top at startup of lhc
DESCRIPTION
Top at startup of LHC. Stan Bentvelsen (NIKHEF) October 2 nd , 2004. Top commissioning studies. Top physics at LHC Top one of ‘easiest’ bread and butter Cross section 830 ±100 pb Used as calibration tool Rich in ‘precision and new physics’ Top mass M t , cross section σ t - PowerPoint PPT PresentationTRANSCRIPT
Top at startup of LHC
Stan Bentvelsen (NIKHEF)October 2nd, 2004
Stan Bentvelsen Commissioning Meeting Freiburg
P 2
Top commissioning studies
Top physics at LHC Top one of ‘easiest’ bread and butter
Cross section 830±100 pb
1. Used as calibration tool
2. Rich in ‘precision and new physics’ Top mass Mt, cross section σt
Resonances decaying into top
Commissioning for the top group: Summarize studies already performed Tasks to do before startup What do we need as input from others? What can/should we provide to collaboration? Goals
Semi-leptonic top channel
detector tools involved:
Lepton reconstruction
Missing ET
Jets + calibration
B-tagging
Work done together with Marina Cobal
Stan Bentvelsen Commissioning Meeting Freiburg
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Initial commissioning studies
First evaluation of statistical uncertainty on σtop and Mtop
Gold-plated channel : single lepton
• pT (lep) > 20 GeV• pT
miss > 20 GeV• ≥ 4 jets with pT > 40 GeV• ≥ 2 b-tagged jets• | mjjb-<mjjb>|< 20 GeV
For initial run at LHC:(L = 1033 cm-2s-1)
Pretty small uncer-tainties after very short time of LHC running!
Period evts dMtop(stat)
1 year 3x105 0.1 GeV
1 month
7x104 0.2 GeV
1 week 2x103 0.4 GeV
/ -st
0.2 %
0.4%
2.5%
P. Grenier
Stan Bentvelsen Commissioning Meeting Freiburg
P 4 Systematic error on Mtop (TDR performance, 10 fb-
1)
Initial performance : uncertainty on b-jet scale expected to dominate
b-jet scale uncertainty Mtop
1% 0.7 GeV 5% 3.5 GeV 10% 7 GeV
Cfr: 10% on q-jet scale 3 GeV on Mtop
Stan Bentvelsen Commissioning Meeting Freiburg
P 5 Various scenarios currently under study
pp collisions What variations in predictions of t-tbar – which generator to use? Underlying event parameterization Background estimation from MC
Try to be as independent from MC as possible.
Detector pessimistic scenarios Partly or non-working b-tagging at startup Dead regions in the LArg Jet energy scale
Get good ‘feel’ for important systematic uncertainties – use data to check data
Software tools Many studies (not all!) only in fast simulation
It is clear we need to redo most important studies with full simulation
Estimate realistic potential for top physics during the first few months of running
Stan Bentvelsen Commissioning Meeting Freiburg
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Status of top event generators
‘Old’ Leading Order MC: Pythia: full standalone MC Herwig: full standalone MC TopRex (include spin correlations – interfaced to Pythia)
‘New’ NLO QCD calculations implemented in MC MC@NLO – interfaced to Herwig shower and fragmentation
This is relevant theoretical improvement Superseeds the old Pythia and Herwig MC’s. Validation done for this generator
Currently DC2 processes 106 MC@NLO t-tbar events Crucial for us to analyse these Waiting for Tier0 exercise to obtain reconstructed objects
Stan Bentvelsen Commissioning Meeting Freiburg
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Generators: MC@NLO, Herwig, Pythia
PT(tt system) Herwig & MCatNLO agree at low PT,
At large PT MCatNLO ‘harder’ PYTHIA completely off
Many more comparisons: see talks in top meeting
Example: distributions on top-anti-top characteristic – PT of the whole system
PT of t-tbar system is balanced by ISR & FSR
Stan Bentvelsen Commissioning Meeting Freiburg
P 8
Underlying event (UE) / minimum bias
Extremely difficult to predict the magnitude of the UEat LHC
Will have to learn much more from Tevatron before startup
Various models exists Herwig’s UE and minimum bias shows much less activity compared
to Pythia. This has always been a problem in Herwig.
Jimmy is developed as alternative model for UE at ep collisions Various ‘tunings’ exist – leading to wildly different results
More work is mandatory here Wish list to generate fully simulated events with Jimmy during DC2
post-production
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Tune of Butterworth
Standard Jimmy
Running at LHC energies
Standard Pythia
Standard Herwig
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P 10
Jimmy UE: Cells & Jets in Atlfast
Herwig vs Jimmy LO t-tbar
At jet-level effectreduced
10 GeV
Cell multiplicity
Cluster multiplicity
Jet multiplicity
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P 11
Reconstruct the top
Top peak for various reconstruction methods
Difference in mass can be as large as 5 GeV
Really need data to check data on UE Study effect better (as said)
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Background events
Top physics background Mistags or fake tags Non-W (QCD) W+jets, Wbbar, Wccbar Wc WW,WZ,ZZ Z tt Single top
AlpGen W+4 jets samples produced Very CPU intense (NIKHEF grid)
Un-weighting to W lepton (e,,) decay Production:
Effective : 2430 pb 380740 unweighted events generated
(2.6 10-5 efficiency) 3.41% (13002) events pass first selection
~ 150 pb-1 W+4jet background available
Largest background is W+4 jet.
This background cannot be simulated by Pythia or Herwig shower process. Dedicated generator needed: e.g. AlpGen. Large uncertainties in rate
Ultimately, get this rate from data itself. For example, measure Z+4 jets rate in data, and determine ratio (Z+4 jets)/(W+4 jets) from MC
W+4 extra light jets
Jet: Pt>10, ||<2.5, R>0.4
No lepton cuts
Initial grid: 200000*3
Events: 150·106
Jobs: 98~1.5 1010
events!
Stan Bentvelsen Commissioning Meeting Freiburg
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Non-W (QCD-multijet) background
Not possible to realistically generate this background Crucially depends on Atlas’ capabilities to minimize
mis-identification and increase e/ separation This background has to be obtained from data itself
E.g. method developed by CDF during run-1:
Rely now on e/ separation of 10-5
Use missing ET vs lepton isolation to define 4 regions:
A. Low lepton quality and small missing ET
Mostly non-W events (i.e. QCD background)
B. High lepton quality and small missing ET
Observation reduction QCD background by lepton quality cuts
C. Low lepton quality and high missing ET
W enriched sample with a fraction of QCD background
D. High lepton quality and high missing ET
W enriched sample, fraction of QCD estimated by (B·C)/(A·D)
Stan Bentvelsen Commissioning Meeting Freiburg
P 14
Detector scenarios: HV problems
Effects of dead HV regions om Mtop
Argon gap (width ~ 4 mm) is split in two half gaps by the electrode HV by Dη x Dφ = 0.2 x 0.2 (or 0.1 x 0.2) sectors, separately in each
half gap ~ 33 / 1024 sectors where we may be unable to set the HV on one
half gap multiply energy by 2 to recover
A.I. Etienvre, J.P. Meyer, J.Schwindling
particle
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100 000 tt events (~ 1.5 days at LHC at low L)
o Simulated using PYTHIA + ATLSIM
/ G3 (initial detector, h < 3.2)
o Reconstructed using ATHENA 7.0.0
Preselection of events:
o At least one recontructed e or μ
with PT > 20 GeV and η < 2.5
o ETmiss > 20 GeV
o At least 4 jets with PT > 20 GeV and h < 2.5
EM clusters
Jets
Analysis
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P 16
Results
If the 33 weak HV sectors die (very pessimistic), the effects on the top mass measurement, after a crude recalibration, are:
o Loss of signal: < 8 %
o Increase in background: not studied
o Displacement of the peak of the mass distribution: -0.2 GeV
This effect on the Top mass is (much) better known than other systematic uncertainties
Mtop(without ) – mtop(with dead regions)
Stan Bentvelsen Commissioning Meeting Freiburg
P 17
Detector scenarios: b-tagging
Precise alignment of ID can be reached only after few months of data taking.
The impact of misalignment can be much larger than having 2 instead of 3 layers
Top events to evaluate b-tagging efficiencies from data Select a pure t-tbar sample with tight kinematical cuts
Count the number of events with at least 1 tagged jet Compare 0 vs 1 vs 2 b-tagged jets in top events Can expect the b-tagging efficiency different in data from MC
In most pessimistic scenario b-tagging is absent at start
Can we observe the top without b-tagging?
Stan Bentvelsen Commissioning Meeting Freiburg
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Non b-tag tops
Selection: Isolated lepton with PT>20 GeV
Exactly 4 jets (R=0.4) with PT>40 GeV
Reconstruction: Select 3 jets with maximal resulting PT
t bjj
M (bjj)V. Kostiouchine
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Non btag: top sample
Signal plus background at initial phase of LHC
Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets
With extreme simple selection and reconstruction the top-peak should be visible at LHC
L = 150 pb-1
(2/3 days low lumi)
Stan Bentvelsen Commissioning Meeting Freiburg
P 20
Extraction of top signal
Fit to signal and background Gaussian signal 4th order polynomal Chebechev background In this fit the width of top is fixed at 12 GeV
Extract
cross section
and Mtop?
150 pb-1
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P 21 Can we see the W? (4 jets sample)
Select the 2 jets with highest resulting PT
W peak visible in signal No peak in background Better ideas well possible!
E.g. utilizing 2 body decay in top rest frame.
Select 2 jets with invariant mass closest to Mw (80.4 GeV)
Large peak in background Enormous bias Not useable!
150 pb-1
Stan Bentvelsen Commissioning Meeting Freiburg
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Fit to W mass
Fit signal and background also possible for W-mass
Not easy to converge fit Fix W width to 6 GeV
150 pb-1 mean σ(stat)
in peak 3.0% 5%
Mtop 167.0 0.8
Mw 77.8 0.7
These numbers for statistical uncertainties are consistent to the earlier study
Stan Bentvelsen Commissioning Meeting Freiburg
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Jet Energy scale / MC dependence
Variation of the jet energy scale to infer systematics Bjet scale: 0.92 – 0.96 – 1.00 –
1.04 – 1.08 Light scale:0.94 – 0.98 – 1.00 – 1.02 – 1.04
(1) (2) (3) (4) (5)
1) Analysis with jet energyscaled
2) All with MC@NLO, Herwigand Pythia;
3) Redo analysis with doubled W+4jet background (stat indep)
Top mass
155
160
165
170
175
180
0 5 10 15 20 25
Scale variations
To
p m
ass
Raw Top Mass
Scaled Top Mass
Cross section
0
200
400
600
800
1000
1200
0 5 10 15 20 25
Cross section
Determine Mtop and σ(top)
‘Raw’, i.e. no correction for jet scale
‘Corrected’, i.e. apply percentage difference of W-peak to the reconstructed top
Dependence on top mass reduced by scaling with W:
Rms Raw: 6.2 GeV
Rms Scaled: 1.2 GeV
Large dependence σ(top) on jet energy scale Via event selection.
Stan Bentvelsen Commissioning Meeting Freiburg
P 24
Some results… (still no b-tag)
Using 150 pb-1 of data:
Statistic uncertainty already smaller than these systematic variations
Note these numbers are very preliminary –
Luminosity uncertainty (15-20%) to be added!
How to judge these values? Systematics overestimated:
since all generators are used, with all energy scales; double counting
W+4jets rate can be measured from data
Systematics underestimated: No real FSR variation No other backgrounds
(e.g. WW, QCD) Trigger Non-uniformities
Need further detailed studies!
Please don’t thrust any of this
without Full simulation
mean Stat std percent
Mtop raw 168,1 0.8 6,2 3,7%
correcte
d 171,9 0.8 1,2 0,7%
σ(top) raw 817,2 5% 94,8 11,6%
Stan Bentvelsen Commissioning Meeting Freiburg
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Lower luminosity?
Go down to 30 pb-1 Both W and T peaks already
observable See something!
30 pb-1 mean σ(stat)
in peak 0.8% 17%
Mtop 170.0 3.2
Mw 78.3 1.030 pb-1
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P 26
Fit the lepton+hadronic top
Full kinematic fit to t-tbar system – no b-tagging Fit the neutrino Px and Py - and get Pz via W constraint Use W-mass constraints Require equal top masses for lepton and hadron side Repeat fit over all possible combinations of jets
Better suited for mass than for cross section Looks promising but further study needed…
E. Bos
Background shows structure
Cut on quality of fit (X2)
Stan Bentvelsen Commissioning Meeting Freiburg
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What with b-tagging on?
Now assume full b-tagging Efficiency 60%, mistag 1%
Background is rapidly decreasing See for example the W-mass peaks for 1 and 2 b-tags Same selection: 4 jet events W reconstructed as dijet mass; |Mjj-80.4| minimal for light-jets j
150 pb-1
Stan Bentvelsen Commissioning Meeting Freiburg
P 28
Reconstruct top mass
Sharp top mass peaks with little background Only use events for which |Mjj-80.4| < 20 GeV
Standard kinematic top reconstruction for 1 and 2 b-tags Background from W+4jets removed by b-tag requirement These results are very sensitive to b mis-tag rate
150 pb-1
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P 29
Mtt at startup?
Can we determine dσ(tt)/dmtt without b-tagging? Interesting to SuSy models that modify this cross section
Mtt is invariant mass over all final state to tt products In principle no
assignment of b-jet to top is needed
Suffer a lotfrom background
No reliable measurement of mtt without b-tagging
W+4jet background
Stan Bentvelsen Commissioning Meeting Freiburg
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Resonances decaying to t-tbar
Observe heavy resonances Xt-tbar during commissioning? Plot invariant mass of 4 jets + lepton + neutrino No intelligence in determining Pz neutrino here
Insert resonance at 2000 GeV Cross section * BR(tt) = 350 pb
‘True’ mtt distributions of resonance and ttbar events
Heavy resonances with large cross sections visible
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Need checks with full simulation
We are eagerly waiting for reconstructed DC2 events Repeat with full simulation for Rome next year
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Top group in Atlas:
Inputs to the top group
Estimate of the single electron trigger efficiency
Can be done by using the Z triggered as single electron
How much time is needed to arrive to a reasonable evaluation of this efficiency?
Estimate of the initial lepton identification efficiency
Estimate of the integrated luminosity
At the beginning the precision on L should be around 10-20%.
The ultimate precision should be < 5%
Eventually: B-tagging efficiency Jet scales
What we have to provide
Top candidates enriched samples A “pure” one, obtained with quite
tight selection criteria A “loose” one: a more “background
enriched” sample, to be used as control sample for background calculations etc…
Estimate of a light jet energy scale correction
Assume 10% for light and b-quark jets, look at effect on Mtop and stop
Assume that at the very beginning only the EM scale is known (means: do not put any weight on the hadronic scale)
Output: provide the MW peak to rescale the light jets
Estimate of the b-tagging efficiency
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Summary / Conclusion
Understand the interplay between using the top signal as tool to improve the understanding of the detector (b-tagging, jet E scale, ID, etc..) and top precision measurements
At LHC Top more easily found than W’s in 4 jet channel Using extreme simple selection, no b-tagging Need more work on background estimation, both from W’s and QCD, e/
ratio, trigger, lepton ID, etc … Remove dependence of results on MC as much as possible
Using few days of data taking (150 pb-1) Current estimate on cross section accuracy of (ball park) 10%
plus luminosity uncertainty Interestingly mass of top via fits to mass peak looks promising
(use W-mass as constraint to jet-scale) Need better ideas to isolate very pure top sample without b-tagging
Its clear we need full simulation Eagerly awaiting reconstructed DC2 events to repeat/extend these studies