Minimum bias studies etc….
Craig Buttar
University of Glasgow
Cosners House Forum on LHC startup April 07
1
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
2
LHC Pilot run: 2007
Hardware
Commissioning
450GeV
Installation
Hardware Commissioning
Machine checkout
450GeV
Beam commissioning
450GeV
Engineering
Run
450GeV
Collisions 450GeV
Ramp commissioning
Kb
43
43
156
156
ib (1010)
2
4
4
10
* 1 5 (m)
11
11
11
11
intensity per beam
8.6 1011
1.7 1012
6.2 1012
1.6 1013
beam energy (MJ)
.06
.12
.45
1.1
2 1028
7 1028
2.6 1029
1.6 1030
event rate 1 1 5 (kHz)
0.7
2.8
10
65
W rate 2 1 5 (per 24h)
0.8
3
11
70
Z rate 3 1 5 (per 24h)
0.08
0.3
1.1
7
Luminosity 1 5
At L=1029, can get ~10M MB events in 1 day with 30% eff
Cosners House Forum on LHC startup April 07
Shutdown
Events/xing
<<1
S. Myers
3
Possible Beam Parameter Evolution at 7TeV/beam
Parameters
Beam levels
Rates in 1 and 5
Rates in 2 (and 8)
kb
N
* 1,5
(m)
Ibeam
proton
Ebeam
(MJ)
Luminosity
(cm-2s-1)
Events/
crossing
Luminosity
(cm-2s-1)
Events/
crossing
43
4 1010
11
1.7 1012
2
1.1 1030
<< 1
1.2 1030
0.15
43
4 1010
2
1.7 1012
2
6.1 1030
0.76
1.2 1030
0.15
156
4 1010
2
6.2 1012
7
2.2 1031
0.76
4.4 1030
0.15
936
4 1010
11
3.7 1013
42
2.4 1031
<< 1
2.6 1031
0.15
936
4 1010
2
3.7 1013
42
1.3 1032
0.73
2.6 1031
0.15
2808
4 1010
2
1.1 1014
126
3.8 1032
0.72
7.9 1031
0.15
2808
5 1010
2
1.4 1014
157
5.9 1032
1.1
1.2 1032
0.24
2808
5 1010
1
1.4 1014
157
1.1 1033
2.1
1.2 1032
0.24
S. Myers
Cosners House Forum on LHC startup April 07
4
Why measure min bias?
Not exactly what the LHC was built for!
But…..
• Physics: measure dN/dh|h=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
5
inelastic events
What are minimum bias events?
pp collisions at
√s = 14TeV
PYTHIA6.323
PHOJET1.12
σtot
101.5 mb
119.1 mb
σelas
22.2 mb
34.5 mb
σsingle dif
(AB→XB)
7.2 mb
5.5 mb
σ single dif
(AB→AX)
7.2 mb
5.5 mb
σdouble dif
10.3 mb
4.1 mb
σnon-dif
54.7 mb
69.5 mb
Trigger bias:
Typically select non-single diffractive
(NSD) events
Cosners House Forum on LHC startup April 07
6
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:

measured
f  f  f
sd
sd
dd
dd

nd inelastic
nd inelastic
fi are acceptances for different physics processes determined by the trigger
Cosners House Forum on LHC startup April 07
7
Event characteristics
• Event characteristics
– Non-single diffractive~non-diffractive
inelastic
– Soft tracks pTpeak~250MeV
– Approx flat distribution in h to |h|~3
and in f
– Nch~30; |h|<2.5
• Trigger rates
 ~70mb (NSD!)
– R~700kHz @ L=1031cm-2s-1
M. Leyton
Cosners House Forum on LHC startup April 07
8
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
9
MBTS
• Trigger scintillation counters mounted on
end of LAr calorimeter covering same
radii as ID
η=2.0
η=3.8
Beam-pipe
+η pannel
interaction
point
MBTS
UA5
•
•
Cosners House Forum on LHC startup April 07
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 simulations10
PHOJET
Noise and Accidental Suppression – Lab tests
•
•
Eric Feng
Noise spectrum fits well to a
Gaussian with σ=2.52pC
Beyond 9pC (almost 4σ), nonGaussian 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
Cosners House Forum on LHC startup April 07
11
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%
Cosners House Forum on LHC startup April 07
Eric Feng
12
13 MHz eventrate
Track Trigger
random trigger
L2 track trigger to validate
random trigger when <<1
events/xing
LVL 1
1 kHz eventrate
pass-through
LVL 2
reduction by a factor 10
Event-Filter
100 Hz eventrate
R. Kwee
Cosners House Forum on LHC startup April 07
storage
13
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
noise spacepoints from ¼ sct-barrel from cosmics data
<sct-spacepoints>=3
simulation of noise in whole SCT
<sct-spacepoints>=0
R. Kwee
Cosners House Forum on LHC startup April 07
14
Rejecting beam-gas
Diffractive
Beamgas
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
Cosners House Forum on LHC startup April 07
To reject beam gas 
need track reconstruction
R. Kwee
15
CMS MB trigger: Trigger Towers
ORCA Simulation
•
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
Cosners House Forum on LHC startup April 07
16
Tracking in MB events
1000 events
dNch/dh
• Acceptance limited in rapidity
and pt
• Rapidity coverage
– Tracking covers |h|<2.5
• pT problem
– Need to extrapolate by ~x2
 Need to understand low pt
charge track reconstruction
h
Black = Generated (Pythia6.2)
Blue = TrkTrack: iPatRec
Red = TrkTrack: xKalman
dNch/dpT
Reconstruct tracks with:
1) pT>500MeV
2) |d0| < 1mm
3) # B-layer hits >= 1
4) # precision hits >= 8
Cosners House Forum on LHC startup April 07
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pT (MeV)
Where is the Momentum Limit ?
• Tracker is in principle sensitive to soft
tracks
50MeV
– Pt = 400 MeV - tracks reach end of TRT
– Pt = 150 MeV - tracks reach last SCT layer
– Pt = 50 MeV - tracks reach all Pixel layers
A.Salzburger
400MeV
Cosners House Forum on LHC startup April 07
150MeV
18
Track reconstruction efficiencies
• MC/Track cuts:
η < 2.5, z0 < 150 mm, d0 < 1 mm
• Efficiency: fraction of MC tracks with > 50%
matched hits
Tracking efficiency >20-50% for pt>200MeV •
Pt=200MeV
pT (MeV)
Cosners House Forum on LHC startup April 07
M.Leyton
19
Track Reconstruction
efficiencies by track insertion
Dan Clemens
efficiency by track insertion
reconstruction with New Tracking
•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
Plot by M.Leyton for
comparison : truth matched
20
Minbias Vertex Reconstruction
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)
X Vertex Residuasl
•
Track multiplicity
Typically ~250μm precision
Cosners House Forum on LHC startup April 07
I. Dawson, K. Prokofiev
21
Tracklet Method
Peter Steinberg, BNL
η
Hits in Layer 1
Exploring methods with
sensitivity at lower pT
with even fewer pixel-hits
Φ
Hits in B-layer
Primary vertex
(standard tracking)
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
Cosners House Forum on LHC startup April 07
22
Peter Steinberg, BNL
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.
Cosners House Forum on LHC startup April 07
23
Peter Steinberg, BNL
“Momentum Reconstruction”
pT = 0.011/ΔΦ → Poor person’s tracking code.
spatial resolution kicks in above ~2 GeV
Cosners House Forum on LHC startup April 07
24
Momentum reconstruction
Peter Steinberg, BNL
Charged primary hadrons
Reconstructed
at low pT (down to 100 MeV) the recontructed and truth pT match well,
at higher pT reconstruction gets distorted
Cosners House Forum on LHC startup April 07
25
Peter Steinberg, BNL
Inclusive dNch/dη
Charged primary hadrons
Reconstructed
Current parameters good for η~0,
large backgrounds at |η|~2
Cosners House Forum on LHC startup April 07
Focus on |η|<1,
look at multiplicity distributions
linear response,good resolution
26
CMS Tracking
Similar to ATLAS pixel tracklets
Cosners House Forum on LHC startup April 07
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What is the underlying event
Hard and soft physics
• ISR
• beam remnants
• central particle production
• colour flow/string drawing
High PT scatter
Beam remnants
ISR
Cosners House Forum on LHC startup April 07
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VBF Signal (HWWll)
•forward tagging jets
•correlated isolated leptons
• low hadronic activity
in central region
•central Higgs production
Tagging jet
W
Z/W
Z/W H
W
Tagging jet
Important discovery channel
For Higgs in mass range
120-200GeV
Cosners House Forum on LHC startup April 07
29
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
Simple
(eff)
LEPACC (eff)
All Vetoes (eff)
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 lepton
Giving a total uncertaintly in the range ~8%
Cosners House Forum on LHC startup April 07
30
Describing the region transverse to the leading jet
Average multiplicity of charged particles
in the underlying event associated to a
leading jet with Ptljet (GeV).
Cosners House Forum on LHC startup April 07
Average pTsum (GeV) of charged particles
in the underlying event associated to a
leading jet with Ptljet (GeV).
31
Min-Max analysis
“Leading Jet”
Jet #1 Direction
f
“Toward”
“TransMAX”
Jet #1 Direction
f
“TransMIN”
“Away”
“Toward”
“TransMAX”
“TransMIN”
“Away”
“Back-to-Back”
Jet #2 Direction
•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
32
Measuring the underlying event at ATLAS
ET jet #1 = 1.63TeV
ET jet #2 = 1.16TeV
Cosners House Forum on LHC startup April 07
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33
First physics studies with the underlying event
ATLAS - √s=900 GeV
ATLAS - √s=14 TeV
Reconstructed tracks
√s=14TeV
Multiplicity of charged particles
with pT > 0.5 GeV and |h|< 1 in
region transverse to leading jet
√s=900GeV
~ 15 days of data taking in 2007 enough to
cover up to pT(leading jet) ~ 40 GeV
Cosners House Forum on LHC startup April 07
 ~ 15 days of data taking in 2007
enough to cover up to pT(leading jet) ~
40 GeV
34
34
Measuring the underlying event in TeV jets
Selecting the underlying event:
charged particles (generator level)
Njets > 1,
|ηjet| < 2.5,
ETjet >10 GeV,
|ηtrack | < 2.5,
pTtrack > 1.0 GeV/c
charged particles (generator level)
J6
J6
J5
J5
J4
J4
J1
J2
J3
J1
Cosners House Forum on LHC startup April 07
J2
J3
35
Selecting the underlying event:
Cosners House Forum on LHC startup April 07
Njets > 1,
|ηjet| < 2.5,
ETjet >10 GeV,
|ηtrack | < 2.5,
pTtrack > 1.0 GeV/c
36
summary
• Minimum bias and underlying event are uncertain at LHC
energies
– Energy extrapolation for SppS and CDF not known
– dN/dh|h=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
37
Extra slides
Cosners House Forum on LHC startup April 07
38
Day zero start-up looks promising!
We are taking REAL DATA !
•
SR1 cosmics test stand
–
–
–
Final TRT barrel
Final SCT barrel
1 sector cabled for both
Cosners House Forum on LHC startup April 07
39
First Results
•
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
Cosners House Forum on LHC startup April 07
40
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
–
–
Cosners House Forum on LHC startup April 07
Example taken from T.Golling
Precision will need Zs and
resonances to fix energy scales,
constrain twists, etc…
M.Elsing Talk in SM meeting
41
Coincidences in MBTS
PYTHIA charged-particle level
Non-diffractive+SD+DD
Diffractive events
Non-diffractive only
Cosners House Forum on LHC startup April 07
42
MBTS response
Non-diffractive inelastic events
(sample 005001)
Single and double diffractive inelastic events
(sample 005002)
Single diffractive
events: energy is
deposited on either
side
Non-diffractive inelastic
events: energy is
deposited on both sides
(typically)
Cosners House Forum on LHC startup April 07
A.Moraes
Double diffractive
events
43
How many events?
• For dN/dh 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
44
Performance goals
Cosners House Forum on LHC startup April 07
45
Machine considerations
Cosners House Forum on LHC startup April 07
46
LHC operational cycle
PHYSICS
BEAM DUMP
9
14000
8
SQUEEZE
PREPARE
PHYSICS
RAMP DOWN
6
START RAMP
MB current
7
PHYSICS
B [T]
5
PREINJECTION
PLATEAU
4
3
INJECTION
2
T0
Ramp down
 18 Mins
1
Pre-Injection Plateau
15 Mins
Tinj
0
-3000
-2000
-1000
0
Time [s]
Cosners House Forum on LHC startup April 07
1000
0
Injection2000
 15 Mins
3000
Ramp
Squeeze
 28 Mins
 5 Mins
Prepare Physics
Physics
 10 Mins
10 - 20 Hrs
47
N 2 k b f
L
L
F Eventrate / Cross  TOT
*
kb f
4 n 
Stage I physics run



Start as simple as possible
Change 1 parameter (kb N  *1 , 5) at a time
All values for



nominal emittance
7TeV
10m * in point 2 (luminosity looks fine)
Parameters
Beam levels
Protons/beam ? 1013
(LEP beam currents)
Stored energy/beam ? 10MJ
(SPS fixed target beam)
Rates in 1 and 5
Rates in 2
kb
N
* 1,5
(m)
Ibeam
proton
Ebeam
(MJ)
Luminosity
(cm-2s-1)
Events/
crossing
Luminosity
(cm-2s-1)
Events/
crossing
1
1010
18
1 1010
10-2
1027
<< 1
1.8 1027
<< 1
43
1010
18
4.3 1011
0.5
4.2 1028
<< 1
7.7 1028
<< 1
43
4 1010
18
1.7 1012
2
6.8 1029
<< 1
1.2 1030
0.15
43
4 1010
2
1.7 1012
2
6.1 1030
0.76
1.2 1030
0.15
156
4 1010
2
6.2 1012
7
2.2 1031
0.76
4.4 1030
0.15
156
9 1010
2
1.4 1013
16
1.1 1032
3.9
2.2 1031
0.77
Cosners House Forum on LHC startup April 07
48
2
•
•
1)
2)
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 -
Pixel system should provide 3
space points down to ~100MeV/c.
Down to ~150MeV/c, particles
can traverse SCT magnetic field,
but reduced efficiency below
300MeV/c due to inelastic
interactions in ID material.
Cosners House Forum on LHC startup April 07
49
3
Physics inefficiencies
What about the charged kaons?
1)
2)
Much larger inefficiencies evident,
due to increased material
interactions and decays.
For pT=300MeV/c, ~10% of K do
not reach final Pixel layer.
→
3)
For pT=100MeV/c, K do not even
make it into the Pixel system!
In the SCT system, 15% of
500MeV/c K do not reach last
SCT layer.
→
Below 300MeV/c no spacepoints in
outer SCT.
Cosners House Forum on LHC startup April 07
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