sm + top physics summary
DESCRIPTION
SM + Top physics Summary. Marina Cobal, ATLAS Overview Week Freiburg, October 2004. Outline. Lot of activity…I had to make some selection! SM results in 2004 TGCs and AQGCs PDFs Underlying Events and Minimum bias Top results in 2004 New spin correlation studies - PowerPoint PPT PresentationTRANSCRIPT
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SM + Top physics Summary
Marina Cobal,ATLAS Overview WeekFreiburg, October 2004
Marina Cobal - Atlas Week, Oct 2004
P 2
Outline Lot of activity…I had to make some selection!
SM results in 2004 TGCs and AQGCs
PDFs
Underlying Events and Minimum bias
Top results in 2004 New spin correlation studies
New study on resonances
Commissioning with the top
Plans
3
TGCs and QGCs TGCs and QGCs
Marina Cobal - Atlas Week, Oct 2004
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Triple Gauge-boson Couplings
Non-abelian SU(2)L×U(1) Y gauge group (foundation of SM!)
WWγ WWZ couplings
most-general C & P conserving WWZ,WWγ vertices are specified by just 5 parameters:
model independent parameterization Probe tool: sensitive to low energy remnants of new physics operating at a higher scale complement to direct searches
big advantage for LHC
S.M in the ZERO , Δκ ,Δκ ,g
s like grow
Z
s like grow
Z1Z
L. Simic
Marina Cobal - Atlas Week, Oct 2004
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Events selection
New study covers the purely leptonic channel:
Selection
PT>25 GeV, |η|<2.5 ET
miss >50 GeV Z mass constraint |MZ-Mll|<15 GeV Veto Jets with PT(jet)>10 GeV and |η(jet)|<3
The expected number of events at 30 fb-1 is 7800, with 11% background.
Background:
μ)e,(lν,νllWW
ZZZ,Z,W,tt
New!!
Marina Cobal - Atlas Week, Oct 2004
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TGC’s in WW processesInclusion of anomalous couplings at WWZ and WWγ vertices yields enhancements in: (WW) at large values in and k PT(W) or PT(e) distributions PT(WW) or PT(e+e-) distributions
NLO corrections are large in the same regions.
Theoretical arguments suggest that anomalous TGC’s are at most of
O (m2W/Λ2)
Λ is scale of new physics.For Λ~1 TeV TGC’s are O(10-2).
Marina Cobal - Atlas Week, Oct 2004
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Confidence intervals for TGC’s
In WW processes 95% confidence intervals for TGC parameters
for 30 fb -1 and ΛFF= 2 TeV are:
WW process WZ and Wγ process
-0.028 < ΔκZ < 0.057 -0.11 < ΔκZ < 0.12
-0.035 < λZ < 0.026 -0.0073 < λZ < 0.0073
-0.077 < Δκγ < 0.15 -0.075 < Δκγ < 0.076
-0.061 < λγ < 0.063 -0.0035 < λγ < 0.0035
-0.13 < Δg1Z < 0.42 -0.0086 < Δg1
Z < 0.011
for HIZ (equal coupling) scheme:
ΔκZ= Δκγ=Δκ λZ= λγ=λ -0.024 < Δκ < 0.025-0.017 < λ < 0.017 WW channel will be competitive
(1d fit, not systematic included) with WZ and Wγ in determining
limits for ΔκV
M. Dobbs,M. Lefebvre: ATL-PHYS-2002 023/022
Marina Cobal - Atlas Week, Oct 2004
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W : Quartic Gauge Couplings LHC able to probe AQGCs through vector boson fusion and triple
vector boson production
Wgood place to start looking for: low partonic centre of mass required suppressed by only one branching ratio
W production is sensitive to possible AQGC of the form WW – this is SM?
Effects of AQGC on the qq to l with l = e or at LHC have been studied by Eboli et al who have a MC with AQGCs implemented
Effects of AQGC can be seen on distributions of PT() and invariant mass of the photon pair, M.
P. Bell
Marina Cobal - Atlas Week, Oct 2004
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Consider only the exclusive channel qq to e Process events using ATLFAST Cross section falls rapidly with the cut on PT
Two photons, PT>15 geV, |n|<2.4
One e with PT > 25, |n|<2.4
Missing ET > 20 GeV R > 0.4, Re > 0.8 MT (l)> 65 GeV
Simulation chain Weighted events from W2GRAD Implemented as external process in PYTHIA within ATHENA-ATLFAST PYTHIA added beam remnants, QCD showers and underlying events and do the
frag/decay etc as usual Accept/reject alg then passes events of unit weight to ATLFAST
W in ATLAS
Eboli et al
PT
Marina Cobal - Atlas Week, Oct 2004
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Results
Background: W +1 jet and
W+2 jet events with one or both
jets are misidentified as a , with
probability 1/Rjet (ATL-PHYS-99-016
gives Rjet=1300 @ low L, Pj = 20GeV)
106 W + jet and W + 2jet events
going through PYTHIA + ATLFAST
If ~ 80% for e and , S=14 evts
in 30fb-1, so about 60 for l+-.
Background ~ 13 evts
PT and M dist. show that the SM background and mis id can still be removed by cutting at PT = 200 GeV, so that Eboli’s conclusions hold even with these backgrounds
Feature of W production In SM. Amplitude for qq W vanishes for cos* = -1/3 ( angle between the q and the W in parton CMS)
This radiation is preserved in the limit of 2 collinear ’s
Vanishes as increases separation criteria for detection
Independent from PT() and same in lab and parton frames
Dip at 0.1 for cos > 0 after suppressing radiative W events
Previous selection + opening angle requirement
Chance in 3 y, clear with 100fb-1
The Radiation Zeroe-e expectation in 30fb-1
[1] x 4
[1]
[2]
[3]
e-e expectation in 100fb-1 x 4
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PDFs PDFs
Marina Cobal - Atlas Week, Oct 2004
P 13Parton parameterizations
Uncertainty on the PDF’s: Propagation of uncertainties on
experimental data to the fitted PDF’s Statistical uncertainties and
(correlated) systematic effects Uncertainties in the theoretical
description of the fit procedure Flavour thresholds, s
Scales uncertainties Nuclear effects Higher twist, …
A number of groups have published the PDF fits with propagated experimental uncertainties: Botje (Eur Phys J C14 (dec 1999)) CTEQ (J. Pumplin et al, hep-ph/0201195) MRST (A. Martin et al, hep-ph/0211080)
Alekhin (S. Alekhin, hep-ph/0011002) Fermi2001 (Giele et al, hep-ph/0104052)
Theoretical uncertainties not treated here
PDF’s obtained from QCD DGLAP evolution fits to data. DIS data from fixed target and
HERA Jet cross sections pp colliders Drell-Yan processes
Z rapidity ZEUS-S W+ rapidity ZEUS-S
Z rapidity MRST02 W+ rapidity MRST02
For LHC:
fits of ZEUS and MRST02in agreement within PDF uncertainties
Both fits use conventional NLOQCD evolution in the DGLAP formalism including data taken at very low-x (down to x=6 10 -5)
Whereas this formalism still fits the data very well, there are theoretical reasons to doubt its validity at low-x
(Devenish and Cooper-Sarkar, ‘Deep Inelastic Scattering’, OUP 2004, Section 6.6.6 and Chapter 9)
A.M. Cooper-Sarkar
Z rapidity MRST02 W+ rapidity MRST02
W+ rapidity MRST03Z rapidity MRST03
PDFs derived from a fit without low-x data : MRST03 conservative partons’.
Compare predictions for W/Z production to those of the ‘standard’ PDFs
The two predictions are very different for LHC(not so different for tevatron)
W and Z rapidity distributions for │η│< 2.4 are potentially sensitive to the treatment of low-x QCD evolution.
R = W- / W+ MRST02
(W)/(W+)=0.74
R = W- / W+ MRST03
(W-)/(W+)=0.79
R = W- / W+
ZEUS-S
(W-)/(W+)=0.75±0.02
The shape of the ratio of W rapidities seems well suited to NOT seeing these differences between different PDFs – minimizing PDF errors-
The magnitude of the ratio differs by ~5%. Is the luminosity error small enough to see this? Is this a good luminosity monitor?
Suppose we WANT to see the differences ? Differences evident for the Z rapidity distributions, once one has moved away from central rapidity. How well can we measure the Z rapidity distribution?
Need to know the efficiency of Z reconstruction for │y│> 1.2 accurately because the difference between MRST02 and MRST03 at central rapidity is mostly normalisation.
The shape differences become most evident for │y│~2
Z reconstruction efficiency vs y
MRST02
MRST03
Similar exercise done with the W’s
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UE and Min Bias UE and Min Bias
Marina Cobal - Atlas Week, Oct 2004
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Min bias events
Definition depends on the experiment trigger! Usually is associated to non-single-diffractive events (NSD) – (see ISR<UA5,E735,CDF…)
Dominated by soft interactions, although there is some contributions from hard scattering
difndifddifselastot ...
σNSD ~ 65 - 73mbσtot ~ 102 - 118 mb
(PYTHIA) (PHOJET)
(PYTHIA)
Marina Cobal - Atlas Week, Oct 2004
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UE in charged jet evolution
Everything except the two outgoing hard scattered jets
In a hard scattering process, the underlying event has a hard
component (ISR+FSR and particles from the outgoing hard
scattered partons) and a soft component (beam-beam
remnants)
Many published data:
Durham HEP database
JetWeb
ljet
CDF analysis:• charged particles: pt>0.5 GeV and |η|<1
• cone jet finder:
7.022 R
Marina Cobal - Atlas Week, Oct 2004
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Comparing different PYTHIA tunings to data
F(z
) =
<n ch
g > P
(nch
g)
z = nchg /<nchg >
High-multiplicity events are described differently by each tuning
√s (GeV)
dN
chg/
dη
at
η=
0
LHC
Marina Cobal - Atlas Week, Oct 2004
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• CDF tuning: tuned particularly to UE data; doesn’t correct for particle decays which affect minimum bias distributions.
Tra
nsv
erse
< N
chg >
Pt (leading jet in GeV)
Rat
io (
MC
/Dat
a)
Comments:
• ATLAS – TDR: not tuned to UE data; doesn’t include double diffraction for minimum bias events; uses a model with a small hadronic core size; doesn’t correct for particle decays which affect minimum bias distributions and doesn’t includea pTmin energy dependence.
dNchg/dη ~ 10
dNchg/dη ~ 15
Central Region(data dNchg/dη ~ 4)
Marina Cobal - Atlas Week, Oct 2004
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Marina Cobal - Atlas Week, Oct 2004
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Data-MC Multiplicity information: ‹nch›, dN/dη, KNO
Transverse region (UE): ‹nch› and ‹pTsum›
Marina Cobal - Atlas Week, Oct 2004
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LHC predictions: PYTHIA6.214 – tuned vs. CDF Tuning
Tra
nsv
erse
< N
chg >
Pt (leading jet in GeV)
LHC
Tevatron
x 2
x 3
dNchg/dη ~ 20
dNchg/dη ~ 30
Central Region(min-bias dNchg/dη ~ 7)
Marina Cobal - Atlas Week, Oct 2004
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LHC predictions
Marina Cobal - Atlas Week, Oct 2004
P 27Summary on UE and min bias
Current min bias and UE data can be described with appropriate
tuning for PYTHIA and PHOJET
PYTHIA6.214-tuned and PHOJET1.12 with its default settings give
the best global agreement to the data. They generate LHC
predictions with ~30% difference for min bias, and ~ a factor of 2 for
UE distributions
More activity in the UE than predicted for an average min bias event!
PYTHIA6.214-tuned predicts an increase (~ a factor of 2) in this
activity when extrapolating from Tevatron to LHC, whereas
PHOJET1.12 suggests the ratio UE/min bias will remain the same
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Top: Resonances Top: Resonances
Marina Cobal - Atlas Week, Oct 2004
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Many theoretical models include resonances decaying to ttbar SM Higgs (but BR smaller with respect to the WW and ZZ decays) MSSM Higgs (H/A, if mH,mA>2mt, BR(H/A→tt)≈1 for tanβ≈1) Technicolor Models, strong ElectroWeak Symmetry Breaking, Topcolor, “colorons” production,
[…]
Generic resonance with 350 GeV<MX<5 TeV ; X < det and X > det
Signal observation above continuum BKG within a 2det mass window
=0.9545 fraction of signal within window Signal must have
a stat significance > 5
must contains ≥ 10 evts
Analysis : determination of X , tt and (fraction of BKG in window)
decaytX BrBr L10).(
New study on Resonances
)()5(. decaytXttBrBr
ttL
E. Cogneras, D. Pallin
Need to look for an alternative method for high MX As for high pt jet method used in Top mass measurement No isolated muon in the preselection (and trigger menu)
Efficiency Detector resolution
Low efficiency above MX ~2 TeV Jet overlap Muon less isolated
Marina Cobal - Atlas Week, Oct 2004
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Analysis repeated for standard model (the BKG!)
Effect of jet calibration in the
continuum:
Resonance mass shift (mtt)
all jets mis-cal. (mtt)= 0.61 10-2 mtt - 0.48 per % of mis-calib.
b jets mis-cal (mtt)= 0.36 10-2 mtt - 0.21 per % of mis-calib.
Typically for a 1TeV resonnance (mtt)= 5.6 GeV (all jets)
(mtt)= 3.4 GeV (b jets)
tt
ttbar continuum
Marina Cobal - Atlas Week, Oct 2004
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Resonance width x < det
Discovery potential Resonance width x = 2 det
Results close to TDR. For 300 fb-1
MX = 5OO GeV/c2 discovery if (.Br)> 1500 fb Needs to investigate newMX = 1 TeV/c2 discovery if (.Br)> 650 fb methods to improve atMX > 3 TeV/c2 discovery if (.Br)> 11 fb high mass
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Top: Spin Correlation Top: Spin Correlation
Marina Cobal - Atlas Week, Oct 2004
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Since no hadronisation: daughter keep spin info Study in semi-leptonic and di-leptonic
Spin analyser:
Leptonic: lepton
Hadronic: (W, b) or least energetic jet (lej)
Interesting angles:
• Θ1 (Θ2) : angle between chosen spin axis and spin
analyzer direction in the t(t) rest frame.
Spin axis is t(t) direction in the parton c.m.s. (helicity basis)
• φ : angle between spin analyzers direction in the t(t) rest
frame
Top Spin CorrelationE. Monnier, P. Pralavorio,
F. Hubaut
Marina Cobal - Atlas Week, Oct 2004
P 35
)coscos1(4
1
)(cos)(cos
121
21
2
Cdd
Nd
N
TopReX 4.05 (SM): LO spin correlation simulation Pythia 6.221 (NC): CTEQ5L and ISR-FSR AlpGen: used for W+jets background Tauola+Photos 2.6: t decay and radiative corrections Atlfast 2.60: ATLAS fast simulation and reconstruction
Unbiased estimator of C : -9 < cos 1 cos 2 > = 0.16
Unbiased estimator of D : -3 < cos φ > = -0.11
)cos1(2
1
cos
1
Dd
dN
N
Variables
C = degree of spin correlation in the helicity base
Marina Cobal - Atlas Week, Oct 2004
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Results for S + B ( stat. syst errors) : 80500 S, S/B=15 C(lej) = 0.21 0.015 0.04 = ~ 5 σ from 0 D(lej) = -0.12 0.01 0.02 = ~ 5 σ from 0 Study with full simulation just started scientific note
C extraction
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Top: Commissioning(see talk of S. Bentvelsen)
Top: Commissioning(see talk of S. Bentvelsen)
Marina Cobal - Atlas Week, Oct 2004
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Scenarios 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
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 top physics potential during first few months of running
S. Bentvelsen, M. Cobal
Marina Cobal - Atlas Week, Oct 2004
P 39 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
Marina Cobal - Atlas Week, Oct 2004
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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!
Marina Cobal - Atlas Week, Oct 2004
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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
Marina Cobal - Atlas Week, Oct 2004
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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
Need full simulation!!
Marina Cobal - Atlas Week, Oct 2004
P 43
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
Marina Cobal - Atlas Week, Oct 2004
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More efforts on..
Mtt reconstruction Top mass reconstruction using full simulation (“In situ”) W-calibration studies Better evaluation of the FSR systematics in the Mtop
reconstruction Single top studies… Involvement in the DC2 validation
(our group has a link-person dedicated to this since 1 year)
Marina Cobal - Atlas Week, Oct 2004
P 45 SM Group: Priorities for November & Rome
LHC Physics Environment: Parton Density Functions
studies just beginning. Expect results for Rome. understand how PDF knowledge will evolve in first years of data (i.e. PDF is largest syst. uncertainty in many studies, but will this be true
after LHC further constrains them?)
Underlying event need uniform tunings for Pythia / Jimmy timescale: DC2. Presentation in November. important e.g. for studies that use MC@NLO for signal, and Pythia for
backgrounds
Fundamental SM Measurements W-mass, Drell-Yan, and AFB in Z0e+e-
fast simulation only thus far specific performance issues need full sim. are performance assumptions realistic?
focus on lepton energy scale (dominates W-mass) forward electron tagging (for AFB)
Marina Cobal - Atlas Week, Oct 2004
P 46 Top Group: Priorities for November & Rome
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
More studies on the QCD W+jet events
Perform a preliminary cross section study for November. Finalize it for Rome
Present a “final” Mtt analysis for Rome
Draft note on commissioning with top events ready before Rome