Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 312–317 (2021) 43–47
www.elsevier.com/locate/nppp
Highlights on top quark measurements from CMS
V.M. Mikunia,∗
a University
of Zurich, Winterthurerstrasse 190, 8057 Zurich
Abstract
Recent results from the CMS Collaboration using top quarks are presented. These results are based on partial
datasets collected by the CMS Collaboration during the LHC Run 2, at a center-of-mass energy of 13 TeV. This document includes the first measurement of tt̄ production in association with charm quarks, the first direct measurement
of the third generation of the CKM matrix elements, the investigation of the running of the top quark mass, search for
CP violation in top quark production, measurement of the forward-backward asymmetry in tt̄ production at the LHC,
and the first global approach in constraining EFT operator coefficients using top quarks.
Keywords: top quarks, CMS, QCD20
1. Introduction
Millions of top quark-antiquark (tt̄) pairs were produced during the LHC Run 2, at a center-of-mass energy of 13 TeV. With this unprecedented dataset, stringent tests of the Standard Model (SM) were carried out
to either perform precise measurements of SM parameters, or to look for deviations that might hint to new
physics effects. In this document, an overview of recent
measurements containing top quarks performed by the
CMS [1] Collaboration are presented. This overview
covers the first measurement of tt̄ production in association with charm quarks (tt̄cc̄), the first direct measurement of the third generation CKM matrix elements,
the investigation of the running of the top quark mass,
search for CP violation in top quark production, measurement of the forward-backward asymmetry in tt̄ production at the LHC, and the first global approach in constraining EFT operator coefficients using top quarks.
∗ Speaker
on behalf of the CMS Collaboration.
Email address: vinicius.massami.mikuni@cern.ch (V.M.
Mikuni )
2. Top quark pair production in association with
charm quarks
The associated production of top quark pairs with
charm quarks (tt̄cc̄) is a challenging process to describe
both theoretically and experimentally. These processes,
together with the associated production with bottom
(tt̄bb̄) and lighter flavor quarks (tt̄LF) often carry large
theoretical uncertainties from missing order corrections,
requiring dedicated measurements to give feedback to
different predictions from simulations. In [2], the fiducial and total cross sections for tt̄cc̄, tt̄bb̄, and tt̄LF were
extracted using 41.5 fb−1 of data collected by the CMS
Collaboration in 2017, taking advantage of the pixel
tracker upgrade to improve the flavor tagging performance between different quark-initiated jets. To reduce
the background from other processes, the dileptonic decays of top quarks are used. To extract simultaneously
the different tt̄ + jets categories, flavor tagging is explored. An artificial neural network (NN) is trained to
classify each event into different categories depending
on the flavor of the additional jets that are not produced
through the top quark decays. The NN output is then
used to extract the different cross sections. The measured results in the fiducial phase space and comparisons with different simulation predictions are shown in
https://doi.org/10.1016/j.nuclphysbps.2021.05.010
2405-6014/© 2021 CMS Collaboration. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
44
V.M. Mikuni / Nuclear and Particle Physics Proceedings 312–317 (2021) 43–47
Figure 1: Measured fiducial cross section for tt̄cc̄, tt̄bb̄, and tt̄LF (left).
The data results (black dots) are compared with different predictions
(blue and red boxes). The ratios between the measured tt̄cc̄ (Rc ) and
tt̄bb̄ (Rb ) cross sections with the tt̄LF cross section are shown in the
right. Figure adapted from [2].
Fig. 1, together with the ratios between tt̄cc̄ and tt̄LF
(Rc ) and tt̄bb̄ and tt̄LF (Rb ) cross sections. The relative
uncertainties are 19%, 13%, and 8% for the tt̄cc̄, tt̄bb̄,
and tt̄LF, respectively. The main experimental uncertainties are the flavor tagging response and jet energy
corrections, while the largest theoretical uncertainties
are determined through variations of the renormalization and factorization scales and the matrix element to
parton shower matching. Both experimental and theoretical uncertainties are of similar size.
Figure 2: Leading-order Feynman diagrams for single top quark production via the t channel featuring a tWb vertex in production and a
tWq in decay, with q being an s or d quark. The l refers to e or μ
leptons. Figure adapted from [4].
The results are μb = 0.99 ± 0.03 (stat + prof) ±
0.12 (nonprof) and μ sd < 87 at 95% confidence level
(CL). The stat + prof uncertainty refers to the statistical
and profiled uncertainties while nonprof refers to uncertainties that were not profiled during the maximum
likelihood fit.
3. Direct measurement of the third generation of the
CKM matrix elements
4. Investigation of the running of the top-quark
mass
The transition between different quark flavors is described in the SM by the Cabibbo–Kobayashi–Maskawa
(CKM) matrix. While indirect measurements of third
generation CKM matrix elements have been performed
before [3], they often rely on additional assumptions.
The first direct measurement of third generation CKM
matrix elements [4] was performed using the dataset
collected in 2016. The measurement targets single top
t-channel events. This channel is sensitive to the magnitude of the CKM matrix elements |Vtb |, |Vts |, and |Vtd |
both at production and decay of top quarks, as shown in
Fig. 2.
To reduce the background from other processes, leptonic decays of top quarks are used. The data is then partitioned in regions of different jet and b-jet multiplicities
to enhance the sensitivity to |Vtb | and to |Vts |2 + |Vtd |2 . A
maximum likelihood fit is then performed to measure
the production cross sections and branching fractions
of single top quark t-channel processes that depend on
Vtb , Vtd , and Vts in production and decay. These measurements are reported as signal strength parameters μ
defined as the ration of measured quantities to the SM
prediction.
Beyond leading order in perturbation theory, fundamental parameters in quantum chromodynamics (QCD)
are subject to renormalization effects. Among those
are the quark masses, which depend on the scale they
are evaluated. This scale dependence is then described
by the renormalization group equations (RGEs). In
the modified minimal subtraction (MS) renormalization
scheme, the quark mass dependence (”running”) with
scale μ is
μ2
dm(μ)
= −γ(α s (μ))m(μ)
dμ2
(1)
where γ(α s (μ)) is the mass anomalous dimension and α
is the strong coupling constant.
The investigation of the running of top quark mass [5]
was performed using the data collected in 2016, select
tt̄ events decaying leptonically. To extract the values of
the top quark mass at different scales, a measurement
of the invariant mass of tt̄ pairs (mtt̄ ) is performed. The
mtt̄ spectrum is then unfolded to parton level in four different bin intervals to extract the top quark mass components at different scales. The results of the measured
V.M. Mikuni / Nuclear and Particle Physics Proceedings 312–317 (2021) 43–47
45
Figure 3: Extracted running of the top quark mass mt (μ)/mt (μre f )
compared to the RGE prediction at one-loop precision (n f =5) evolved
from the initial scale μ0 = μre f = 476 GeV. Figure adapted from [5].
quantities over a reference scale μre f = 476 GeV are
shown in Fig. 3.
The extracted result is found to be compatible with
the prediction from the RGEs where the no-running hypothesis is excluded at 95% CL.
5. Search for CP violation in top quark production
The matter-antimatter asymmetry in the universe is
still an open problem. To explain this asymmetry,
sources of charge conjugation and parity transformation (CP) beyond the ones described in the SM are required. A search for new sources of CP violation using top quark decays [6] was performed, using the data
collected in 2016 to select top quark pairs decaying to
leptons. A possible source for CP violation could arise
from the interaction of the chromoelectric dipole moment (CEDM) of top quarks, as proposed in [7, 8]. In
this scenario, the CP violation can be probed by measuring the asymmetries Ai defined as
N(Oi > 0) − N(Oi < 0)
,
Ai =
N(Oi > 0) + N(Oi < 0)
Figure 4: Asymmetry as a function of dtG for O1 . The inner and outer
bands correspond to the uncertainties at the 68% and 95% confidence
level, respectively, of the linear fit results. The square points are the
asymmetries measured with the simulated samples corresponding to
the different assumed dtG values. The horizontal line indicates the
measured asymmetry, and the shaded region around it the total statistical and systematic uncertainty. Figure adapted from [6].
All measured values are consistent with the SM prediction.
6. Measurement of the forward-backward asymmetry in tt̄ production
A different kind of asymmetry that can be measured
at the LHC is related to the angular distribution between
top quarks in tt̄ production (forward-backward asymmetry [9]). This measured is motivated by different beyond
the Standard Model (BSM) scenarios that could lead to
an enhancement of the asymmetry AFB defined as
AFB =
(2)
where Oi are the determinants of the Levi-Civita tensor
when evaluated using reconstructed b (anti-) quark jets
and (anti) leptons or top (anti-) quarks and (anti) leptons, which defines O1 and O3 , respectively. By measuring the asymmetry in these two operators, limits to
the dimensionless CEDM (dtG ) can be extracted by noting the linear relationship between the asymmetries and
dtG . An example of the measured asymmetry and dtG
value is shown in Fig. 4 for the O1 operator.
The numerical results for O1 and O3 are listed in
Tab. 1
σ(c∗ > 0) − σ(c∗ < 0)
,
σ(c∗ > 0) + σ(c∗ < 0)
(3)
with c∗ = cos θ∗ , where θ∗ is the production angle of the
top quark relative to the direction of the initial-state parton in the tt̄ center-of-mass frame. This measurement
uses the dataset collected in 2016, targeting tt̄ events
where one of the top quarks decays leptonically and the
other decays hadronically. To enlarge the data sample,
final states with low and high Lorentz boosts are used.
The top-quark pairs are then reconstructed using a kinematic fit. The asymmetry is then extracted using observables that are sensitive to the quark-antiquark (qq̄) production of top quark pairs with a maximum likelihood
fit. From these distributions, different asymmetry values
V.M. Mikuni / Nuclear and Particle Physics Proceedings 312–317 (2021) 43–47
46
Table 1: Measured dtG and CEDM of O1 and O3 with their uncertainties.
Operator
O1
O3
dtG
0.10 ± 0.12(stat) ± 0.12(syst)
0.00 ± 0.13(stat) ± 0.10(syst)
CEDM (10−18 gs .cm)
0.58 ± 0.69(stat) ± 0.70(syst)
−0.01 ± 0.72(stat) ± 0.58(syst)
ChargeFlips
Fakes
Diboson
Triboson
Convs
ttH
ttll
ttlν
tllq
tHq
Total unc.
Data
Figure 6: Expected yields postfit. The postfit values of the WCs
are obtained from performing the fit over all WCs simultaneously.
”Convs” refers to the photon conversion background, ”ChargeFlips”
is the lepton charge mismeasurement background, and ”Fakes” is the
background from misidentified leptons. Figure adapted from [10].
Figure 5: Neyman construction for the AFB parameter of interest in
groups of 1000 pseudo-experiments generated with systematic uncertainty nuisance parameters allowed to vary. The horizontal dotted
lines indicate the values of the parameters determined from the fits and
the vertical dotted lines indicate where these values intersect with the
central value and uncertainty contours from the pseudo-experiment
groups. Figure adapted from [9].
are injected to generate template predictions used to fit
the data. The measured forward-backward asymmetry
is shown in Fig. 5.
7. Global approach in constraining EFT operator
coefficients using top quarks
A different approach for BSM searches is to use the
effective field theory (EFT) framework by assuming the
SM to be a low-energy approximation of a high-energy
theory. This approach results in additional high-order
terms in the SM Lagrangian that are still consistent with
symmetries and conservation laws. This effective Lagrangian (Leff ) for some high energy mass scale Λ can
then be described as:
Leff = LSM +
c(d)
i
O(d) ,
d−4 i
Λ
d,i
parameters known as Wilson coefficients (WCs). This
measurement uses the data collected in 2017 and targets tt̄ events with additional leptons [10]. The dataset
is then subdivided into categories of lepton and jet multiplicities that are sensitive to different WCs. A set of
16 WCs are investigated, associated to dimension-6 operators. The expected yield in each category is then parameterized in terms of the WC associated with effective
field theory operators relevant to the dominant processes
in the selected data. The result of the fit that determines
the yield of the different processes is shown in Fig. 6.
In order to place limits to the different WCs, two different approaches were tested while performing the fit:
scanning one WC while the other coefficients are treated
as nuisance parameters in the maximum likelihood fit,
or to scan a WC while the other coefficients are assumed
to be 0. The result of these two approaches are shown in
Fig. 7. In both approaches, the SM value of 0 was not
excluded.
8. Summary
(4)
where LSM is the SM Lagrangian, O(d)
i the effective
(d)
operators with dimension d, and ci the dimensionless
A diverse number of recent results from the CMS
Collaboration using top quarks were presented. The
large dataset collected during LHC Run 2 resulted in the
first measurement of the associated production of top
V.M. Mikuni / Nuclear and Particle Physics Proceedings 312–317 (2021) 43–47
47
quarks with charm jets, the first direct measurement of
third generation CKM matrix elements using single top
production, and the investigation of the running of the
top-quark mass. Different new physics searches were
also performed by directly testing new models, like the
search for new sources of CP violation in the Standard
Model affecting top quarks, or by measuring Standard
Model parameters that are sensitive to new physics effects, like the measurement of the top quark-antiquark
angular asymmetry at the LHC. A more general framework to look for new physics described by the effective
field theory (EFT) framework was also used to provide
the first global approach to EFT constraints using top
quark events. In all measured results, a good agreement
with the expected predictions from the Standard Model
was observed.
References
Figure 7: Observed WC 1σ (thick line) and 2σ (thin line) confidence
intervals. Solid lines correspond to the other WCs profiled, while
dashed lines correspond to the other WCs fixed to the SM value of
zero. Figure adapted from [10].
[1] CMS Collaboration, The CMS Experiment at the CERN LHC,
JINST 3 (2008) S08004. doi:10.1088/1748-0221/3/08/S08004.
[2] CMS Collaboration, Measurement of the cross section of top
quark pair production with additional charm jets using the dileptonic final state in pp collisions at 13 TeV, Tech. Rep. CMSPAS-TOP-20-003, CERN, Geneva (2020).
URL http://cds.cern.ch/record/2730232
[3] M. Tanabashi, et al., Review of Particle Physics, Phys. Rev. D
98 (3) (2018) 030001. doi:10.1103/PhysRevD.98.030001.
[4] CMS Collaboration, Measurement of CKM matrix elements
in single top
√ quark t-channel production in proton-proton collisions at s = 13 TeV, Phys. Lett. B 808 (2020) 135609.
arXiv:2004.12181, doi:10.1016/j.physletb.2020.135609.
[5] CMS Collaboration, Running of √the top quark mass
from proton-proton collisions at
s = 13TeV, Phys.
Lett. B 803 (2020) 135263.
arXiv:1909.09193,
doi:10.1016/j.physletb.2020.135263.
[6] CMS Collaboration, Search for CP violating anomalous√ top
quark coupling in pp collisions with the CMS detector at s =
13 TeV, Tech. Rep. CMS-PAS-TOP-18-007, CERN, Geneva
(2020).
URL http://cds.cern.ch/record/2730231
[7] O. Antipin, G. Valencia, t-odd correlations from cp violating anomalous top-quark couplings revisited, Phys. Rev. D 79
(2009) 013013. doi:10.1103/PhysRevD.79.013013.
[8] A. Hayreter, G. Valencia, T-odd correlations from the
top-quark chromoelectric dipole moment in lepton plus
jets top-pair events, Phys. Rev. D 93 (2016) 014020.
doi:10.1103/PhysRevD.93.014020.
[9] CMS Collaboration, Measurement of the top quark forwardbackward production asymmetry and the anomalous chromoelectric
and chromomagnetic moments in pp collisions at
√
s = 13 TeV, JHEP 06 (2020) 146. arXiv:1912.09540,
doi:10.1007/JHEP06(2020)146.
[10] CMS Collaboration, Using associated top quark production
to probe for new physics within the framework of effective
field theory, Tech. Rep. CMS-PAS-TOP-19-001, CERN, Geneva
(2020).
URL http://cds.cern.ch/record/2725399