Measurements of the charge asymmetry in top-quark pair production in the dilepton final state at $\sqrt{s}=8$ TeV with the ATLAS detector

0.36 MB·

by  Roger Naranjo

January 6, 2017
Measurements of the charge asymmetry in top-quark pair
√
production in the dilepton final state at s = 8 TeV with the
ATLAS detector
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2 Roger Naranjo1 on behalf of the ATLAS Collaboration

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DESY
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Notkestrasse 85, 22607 Hamburg. Germany
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Measurements of the top–antitop quark pair production charge asym-
e
h metry in the dilepton channel, characterized by two high-p leptons (elec-
T
[
trons or muons), are presented using data corresponding to an integrated
√
1 luminosity of 20.3 fb−1 from pp collisions at a center-of-mass energy s =
v
8 TeV collected with the ATLAS detector at the Large Hadron Collider at
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7 CERN.Inclusiveanddifferentialmeasurementsasafunctionoftheinvari-
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ant mass, transverse momentum, and longitudinal boost of the tt system
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0 are performed both in the full phase space and in a fiducial phase space
.
1 closely matching the detector acceptance. Two observables are studied:
0 A(cid:96)(cid:96) based on the selected leptons and Att based on the reconstructed tt fi-
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nal state. No significant deviation from the Standard Model expectations
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: is observed.
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PRESENTED AT
9th International Workshop on Top Quark Physics
Olomouc, Czech Republic, September 19–23, 2016
1Work supported by the Helmholtz Association of German Research Centres.
1 Introduction
The measurements of the charge asymmetry provide a good precision test of the Stan-
dard Model (SM). In the SM, the asymmetry is produced by interferences between
the Born and one-loop diagram of the qq → tt processes and between qq → ttg dia-
grams with initial-state and final-state radiation. In the tt rest frame, this asymmetry
causes the top quark to be preferentially emitted in the direction of the initial quark,
and causes the antitop quark to be emitted in the direction of the initial antiquark.
In the pp collision at the LHC, valence quarks carry on average a larger fraction of
the proton momentum than sea antiquarks, hence top antiquarks produced through
qq annihilation are more central than top quarks. In dileptonic events, the charge
asymmetry can be measured in two complementary ways: using the pseudorapidity
of the charged leptons or using the rapidity of the top quarks. The asymmetry based
on the charged leptons uses the difference of the absolute pseudorapidity values of the
positively and negatively charged leptons, |η | and |η |. The leptonic asymmetry is
(cid:96)+ (cid:96)−
defined as
N(∆|η| > 0)−N(∆|η| < 0)
A(cid:96)(cid:96) = with ∆|η| = |η |−|η |, (1)
C N(∆|η| > 0)+N(∆|η| < 0) (cid:96)+ (cid:96)−
where N(∆|η| > 0) and N(∆|η| < 0) represent the number of events with positive
and negative ∆|η|, respectively. For the tt charge asymmetry the absolute values
of the top and antitop quark rapidities (|y | and |y |, respectively) are used. The tt
t t
charge asymmetry is defined as
N(∆|y| > 0)−N(∆|y| < 0)
Att = with ∆|y| = |y |−|y |, (2)
C N(∆|y| > 0)+N(∆|y| < 0) t t
where N(∆|y| > 0) and N(∆|y| < 0) represent the number of events with positive
and negative ∆|y|, respectively.
In these proceedings, the inclusive and differential measurements of the leptonic
and tt charge asymmetry in the dilepton channel using data collected by the ATLAS
detector[1]correspondingtoanintegratedluminosityof20.3fb−1 fromppcollisionsat
√
a center-of-mass energy s = 8 TeV are presented [2]. The differential measurements
are performed as a function of the mass (m ), transverse momentum (ptt) and boost
tt T
(βtt) of the tt system. The measurements are performed in a fiducial region and in
z
the full phase space.
2 Event Selection and Reconstruction
Events are required to have exactly two leptons of opposite electric charge and at
least two jets with p > 25 GeV within |η| < 2.5. In all three final states, exactly
T
1
two isolated leptons with opposite charge and an invariant mass m > 15 GeV
(cid:96)(cid:96)
are required. In the same-flavor channels (ee and µµ), the invariant mass of the
two charged leptons is required to be outside of the Z boson mass window such that
|m −m | > 10 GeV. Furthermore, itisrequiredthatmissingtransversemomentum
(cid:96)(cid:96) Z
isgreaterthen30GeVandatleastoneofthejetsmustbeb-tagged. Intheeµchannel,
the scalar sum of the p of the two leading jets and leptons is required to be larger
T
than 130 GeV.
The main background contribution comes from Drell–Yan production of Z/γ∗ →
(cid:96)(cid:96), which is estimated by a combination of simulated samples and corrections derived
from data. The smaller contributions from diboson and single-top-quark production
are evaluated purely via simulations. Contributions arising from events including a
jet or a lepton from a semileptonic hadron decay misidentified as an isolated charged
lepton as well as leptons from photon conversions, are estimated using simulated
samples, modified with corrections derived from data.
The tt system is reconstructed in order to perform the inclusive and differential
measurements of Att. The system is reconstructed using the KIN method. The KIN
C
method assumes the mass of the top quarks (172.5 GeV) and W mass (80.4 GeV),
and solve the system of equations obtained from momentum convervation numerically
using the Newton-Rhapson method. The reconstruction efficiency is above 90%. A
comparison between observations and expectations is shown in Fig. 1 after event
reconstruction. A good agreement within the uncertainties is observed.
Events 4567000000000000 AsT =L A8S TeV,20.3 fb-1 dtZSDNUtaiiPnnbtc ago&els erfota-atnkoinept yleptons Events / 20 GeV 5678000000000000 AsT =L A8S TeV,20.3 fb-1 dtZSDNUtaiiPnnbtc ago&els erfota-atnkoinept yleptons Entries / 10 GeV1680000000000 AsT =L A8S TeV,20.3 fb-1 dtZSDNUtaiiPnnbtc ago&els erfota-atnkoinept yleptons
4000
3000
3000 4000
2000 2000
2000
1000 1000
DataExpected01..821 |bz,tt| DataExpected01..821 mtt [GeV] DataExpected01..821 pT,tt [GeV]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 300 400 500 600 700 800 900 1000 0 50 100 150 200 250 300
|b | m [GeV] p [GeV]
z,tt tt T,tt
Figure 1: Comparison between observations and expectations for the boost (left)
mass (middle), and p of the tt system. The total uncertainty on the distributions
T
are shown [2].
3 Unfolding
The measurements of A(cid:96)(cid:96) and Att are corrected in order to remove the effects in-
C C
troduced by the detector. This correction is performed by using the Fully Bayesian
2
Unfolding (FBU) method [5]. The measurements are unfolded back to a stable par-
ticle level in a fiducial region closely matching the detector acceptance, and back to
parton level in the full phase space. The combined measurement of the three decay
channels is performed during the unfolding procedure. The asymmetries are com-
puted using the posterior probability density obtained as an output of the unfolding
procedure. Systematic uncertainties related with detector modeling and background
modeling are evaluated during the unfolding by using a marginalization procedure.
4 Results
Figure 2 shows the inclusive and differential measurements performed for A(cid:96)(cid:96) and Att
C C
at parton level in the full phase space. The total uncertainty on the measurements
is shown. The main source of uncertainty on the different measurements is the sta-
tistical uncertainty, followed by the signal modeling uncertainty. The measurements
that involve the reconstruction of the tt system are also affected by a reconstruction
uncertainty which is approximately half of the size of the statistical uncertainty. The
uncertainties corresponding to the detector and background modeling do not con-
tribute significantly to the total uncertainty. The results are compatible with the SM
predictions [3]. A similar behavior is observed on the uncertainties in the measure-
ments performed in the fiducial region, however, there is a reduction in the modeling
uncertainties. Figure 3 shows the unfolded distribution for the ∆|y| and ∆|η| observ-
ables in the fiducial region. The distribution is in agreement with SM predictions.
Figure 4 shows the A(cid:96)(cid:96) and Att measurements in comparison with several models be-
C C
yond the SM [4] in the full phase space. In these models, the values of the asymmetry
are expected to be different from the SM expectations. The ellipses correspond to
the 1σ and 2σ total uncertainty on the measurements. The correlation between A(cid:96)(cid:96)
C
and Att is about 48%. The measurements are compatible with the SM and do not
C
exclude the two sets of BSM models considered.
References
[1] ATLAS Collaboration, 2008 JINST 3 S08003.
[2] ATLAS Collaboration, Phys. Rev. D 94, no. 3, 032006 (2016).
[3] W. Bernreuther and Z. G. Si, Phys. Rev. D 86 (2012) 034026.
[4] J. A. Aguilar-Saavedra, JHEP 1408 (2014) 172.
[5] Georgios Choudalakis. arXiv:1201.4612 [physics.data-an].
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ATLAS ATLAS data ATLAS ATLAS data
s = 8 TeV,20.3 fb 1 PBOerWnrHeuEtGhe hr v&q +SPi. YPTRHDIA 866, 034026. s = 8 TeV,20.3 fb 1 PBOerWnrHeuEtGhe hr v&q +SPi. YPTRHDIA 866, 034026.
Inclusive 0.008 ± 0.006 Inclusive 0.021 ± 0.016
0 500 GeV 0.009 ± 0.012 0 500 GeV 0.030 ± 0.043
m m
tt tt
500 2000 GeV 0.009 ± 0.019 500 2000 GeV 0.016 ± 0.024
0 0.6 0.010 ± 0.010 0 0.6 0.038 ± 0.036
βtt βtt
z 0.6 1.0 0.004 ± 0.013 z 0.6 1.0 0.007 ± 0.025
0 30 GeV 0.002 ± 0.025 0 30 GeV 0.026 ± 0.042
ptt ptt
T 30 1000 GeV 0.015 ± 0.018 T 30 1000 GeV 0.053 ± 0.034
−0.15 −0.1 −0.05 0 0.05 0.1 0.15 −0.2 −0.15 −0.1 −0.05 0 0.05 0.1 0.15 0.2
AlCl in the full phase space AtCt in the full phase space
Figure 2: Summary of the inclusive and differential measurements of the tt asym-
metry (left) and lepton asymmetry (right) performed in the full phase space. The
measurements are compared with theoretical predictions [2].
dσ1 σ||dη∆0.6 AsT =L A8S TeV,20.3 fb 1Particle PdleaOvtWaelHEG hvq+PYTHIA6 σd1 σ∆|y|d0.6 AsT =L A8S TeV,20.3 fb 1Particle PdleaOvtWaelHEG hvq+PYTHIA6
0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2
0.1 0.1
0 0
DataExpected01..911−5 −4 −3 −2 −1 0 1 2 3 4∆|η|5 DataExpected01..911−5 −4 −3 −2 −1 0 1 2 3 4∆|y|5
Figure 3: Data distribution after the unfolding procedure compared with the predic-
tion for the inclusive ∆|η| (left) and ∆|y| (right) observables in the fiducial volume.
The data/expected ratio is also shown [2].
ll AC ATLAS ll AC ATLAS
0.03 0.03
s = 8 TeV,20.3 fb 1 s = 8 TeV,20.3 fb 1
0.02 Inclusive Parton level 0.02 Inclusive Parton level
0.01 0.01
0 0
−0.01 −0.01
ATLAS data Bernreuther & Si. PRD 86, 034026. ATLAS data Bernreuther & Si. PRD 86, 034026.
−0.02 ATLAS 1σ hheeaavvyy oocctteett,, LREIGFHTT −0.02 ATLAS 1σ lliigghhtt oocctteett,, LREIGFHTT
ATLAS 2σ heavy octet, AXIAL ATLAS 2σ light octet, AXIAL
−0.02 −0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 −0.02 −0.01 0 0.01 0.02 0.03 0.04 0.05 0.06
AtCt AtCt
Figure 4: Comparison of the inclusive A(cid:96)(cid:96) and Att measurement values in the full
C C
phase space to the SM and to two benchmark BSM models [2].
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