Transcript MET studies in Z events

Measurement of
Top Quark Properties
at the TeVatron
Jessica Levêque
University of Arizona
on behalf of the CDF and DØ collaborations
Jessica Levêque
Rencontres de Moriond QCD 2006
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Top Quark Physics Overview
Since its discovery in 1995 at Fermilab, the top
quark has been intensively studied but most of
its properties are still poorly understood due to
statistically limited samples.
Direct measurements of the top quark
properties are necessary : the constraints
already set by precision electroweak
measurements leave plenty of room for exotic
behaviors regarding the charge, mass,
branching fractions, decay and production
modes.
Top quark properties covered in this talk :

Lifetime
L = 318 pb-1

Charge
L = 370 pb-1

Search for resonant production
L = 682/370 pb-1

Search for “heavy” top quark
L = 760 pb-1

W boson helicity
L = 370 pb-1
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 2
Top Quark at TeVatron
At the TeVatron, the top quark is
mainly produced in pairs via strong
interaction. In the following, we
will assume :
s = 6.8 ± 0.8 pb [*]
B(t→Wb) = 100%
mtop =175 GeV/c2
85%
15%
The top quark properties are measured in
events where at least one of the W
bosons decays into leptons (e, m, t→ e,m)
 Large branching fraction ~ 40%
 Limited physics background
Most of the analyses presented here use
b-jets identification to increase the
fraction of signal in the selected data
samples :
 cut on the decay length of the
secondary vertex found in jets
[*] Phys. Rev. D 68, 114014
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 3
Top Quark Lifetime at CDF
Top quark lifetime constrained in Standard Model (SM) to
be less than 10-24 s (ct < 3.10-10 mm) in absence of a 4th
quark generation.
First direct measurement of the top quark lifetime:
- Confirm the identity of the “top” candidates
- Detect unexpected production modes through new
long-lived particles
Method : measure the distance between the initial
collision point and the W boson decay vertex via the
lepton impact parameter d0
Signal templates generated for top quark lifetimes ct
between 0 and 500 mm, combined with background
distributions and corrected with the detector resolution to
model the impact parameter distribution in data.
Detector resolution measured with Z/g* → e +e-, m+mImpact parameter of the lepton measured by fitting the
data with templates using a maximum likelihood method
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 4
Top Quark Lifetime at CDF
Top quark selection :
 1 lepton (e or m), 3 or more jets
 1 b-jet tagged with secondary vertex
 Missing transverse energy
 g / Ks / L conversions removed
97 electron tracks and 60 muon tracks selected
Maximum likelihood attained
with SM template ct = 0 mm
Limit : ct < 52.5 mm @ 95% C. L.
For comparison :
•
b-hadrons ct ~ 450 mm
•
detector resolution ~ 40 mm
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 5
Top Quark Charge at DØ
The SM predicts a top quark charge of +2/3e
Exotic model with a 4th generation of quarks and
a Higgs triplet predict the existence of an
additional heavy quark with a charge -4/3e [*]
Top quark selection :
 1 lepton (e or m), at least 4 jets
 2 b-jets tagged with secondary vertex
 Kinematic fit used to reconstruct the top
quarks and find the correct (lepton, b-jet)
pairing
Top (anti-top) charge defined as the jet charge
plus (minus) the lepton charge
L = 370 pb-1
b-jet charge measured with a jet-charge
algorithm using tracks with PT > 0.5 GeV/c
Jet-charge algorithm calibrated with data
using semi-leptonic b-jet decays to determine
the charge (corrected for c-jets contamination
and B mixing)
- 17 top pair candidates selected in data
- 2 measurements per event
[*] Phys. Rev. D 59, 091503 / 61, 037301 / 65 053002
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 6
Top Quark Charge at DØ
To test the Standard Model, a likelihood ratio is
computed from data :
psm(q i) = probability to observe the quark
charge qi in the SM
pex(qi) = probability to observe the quark charge
qi in the exotic scenario
The value Ldata = 11.5 is then compared to the
expected distributions Lsm and Lex derived
from Monte-Carlo pseudo-experiments.
The number of background events is allowed to
fluctuate according to a Binomial distribution.
Jessica Levêque
Exotic quark with charge 4/3e
excluded @ 94% C. L.
Rencontres de Moriond QCD 2006
Page 7
Search for tt resonances
Test an exotic model [*] predicting the top
quark pair production through the decay of
a new heavy gauge bosons.
The measured top quark pair production
cross-section would be higher than expected
in the SM and resonances would appear in
the tt- invariant mass distribution.
Monte Carlo
templates
X?
The search is model dependant.
We consider values for the resonance mass
MX in the range [350 - 1000] GeV/c2
and assume a width Gx = 0.012 Mx
[*] Phys. Rev. D 92 221801 / Phys. Rev. D 85 2065
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 8
Search for tt resonances at CDF
Top quark selection :
 1 lepton (e or m), at least 4 jets
 Large missing transverse energy
Top quarks reconstructed with matrix element
information and “transfer functions” describing
the correlation between partons and jet energies.
Background and top quark pair production
normalized to SM expectations. The QCD
contribution is estimated from data.
No excess observed
Exclude the production of a
leptophobic Z’ with mass
Mx < 725 GeV/c2 @ 95% C. L.
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 9
Search for tt resonances at DØ
Top quark selection :
 1 lepton (e or m), at least 4 jets
 At least 1 b-jet tagged with secondary vertex
 Missing transverse energy
Use a constrained kinematic fit to reconstruct the
top quarks 4-vectors.
Top quark and background processes normalized
to SM predictions. The QCD multijets
contribution is estimated from data.
No excess observed
Exclude the production of
a leptophobic Z’ with mass
Mx < 680 GeV/c2
Jessica Levêque
Rencontres de Moriond QCD 2006
@ 95% C. L.
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Search for heavy top quark at CDF
Looking for a 4th generation of heavy quarks.
Assumptions : produced in pairs, larger mass than the top quark, decaying promptly into W+b
[*]
Event selection : 1 isolated lepton, at least 4 jets,
large missing transverse energy
t’ reconstructed mass determined with the mass
template method.
Signal Monte-Carlo was generated with Mt’
varying in the range [175 - 400] GeV/c2
To discriminate the new physics signal from the
SM processes, 2 variables are used :
- Reconstructed quark mass Mreco
- Total transverse energy in the event HT
[*] “beautiful mirror model” hep-ph/0109097
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 11
Search for heavy top quark at CDF
The contribution of the t’ signal is measured
with a binned likelihood fit to the 2D
function (HT, M t’ )
The tt- contribution is normalized to the SM
cross-section. The W+jets production rate is
a free parameter of the fit.
6.8 events from SM are expected in the tails
7 events are selected in data
No evidence for t’ signal observed
Mt’ < 258 GeV/c2
excluded @ 95% C. L.
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 12
W boson helicity
SM prediction :
Assuming a massless bottom quark, the V-A
structure of the weak interaction in the SM
strongly suppress the fraction of right-handed
W boson produced in t→Wb decay
A measured value of f+ ≠ 0 would be
a unambiguous signature of new physics
Left-handed
f- ~ 0.30
Experimentally, the W boson helicity is
measured from the cos q* distribution :
Longitudinal
f0 ~ 0.70
Right-handed
f+ ~ 3.6 10-4
SM prediction
q* = angle between the charged lepton and
the top quark boost direction measured in the
W rest frame.
2
cos θ 
*
Jessica Levêque
2 M lb
m m
2
t
2
w
1
Rencontres de Moriond QCD 2006
Page 13
W boson helicity at DØ
Dilepton selection : 2 oppositely charged leptons (e or m), at
least 2 jets, large missing transverse energy. Additional
kinematic cuts to suppress Z→ℓ+ℓ- events.
Lepton+jets selection : at least one lepton and 4 jets. Cut on
likelihood discriminant using topological and b-tagging
variables to increase the signal fraction.
Kinematic fit used to reconstruct the top quark pairs and the
W rest-frame, assuming mtop = 175 GeV/c2
In dilepton events, the system is underconstrained and gives
two possible angle measurements : both are used in the fit to
increase the statistics. The resulting distribution still shows
discrimination between V-A and V+A models.
Combined result from maximum likelihood fit :
+
f  0.08  0.08 (stat)  0.06 (syst)
+
f  0.24 @ 95% C. L.
Jessica Levêque
Rencontres de Moriond QCD 2006
Page 14
Summary
All top properties measurements consistent
within errors with the Standard Model
predictions
The analyses are still statistically limited but
were intensively optimized and already
reached interesting precisions
CDF and DØ have now more than
1 fb-1 of data recorded on tape
currently being analyzed
Best result
Charge
Rule out +4/3e model
@ 94% C. L.
Lifetime
Search for
heavy t’ quark
Updates expected from both experiments
for summer conferences
If the top quark is hiding something,
we may figure it out next summer !
Jessica Levêque
Measurement
Search for tt
resonance
W boson
helicity
Rencontres de Moriond QCD 2006
ct < 53 mm @ 95% C. L.
Exclude Mt’ in [196-207]
GeV/c2 @ 95% C. L.
Mx > 725 GeV/c2
@ 95% C. L.
+
f  0.08  0.08  0.06
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