2011 QCD and High Energy Interactions - Rencontres de Moriond ...

Events Events Events
160
140
120
100
80
60
40
20
0
ATLAS
Preliminary
∫
-1
L = 35 pb
all channels
data
tt
single top
DY + jets
diboson
fake leptons
uncertainty
0 1 2 3 ≥ 4
Number of jets
[pb]
t
t
σ
2
10
10
NLO QCD (pp)
Approx. NNLO (pp)
NLO QCD (pp)
Approx. NNLO (pp)
CDF
D0
ATLAS 180 ± 18 pb
-1
(35 pb , Prelim.)
CMS 158 ± 19 pb
-1 (36 pb , Prelim.)
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100
6.5 7 7.5
1
1 2 3 4 5 6 7 8
s [TeV]
Figure 1: Selected dilepton events superimposed on expectations from simulation and data-driven estimations
(left). Cross-section measurements at Tevatron and LHC compared to approximate NNLO predictions (right).
candidate events. Using data-driven methods to estimate the QCD and W+jets backgrounds,
a production cross-section of σt = 53 +46
−36 pb is measured, which translates to an upper limit of
162 pb at 95% confidence level. A likelihood function approach is also used to cross-check the
result. Both results are consistent with the SM expectation of 66 pb.
The Wt-channel analysis is based on the selection of events with either one or two leptons,
jets and Emiss T . The expected SM cross-section for this single-top process is 15 pb. A 95%
confidence level limit is set on the Wt-channel production cross section of σWt < 158 pb. In the
dilepton channel, the t¯t background is estimated from data, by considering the one-jet bin as a
control region.
4 Top quark properties
4.1 Mass
The top-quark mass, a fundamental parameter of the SM, is a source of large contributions to
electroweak radiative corrections and, in conjunction with precision electroweak measurements,
can be used to derive constraints on the Higgs boson mass or heavy particles predicted in SM
extensions. The current world average is mtop = 173.3 ± 1.1 GeV.
The main systematic on the determination of the top-quark mass is the uncertainty in the jet
energy scale (JES). Three complementary template analyses have been developed 9 that address
the uncertainty due to the JES in different ways: a 2D analysis that simultaneously determines
mtop and a global jet energy scale factor between data and predictions; a 1D analysis exploiting
a kinematical likelihood fit to all decay products of the t¯t system; a 1D analysis which is based
on the ratio between the per-event reconstructed invariant mass of the top-quark and the mass
of the W-boson associated to the hadronically decaying top-quark candidate. The latter method
yields a top-quark mass measurement of mtop = 169.3 ± 4.0 ± 4.9 GeV.
4.2 W-boson polarisation in top-quark decays
The polarisation states of the W-bosons that emerge from top-quark decays are well defined in
the SM, due to the V − A structure of the charged current weak interactions. These can be
extracted from the angular distributions of the decay products in t → bW → bℓνℓ.
In a first measurement 10 , templates of the cos θ ⋆ distribution are built from simulation and
fitted to selected events with a single charged lepton, E miss
T and at least four jets, where at least
one of them is b-tagged. Here θ ⋆ is the angle between the direction of the lepton and the reversed