 1010104343
1.2 101.2 103030<< 1<< 16.8 106.8 102929221.7 101.7 10121218184 104 1010104343
7.7 107.7 102828<< 1<< 14.2 104.2 1028280.50.54.3 104.3 1011111818101010104343
1.8 101.8 102727<< 1<< 1101027271010--221 101 10101018181010101011
LuminosityLuminosity
(cm(cm--22ss--11))
Events/Events/
crossingcrossing
LuminosityLuminosity
(cm(cm--22ss--11))
EEbeambeam
(MJ)(MJ)
IIbeambeam
protonproton
* 1,5* 1,5
(m)(m)
NNkkbb
FfkN
Ln
b*
2
4
fk
LCrossEventrate
b
TOT/
Protons/beam ? 1013
(LEP beam currents)
Stored energy/beam ? 10MJ(SPS fixed target beam)
Cosners House Forum on LHC startup April 07
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Physics inefficiencies
• What fraction of primary particles reach the various detector layers, versus pT?
• Charged particle component of MB events dominated by +-, with ~10% K+-. First look at -
1) Pixel system should provide 3 space points down to ~100MeV/c.
2) Down to ~150MeV/c, particles can traverse SCT magnetic field, but reduced efficiency below 300MeV/c due to inelastic interactions in ID material.
2
Cosners House Forum on LHC startup April 07
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Physics inefficiencies
What about the charged kaons?
1) Much larger inefficiencies evident, due to increased material interactions and decays.
2) For pT=300MeV/c, ~10% of K do not reach final Pixel layer.
→ For pT=100MeV/c, K do not even make it into the Pixel system!
3) In the SCT system, 15% of 500MeV/c K do not reach last SCT layer.
→ Below 300MeV/c no spacepoints in outer SCT.
3