Transcript Recent top quark results from DØ Amnon Harel for the DØ Collaboration

Recent top quark
results from DØ
Amnon Harel
[email protected]
for the DØ Collaboration
The top quark
• Extremely massive: mt ~ gold atom
• Discovered in 1995 by CDF and DØ at
the Fermilab Tevatron
– Only o(104) were ever produced
• Only quark that decays before hadronizing
– Can measure “bare” quark properties
Two such measurements by the DØ Collaboration:
1. Measurement of W helicity in tWb
2. Measurement of top mass using
the matrix element method
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The experiment
CDF
p
Tevatron
DØ
p
Highest energy
in the world !
Highest instantaneous
luminosity in the world !
Enjoying it
while it
lasts
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Top quark measurements at DØ
Top Pair Production
µ+
production cross section
production asymmetry
resonant production?
ttH production? p
~ production?
t1
W Helicity
b
W+
t-
t
Wq
νµ
p
b
Branching ratios:
non SM decays,
CKM matrix
top mass (1/6)
top charge
q’
µ+
Single Top Production
Production Cross Section
Anomalous Couplings
W’ Search
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b
p
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W+
t b
νµ
p
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Data samples
Run IIb,
dilepton
New 1.7fb-1
Run IIb,
l+jets
New 1.2fb-1
DØ upgrade
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A model-independent measurement of
W helicity
in top decays
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W helicity
So far measurements support the SM prediction: f(tWb) = ~100%
Breaking it down by W helicity states:

p
Left handed
λ=-1
 t  W b 
 t  Wb 
SM: 30.3%
f 

p
Longitudinal
λ=0
t  W0b 
t  Wb 
SM: 69.6%
f0 

s

s

p
Right handed
λ=1
t  W b 
t  Wb 
SM: 0.1%
f 

s
Distinguish between helicity states by
reconstructing cos q*:
e
q*
b
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t
ne
W
Arbitrary Normalization
SM uncertainties << Experimental uncertainties  can’t constrain SM parameters
Firm SM prediction, in particular: tiny f+  looking for new physics
Left-handed
Right-handed
Longitudinal
SM
cosq*
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W helicity
l+jets sample
b
W+
t
t
W-
q
q’
b
Multijet production
Estimated from data with
leptons that almost pass our ID
isolated, pT>20 GeV,
ν |η|<1.1(e) / 2.0(μ)
MET>20GeV,
triangle cut on
ΔΦ(l,MET)
Events
l+
note any b-tags
Run IIb,
l+jets
W(lν)+jets production
Same final state
4 Jets (pT>20 GeV, |η|<2.5)
Discriminant
• Signal and W+jets templates from MC.
• Matched ALPGEN + Pythia
• V+A and V-A signal MC reweighted to
yield desired cos q * distributions
• Data and MC are compared in control samples;
corrections applied for residual discrepancies
• Their amounts from fit to data sample.
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1075
108
505
63
34
376
Discriminant
Combines kinematic and b-ID information
Chose variables that:
• discriminate between signal and W+jets
• are well modeled
• are weakly correlated with cos q *
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W helicity
l+jets reconstruction
Partons l+
b
Observed objects
ν
W+
t
t
W-
q
b
q’
Parton-level
Particle-level
QCD
Simulation
Detector-level
Experimental resolutions
& b-ID probabilities
• fit partons to the measured objects, minimizing a χ 2
• Constraints: mt1 = mt2 = 172.5 GeV; mW1 = mW2 = 80.4 GeV
• Do the fit for every combination of assigning a jet to a parton
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W helicity
Left-handed
Right-handed
Excellent cos q *
reconstruction!
Longitudinal
cosq*
Reconstructed
Fraction
Leptonic W
Fitting f0, and f+ rather than V-A vs. V+A
 Can also use the hadronic W to fit f0
Fraction
Arbitrary Normalization
l+jets
reconstruction
results
Parton level
Can’t distinguish up
and down type quarks
Acceptance
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Hadronic W
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W helicity
eμ sample
µ+
b
2 Jets
(pT>20 GeV, |η|<2.5)
maybe a couple
Discriminant construction and fit
- of b-tags…
b
e-
Isolated e, pT>15 GeV,
|η|<1.1 / 1.5<|η|<2.5
procedures similar to those in l+jets
Events
νe
νµ
W+
t
t
W-
Isolated μ, pT>15 GeV, |η|<2.0
A strong experimental signature
 no MET requirements
 looser lepton ID requirements
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cut value
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Discriminant
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W helicity
eμ reconstruction
ν
µ+
νµ
W+
• 500 times per event
t
t
b
W-
Arbitrary Normalization
νe
e-
Parton level
Left-handed
“resolution sampling”
• smear objects within their resolution
• for each b-jet & l combination and smearing,
solve algebraically for cosq *
• use the 2 MET components + 4 mass constraints
• 0-8 solutions
• average all solutions
Reconstructed
Fraction
b
With two s, reconstruction is harder.
Right-handed
Longitudinal
cosq*
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cosq*
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W helicity
Results
Consistencies
• first 1fb-1 vs. newer data: 49%
• e+jets vs. μ+jets: 12%

p
SM
• l+jets vs. di-lepton: 1.6%
Physically allowed region
Dominant systematics
•Signal modeling
68%
95%
Cor  f 0 , f    0.8
• data vs. SM: 23%
• underlying event
• additional collisions
• MC generator

p
•Background modeling
• shape and yield in low discriminant
sample
Longitudinal: f0 = 0.490 ± 0.106(stat.) ± 0.085(syst.)
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Right handed: f+ = 0.110 ± 0.059(stat.) ± 0.052(syst.)
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A measurement of
top mass
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mtop
l+
Analysis basics
b
W+
l+jets selection similar to W helicity measurement, but:
• exactly 4 jets
t
t
• at least 1 b-tagged jet (70% efficient for top pair signal)
Base observable: 3 jet invariant mass
• very sensitive to JES uncertainties
Wq
b
q’
 hence, use the W peak to constraint JES
 mW
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 m top
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ν
mtop
Matrix element method
• Developed for l+jets by DØ, yields the most precise measurements
• Now used everywhere (dileptons, single top, Higgs search)
Goal: Use all the measured
4-vectors, x, in each event
M
2
dσ
Partonic differential Cross Section,
based on LO Matrix Element
1
dq1dq2 f (q1 ) f (q2 ) d ( y | mt ) T ( x , y , JES )
Ptt , ( x | mt , JES ) 

 tt ( m t )
for a particular
assignment = which
jet goes with which
quark
Normalization
acceptance &
efficiency
Initial State
momentum fractions
of incoming quarks
Transfer Functions
probability to measure x
from parton-level y
• Ptt = sum over all 24 possible assignments (α) in l+jets events,
weighted with b-tagging event probabilities
• Event probability: Pevent  x | mt , JES   f  Ptt  x | mt , JES   (1  f )  Pbkg  x | JES 
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mtop
Matrix element method II
Pbkg
•
•
Pevent
Pbkg
Top Mass
Top Mass
Pbkg
Pevent
Ptt
Top Mass
=
Probability
Pevent
Ptt
Probability
Ptt
Probability
Probability
Combine events by multiplying event probabilities & extract the most likely mass value
Top Mass
Calibrate method
with ensemble of
simulated datasets
• account for approximations made
with 2.2 fb -1
mt = 172.2±1.0(stat)±1.4(syst)GeV
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In closing
Presented two recent top quark measurements from DØ
To see more: http://www-d0.fnal.gov/Run2Physics/WWW/results/top.htm
Data sets are well understood and large enough to probe to new
physics and to measure SM parameters
• so far all measurements are consistent with the SM
Top mass already measured to a precision exceeding expectations
Looking forward to results with bigger data sets
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Back up slides
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W helicity
l+jets sample composition
Fit to preselected
Source
Data
Final sample
e+jets
μ+jets
e+jets
μ+jets
577
498
251
247
Signal
192.2 ± 17.4 186.2 ± 17.3 171.3 ± 4.2 162.7 ± 5.1
W+jets
285.0 ± 23.9 301.9 ± 22.4
55.2 ± 3.4
75.6 ± 4.7
Multijet
111.2 ± 9.6
10.7 ± 10.0
35.5 ± 2.9
5.0 ± 2.2
Cut value
0.5
0.2
N/A
N/A
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W helicity
Dilepton sample composition
Source
Preselected
Final
Exp. signal
50.5 ± 2.6
49.0 ± 2.6
Z/γ*ττ
17.7 ± 4.2
5.0 ± 2.9
Fake l
12.5 ± 4.4
4.5 ± 2.9
Dilepton
3.6 ± 0.7
1.9 ± 0.8
Observed
100
77
Total background normalized to fit results. (N=1.05, and uncertainties inflated)
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W helicity
Fit to data
Comparing the global best-fit model to data in l+jets
and in di-leptons
These plots are for
the subset of data
since the 1fb-1 PRL
PRL 100, 062004 (2008)
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mtop
Top quark mass
EW fit
CDF
DØ
World Average
Fundamental parameter
of the standard model
Quigg hep-ph/0404228
Implications on mH & MW
2
MW  ln m H
MW  mt2
Update of
hep-ex/0612034
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mtop
World average mtop
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Even more
back up slides
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t quark
Why is the Top Quark Interesting?
Standard • It’s heavy: so it decays so quickly that the strong
Model
force does not confine it
1
 25
 t  4  10 sec 
 3  10 24 sec
 QCD
 It is the only bare quark.
• It’s heavy: effects the mass and decays of other
particles (in particular, we’ll talk about implications for mH)
New
Physics
• It’s heavy: we already know a lot about what’s
happening at low energies.
 Look for new physics at high energies.
• It’s heavy: Yukawa coupling to Higgs λt ~ 1
– Does it play a special role in EW symmetry breaking?
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Apparatus
The DØ Detector
Toroid
Endcap
calorimeter
Muon
Detectors
All used to measure top quarks:
Tracking and vertexing (momenta, b-jet ID)
Calorimetry (jets, electrons, pT imbalance)
Muon detectors
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The channels
Lepton (isolated,
pT>20 GeV, |η|<1.1/2.0)
MET (>20GeV, triangle
cut on ΔΦ(l,MET) )
4 Jets (pT>20 GeV,
|η|<2,5) + maybe a
couple of b-tags…
Electron (isolated,
|η|<1.1 or 1.5<|η|<2.0,
pT>15 GeV)
2 Jets (pT>20 GeV,
|η|<2,5) + maybe a
couple of b-tags…
Muon (isolated,
|η|<2.0, pT>15 GeV)
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T
A
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