Flavour-changing neutral scalar interactions of the top quark N. F. Castro1and K. Skovpen2 1Laborat orio de Instrumenta c ao e F sica Experimental de Part culas LIP
2025-05-06
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Flavour-changing neutral scalar interactions of the top quark
N. F. Castro1and K. Skovpen2
1Laborat´orio de Instrumenta¸c˜ao e F´ısica Experimental de Part´ıculas (LIP),
Departamento de F´ısica, Escola de Ciˆencias, Universidade do Minho, 4710-057 Braga,
Portugal
2Ghent University, Sint-Pietersnieuwstraat 33, 9000 Gent, Belgium
Abstract
A study of the top quark interactions via flavour-changing neutral current (FCNC)
processes provides an intriguing connection between the heaviest elementary particle of
the standard model (SM) of particle physics and the new scalar bosons that are predicted
in several notable SM extensions. The production cross sections of the processes with
top-scalar FCNC interactions can be significantly enhanced to the observable level at the
CERN Large Hadron Collider. The present review summarizes the latest experimental
results on the study of the top quark interactions with the Higgs boson via FCNC and
describes several promising directions to look for new scalar particles.
1
arXiv:2210.09641v1 [hep-ex] 18 Oct 2022
1 Introduction
Conservation laws and flavour-symmetry structures represent the core element of any theo-
retical model that provides a description of interactions involving elementary particles. An
experimental study of fundamental interactions is an excellent probe of higher-order sym-
metries, potentially leading to a construction of a more complete model of nature with re-
solving the remaining unanswered questions of the remarkably successful Standard Model
(SM). The SM theory of weak interactions allows flavour-violating processes in the quark
sector through the charged weak currents. Such flavour-changing transitions proceed via an
exchange of a W boson between the two fermionic states. The weak eigenstates are treated
as left-handed doublets, allowing transitions between the up- and down-type quarks, while
the mass eigenstates are represented by a superposition of weak eigenstates connected via
an unitary matrix. The rotation from one type of state to another is then expressed as
the Cabibbo-Kobayashi-Maskawa (CKM) matrix which governs the flavour-mixing processes
through the flavour-changing charged weak transitions [1]. The processes, where a fermion
changes its flavour via an exchange of a neutral boson, are therefore absent at tree level in
the SM due to the unitarity of rotational matrices and are called flavour-changing neutral
currents (FCNC) [2].
The effect of flavour mixing in the quark sector was first introduced using a three-quark
model that only included the u, d, and s quarks [3]. Experimental studies of the KL→µ+µ−
decays and neutral kaon mixing processes however indicated important difficulties in satisfying
theoretical predictions for FCNC transitions [4]. The problem was theoretically solved in
1970s by introducing the fourth type of quark, the charm (c) quark, in order to restore the
quark-lepton symmetry of the weak interaction. It was shown that an additional contribution
associated with an exchange of a c quark at one-loop level almost completely cancels the
respective contributions connected to the lighter quarks. This effect leads to a significant
suppression of FCNC transitions at higher orders — the Glashow-Iliopoupos-Maiani (GIM)
mechanism. The discovery of the c quark, just a few years later, confirmed these theoretical
speculations [5]. The four-quark model was later extended to include five quark flavours,
after the discovery of the bottom (b) quark [6]. It took a bit longer for the top (t) quark to
be experimentally observed in 1995, completing the SM to contain six quark flavours [7, 8].
In a full representation of the quark sector, the tree-level transitions between different quark
flavours are only allowed through the weak flavour-changing charged interaction, while the
tree-level FCNC transitions are completely missing in the SM and are only possible as loop
corrections.
The FCNC effects are predicted in the leptonic sector as lepton flavour violating transi-
tions. However, the probability to observe such processes is expected at the level of '10−54,
in the case of the µ→eγ decay, due to an extreme suppression from the neutrino mass dif-
ference to the power of four and is experimentally inaccessible [9–11]. The FCNC transitions
in the decays of hadronic states with s, c, or b quarks are observed experimentally [12–17].
The studies of these processes are however affected by the large uncertainties in theoreti-
cal calculations of branching ratios of hadron decays, mainly driven by the non-perturbative
long-distance strong interaction contributions.
The lifetime of the top quark (τt'5×10−25 s), that is shorter than the typical formation
time of the bound states (τhad = 1/ΛQCD '10−24 s), makes the processes with the top quark
production an excellent probe to search for FCNC effects. The absence of hadronic activity
leading to the formation of bound states involving top quarks makes the study of the FCNC
2
processes less affected by radiative QCD corrections. The FCNC effects can be probed in the
top quark production processes, as well as in the decays of the top quarks. The amplitude
of an FCNC transition is proportional to the squared mass of the quark involved in the loop
diagram. A remarkable suppression of the top quark FCNC decays is explained by the fact
that the only possible one-loop contributions are associated with the lighter quarks, leading to
the branching fractions of B(t →cX) '10−15 −10−12 [18], where X represents either a gluon
(g), photon (γ), Z or a Higgs boson (h). Theoretical predictions for the top quark FCNC
effects are available with the next-to-leading order (NLO) precision [19, 20], as well as the
approximate next-to-next-to-next-to-leading order calculations for some of these processes [21,
22].
The study of the flavour structure of the SM is one of the strongest probes of the be-
yond the SM (BSM) theories. A strong suppression of the top quark FCNC transitions is a
perfect condition to search for various possible deviations from the SM predictions. Several
experimental studies of the properties of FCNC decays of b hadrons have sparked a series
of intriguing anomalies in the measured probabilities of the rare b →s`+`−FCNC transi-
tions, as well as in the measurements of the ratios B(B+→K+µ+µ−)/B(B+→K+e+e−) [23],
B(B0→K∗0µ+µ−)/B(B0→K∗0e+e−) [24], as well as the branching fractions [25–27]. A com-
mon analysis of these results reveals a potential tension with respect to the SM [28–32]. Ex-
perimental searches for FCNC effects in the top quark sector represent therefore an important
channel to probe the anomalous interactions of the third-generation quarks.
2 Experimental studies of the top quark FCNC processes
The top quark FCNC effects can be probed directly in the production of a single top quark, as
well as in the top quark decays. Studies of the top quark FCNC decays are typically associated
with similar sensitivities to the top quark FCNC couplings with an up and a charm quark.
Experimental sensitivities to these couplings mainly differ in terms of the performance of
various reconstruction methods used for identification of hadronic jets originating from quarks
of different flavour. At hadron colliders, the single top quark FCNC production process is
mostly sensitive to the top quark FCNC coupling with an up quark (or an up antiquark) due
to an enhanced sensitivity due to the proton distribution function of the colliding protons (or
antiprotons). The importance of these two production channels depends on a specific type of
the top FCNC coupling that is probed in an experiment.
Before the LHC, the top FCNC couplings were studied in electron-positron collisions
at LEP2 [33–36], in deep inelastic scattering processes at HERA [37–41], and in proton-
antiproton collisions at Tevatron [42–45]. The electron-positron colliders allow for a study
of the top-γand top-Z couplings in the processes with the production of a single top quark,
e+e−→t¯c(¯u). The study of deep inelastic scattering of electrons on protons has an enhanced
sensitivity to the same type of couplings in the processes of ep →et + X, as well as to the
top-gluon FCNC couplings in the ep →etq(g) + X processes. The obtained experimental
constraints were recently improved by almost one order of magnitude after the analysis of the
LHC proton-proton collision data [46–55].
The top-Higgs FCNC transitions receive the largest suppression in the SM with respect
to the other top quark FCNC processes because of the large mass of the Higgs boson. These
transitions are among the rarest processes predicted in the SM in the quark sector, and
therefore, the study of these processes is associated with a generally enhanced sensitivity to
3
potential new physics effects. The discovery of the Higgs boson at the LHC paved a way to a
comprehensive study of the top-Higgs FCNC processes at the ATLAS and CMS experiments,
which resulted in the first experimental constraints on these anomalous couplings [56–64].
The direct searches for the top-Higgs FCNC effects are performed in top quark decays, as
well as in the associated production of single top quarks with a Higgs boson. Many of the
performed studies were targeting the top quark FCNC decays in t¯
t events. In recent studies
of the 13 TeV data, the analysis of the single top quark associated production with a Higgs
boson was also included [61–64].
2.1 h→γγ
Search channels that are relevant to the top-Higgs FCNC couplings are usually defined based
on the Higgs boson decay channels. The Higgs boson decays to pairs of photons provide a
clean experimental environment to look for the top-Higgs FCNC effects. In addition to the
two photons, these final states consist of up to one isolated lepton with additional hadronic
jets. The analysis strategy is primarily based on the reconstruction of the Higgs boson di-
photonic invariant mass. The contributions from various background processes are fitted in
the mass sidebands in data, followed by its extrapolation to the signal region. In these fits,
the background contributions that are associated with the SM Higgs boson production must
be accounted for, representing one of the dominant resonant backgrounds in the search region.
The uncertainty associated with the choice of the fit function, the statistical uncertainty in
data, as well as the background contributions from the SM processes involving the Higgs
boson, represent the main uncertainties in the study of these final states.
The searches for top-Higgs FCNC processes in the h →γγ channel were carried out by
ATLAS [58] and CMS [62] in the single-lepton and hadronic final states, including a pair
of photons. The integrated luminosity of recorded 13 TeV data corresponds to 36 fb−1and
137 fb−1, respectively. The identification of isolated photon objects and the common vertex of
the photon pair is the core part of the analysis. The photon and the common vertex identifica-
tion algorithms are based on the multivariate analysis (MVA) approaches. The obtained mass
resolution allows to observe a resonance structure in the diphoton invariant mass spectra in
simulated signal events corresponding to the Higgs boson decay. The contributing nonresonant
background processes include the diphoton production with jets, as well as the top quark pair
and the vector boson production processes with additional photons. The SM production of
the Higgs boson represents the dominant resonant background. The nonresonant backgrounds
are estimated directly from data by performing a fit to the reconstructed diphoton invariant
mass spectrum. The fitted function represents the sum of a double-sided Crystal Ball func-
tion that corresponds to the signal prediction, the resonant background from the SM Higgs
production, and a parameterized function describing the nonresonant background obtained
in a data control region. The main uncertainties include the b tagging and jet energy correc-
tions, as well as photon identification systematic uncertainties. The uncertainty in the limited
number of events in data also represents an important limiting factor in the final sensitivity
in these searches. An additional contribution to the total systematic uncertainty is associated
with theoretical uncertainties in the prediction of the resonant background processes with the
SM Higgs boson production. The unbinned likelihood fit to data using the described signal
and background diphoton mass spectra is performed, and the constraints are set on the top
quark FCNC decay branching fractions. The observed (expected) limits obtained by ATLAS
are B(t →hc) <2.2 ×10−3(1.6 ×10−3) and B(t →hu) <2.4 ×10−3(1.7 ×10−3). The
4
observed (expected) constraints obtained in the CMS analysis are B(t →hc) <7.3 ×10−4
(5.1 ×10−4) and B(t →hu) <1.9 ×10−4(3.1 ×10−4). An enhanced sensitivity obtained
in the CMS analysis is explained by a larger data sample used in the study, as well as due to
the inclusion of the top-Higgs FCNC process with an associated production of a single top
quark and a Higgs boson. The latter has led to an improved sensitivity to B(t →hu).
[GeV]
γγ
m
100 110 120 130 140 150 160
Events / 2 GeV
0
5
10
15
20
25
30
35
40
Data
Continuum bkg.
+ SM Higgs
)γγ cH(→+ t
ATLAS -1
= 13 TeV, 36.1 fbs
Hadronic category 2
0
20
40
60
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100
120
140
S/(S+B) weighted events / GeV
Data
S+B model
B component
σ1 ±
σ2 ±
CMS (13 TeV)
-1
137 fb
γγ →H
100 110 120 130 140 150 160 170 180
(GeV)
γγ
m
20−
0
20 Background component subtracted
Figure 1: Distributions with the invariant diphoton mass showing the results of the fit to
data in the top-Higgs FCNC study of the h →γγ channel at (left) ATLAS [58] and (right)
CMS [62]. The ATLAS results are presented for hadronic final states, while the CMS results
include a combination of all considered channels with events weighted by the associated
significance of each event category.
2.2 h→WW/ZZ/τ τ
Multilepton final states arise from the Higgs boson decays to a pair of W or Z bosons, as
well as to τleptons. Event categories in these studies are associated with the final states
including two same-sign and three leptons. The same-sign lepton channel has the dominant
background contributions originating from the processes with nonprompt and misidentified
leptons, while the three-lepton channel is mainly affected by the presence of diboson events
as well as nonprompt leptons. These backgrounds are estimated from data. The search
channels involving one hadronic τlepton identified in the Higgs boson decay receive dominant
background contributions from the processes with misidentified τlepton decays, as well as
from events with the SM production of top quarks. In the case when the decays of both τ
leptons result in hadronic final states, a significant background contribution is also associated
with the Z boson decays to the pairs of τleptons.
The searches for top-Higgs FCNC couplings in the multilepton channels were performed
at ATLAS [57] and CMS [60] using 36 fb−1of 13 TeV and 20 fb−1of 8 TeV data, respectively.
Events are split into the final states with two same-sign (2lSS) and three (3l) leptons. The
dominant backgrounds are associated with the nonprompt and misidentified leptons, as well
as with the leptons originating from photon conversions. The prompt-lepton backgrounds
correspond to events with an associated production of top quark pairs and a W, a Z, or a
5
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Flavour-changingneutralscalarinteractionsofthetopquarkN.F.Castro1andK.Skovpen21LaboratoriodeInstrumentac~aoeFsicaExperimentaldePartculas(LIP),DepartamentodeFsica,EscoladeCi^encias,UniversidadedoMinho,4710-057Braga,Portugal2GhentUniversity,Sint-Pietersnieuwstraat33,9000Gent,BelgiumAbstractAst...
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