1

92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

92. Dynamical Electroweak Symmetry Breaking:
Implications of the H 0
Revised August 2023 by K.M. Black (Wisconsin U.), R. Sekhar Chivukula (UC San Diego) and A.
Lister (U. of British Columbia).

92.1 Introduction and Phenomenology
In theories of dynamical electroweak symmetry breaking, the electroweak interactions are broken
to electromagnetism by the vacuum expectation value of a composite operator, typically a fermion
bilinear. In these theories, the longitudinal components of the massive weak bosons are identified
with composite Nambu-Goldstone bosons arising from dynamical symmetry breaking in a stronglycoupled extension of the standard model. Viable theories of dynamical electroweak symmetry
breaking must also explain (or at least accommodate) the presence of an additional composite
scalar state to be identified with the H 0 scalar boson [1, 2] – a state unlike any other observed
previously.
Theories of dynamical electroweak symmetry breaking can be classified by the nature of thecomposite singlet state to be associated with the H 0 and the corresponding dimensional scales f ,the analog of the pion decay-constant in QCD, and Λ, the scaleof the underlying strong dynamics.1√
Of particular importance is the ratio v/f , where v 2 = 1/( 2GF ) ≈ (246 GeV)2 , since this ratio
measures the expected size of the deviations of the couplings of a composite Higgs boson fromthose expected in the standard model. The basic possibilities, and the additional states that they
predict, are described below.
92.1.1 Technicolor, v/f ' 1, Λ ' 1 TeV
Technicolor models [7–9] provided the first examples of theories of dynamical electroweak symmetry breaking. These theories incorporate a new asymptotically free gauge theory (“technicolor”)
and additional massless fermions (“technifermions” transforming under a vectorial representationof the gauge group). The global chiral symmetry of the fermions is spontaneously broken by theformation of a technifermion condensate, just as the approximate chiral symmetry in QCD is broken down to isospin by the formation of a quark condensate. The SU(2)W × U(1)Y interactions areembedded in the global technifermion chiral symmetries in such a way that the only unbroken gauge
symmetry after chiral symmetry breaking is U(1)em .2 The theories naturally provide the NambuGoldstone bosons “eaten” by the W and Z bosons. There would also typically be additional heavy
states (e.g. vector mesons, analogous to the ρ and ω mesons in QCD) with TeV masses [13, 14],and the W W and ZZ scattering amplitudes would be expected to be strong at energies of order
1 TeV.
There are various possibilities for the scalar H 0 in technicolor models. First, the H 0 could
be identified as a singlet scalar resonance, analogous to the σ particle expected in pion-scattering
in QCD [15, 16]. Alternatively, the H 0 could be identified as a dilaton, a (pseudo-)Goldstoneboson of scale invariance in theories of “walking technicolor” [17–21].3 Finally, the H 0 could be
identified as an additional isosinglet state if the chiral symmetry breaking pattern of the technicolor
theory provides for such a state.4 In all of these cases, however, one expects large deviations in
1
In a strongly interacting theory “Naive Dimensional Analysis” [3, 4] implies that, in the absence of fine-tuning,Λ ' g ∗ f where g ∗ ' 4π is the typical size of a strong coupling in the low-energy theory [5, 6]. This estimate ismodified in the presence of multiple flavors or colors [7] .
2
For a review of technicolor models, see [10–12].
3
If both the electroweak symmetry and the approximate scale symmetry are broken only by electroweak doublet
condensate(s), then the decay-constants for scale and electroweak symmetry breaking may be approximately equal –differing only by terms formally proportional to the amount of explicit scale-symmetry breaking.
4
In this case, however, the coupling strength of the singlet state to W W and ZZ pairs would be comparable

S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024)
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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

the couplings of this particle from those of the standard model Higgs boson. Since the couplings
observed for the H 0 approximate those of the Higgs boson to the 10% level, models of this kindare very highly constrained.
92.1.2 The Higgs doublet as a pseudo-Nambu-Goldstone Boson, v/f < 1, Λ > 1 TeV
In technicolor models, the symmetry-breaking properties of the underlying strong dynamics necessarily breaks the electroweak gauge symmetries. An alternative possibility is that the underlying
strong dynamics itself does not break the electroweak interactions, and that the entire quartet ofbosons in the Higgs doublet (including the state associated with the H 0 ) are composite (pseudo-)
Nambu-Goldstone particles [23–25]. In this case, the underlying dynamics can occur at energies exceeding 1 TeV and additional interactions with the top-quark mass generating sector (and possiblywith additional weakly-coupled gauge bosons) cause the vacuum energy to be minimized when thecomposite Higgs doublet gains a vacuum expectation value [26,27]. In these theories, the couplingsof the remaining singlet scalar state would naturally be equal to that of the standard model Higgs
boson up to corrections of order (v/f )2 and, therefore, constraints on the size of deviations of theH 0 couplings from that of the standard model Higgs [28] give rise to lower bounds on the scales fand Λ.5
The electroweak gauge interactions, as well as the interactions responsible for the top-quark
mass, explicitly break the chiral symmetries of the composite Higgs model and lead genericallyto sizable corrections to the mass-squared of the Higgs-doublet – the so-called “Little Hierarchy
Problem” [29]. “Little Higgs” theories [30–33] are examples of composite Higgs models in which
the (collective) symmetry-breaking structure is selected so as to suppress these contributions to the
Higgs mass-squared.
Composite Higgs models typically require a larger global symmetry of the underlying theory, andhence additional relatively light (compared to Λ) scalar particles, extra electroweak vector bosons(e.g. an additional SU(2)×U(1) gauge group), and vector-like partners of the top-quark of charge+2/3 and possibly also +5/3 [34]. In addition to these states, one would expect the underlying
dynamics to yield additional scalar and vector resonances with masses of order Λ. If the theory
respects a custodial symmetry [35], the couplings of these additional states to the electroweak and
Higgs boson will be related – and, for example, one might expect a charged vector resonance tohave similar branching ratios to W Z and W H. Different composite Higgs models utilize different
mechanisms for arranging for the hierarchy of scales v < f and arranging for a scalar Higgs selfcoupling small enough to produce an H 0 of mass of order 125 GeV, for a review see [36] . If theadditional states in these models carry color, they can provide additional contributions to Higgs
production via gluon fusion [37]. The extent to which Higgs production at the LHC conforms withstandard model predictions provides additional constraints (typically lower bounds on the massesof the additional colored states of order 0.7 TeV) on these models.
In addition, if the larger symmetry of the underlying composite Higgs theory does not commute
with the standard model gauge group, then the additional states found in those models – especiallythose related to the top-quark, which tend to have the largest couplings to the electroweak sector –may be colorless. For example, in twin Higgs models [38], the top-partners carry no standard model
charges. The phenomenology of the additional states in twin Higgs theories is rather different, since
to the couplings to gluon and photon pairs, and these would all arise from loop-level couplings in the underlying
technicolor theory [22].
5
In these models v/f is an adjustable parameter, and in the limit v/f → 1 they reduce, essentially, to thetechnicolor models discussed in the previous subsection. Our discussion here is consistent with that given there,
since we expect corrections to the SM Higgs couplings to be large for v/f ' 1. Current measurements constrain
the couplings of the H 0 to equal those predicted for the Higgs in the standard model to about the 10% level [28],suggesting that f must have values of order a TeV or higher and, therefore, a dynamical scale Λ of at least several
TeV.
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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

3lacking color the production of these particles at the LHC will be suppressed – and, their decays
may occur only via the electroweak symmetry breaking sector, leading to their being long-lived.
92.1.3 Top-Condensate, Top-Color, Top-Seesaw and related theories, v/f < 1, Λ >1 TeV
A final alternative is to consider a strongly interacting theory with a high (compared to a TeV)
underlying dynamical scale that would naturally break the electroweak interactions, but whosestrength is adjusted (“fine-tuned”) to produce electroweak symmetry breaking at 1 TeV. This
alternative is possible if the electroweak (quantum) phase transition is continuous (second order)
in the strength of the strong dynamics [39]. If the fine-tuning can be achieved, the underlying
strong interactions will produce a light composite Higgs bound state with couplings equal to thatof the standard model Higgs boson up to corrections of order (1 TeV/Λ)2 . As in theories in whichelectroweak symmetry breaking occurs through vacuum alignment, therefore, constraints on the sizeof deviations of the H 0 couplings from that of the standard model Higgs give rise to lower bounds
on the scale Λ. Formally, in the limit Λ → ∞ (a limit which requires arbitrarily fine adjustmentof the strength of the high-energy interactions), these theories are equivalent to a theory with afundamental Higgs boson – and the fine adjustment of the coupling strength is a manifestation of
the hierarchy problem of theories with a fundamental scalar particle.
In many of these theories the top-quark itself interacts strongly (at high energies), potentiallythrough an extended color gauge sector [40, 40–44]. In these theories, top-quark condensation (orthe condensation of an admixture of the top with additional vector-like quarks) is responsible forelectroweak symmetry breaking, and the H 0 is identified with a bound state involving the third
generation of quarks. These theories typically include an extra set of massive color-octet vectorbosons (top-gluons), and an extra U (1) interaction (giving rise to a top-color Z0 ) which couple
preferentially to the third generation and whose masses define the scale Λ of the underlying physics.
92.1.4 Flavor
In addition to the electroweak symmetry breaking dynamics described above, which gives riseto the masses of the W and Z particles, additional interactions must be introduced to produce
the masses of the standard model fermions. Two general avenues have been suggested for these
new interactions. In “extended technicolor” (ETC) theories [45, 46], the gauge interactions in theunderlying strongly interacting theory are extended to incorporate flavor. This extended gauge
symmetry is broken down (possibly sequentially, at several different mass scales) to the residualstrong interaction responsible for electroweak symmetry breaking. The massive gauge-bosons corresponding to the broken symmetries then mediate interactions between mass operators for thequarks/leptons and the corresponding bilinears of the strongly-interacting fermions, giving rise to
the masses of the ordinary fermions after electroweak symmetry breaking.
In the case of “partial compositeness” [47], the additional flavor-dependent interactions arise
from mixing between the ordinary quarks and leptons and massive composite fermions in thestrongly-interacting underlying theory. Theories incorporating partial compositeness include additional vector-like partners of the ordinary quarks and leptons, typically with masses of order a TeVor less.
In both cases, the effects of flavor interactions on the electroweak properties of the ordinary
quarks and leptons are likely to be most pronounced in the third generation of fermions.6 The
additional particles present in these theories, especially the additional scalars, often couple morestrongly to heavier fermions.
Moreover, since the flavor interactions must give rise to quark mixing, we expect that a generic6

Indeed, from this point of view, the vector-like partners of the top-quark in top-seesaw and little Higgs models
can be viewed as incorporating partial compositeness to explain the origin of the top quark’s large mass.

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492. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

theory of this kind could give rise to large flavor-changing neutral-currents [46]. In ETC theories,
these constraints are typically somewhat relaxed if the theory incorporates approximate generational flavor symmetries [48], the theory has a slowly running coupling constant or “walks” [17–21],
or if Λ > 1 TeV [49]. In theories of partial compositeness, the masses of the ordinary fermionsdepend on the scaling-dimension of the operators corresponding to the composite fermions withwhich they mix. This leads to a new mechanism for generating the mass-hierarchy of the observed
quarks and leptons that, potentially, ameliorates flavor-changing neutral current problems and canprovide new contributions to the composite Higgs potential which allow for v/f < 1 [50–54].
Alternatively, one can assume that the underlying flavor dynamics respect flavor symmetries(“minimal” [55, 56] or “next-to-minimal” [57] flavor violation) that suppress flavor-changing neutral currents in the two light generations. Additional considerations apply when extending thesearguments to potential explanation of neutrino masses (see, for example, [58, 59]).
92.1.5 Theoretical Considerations
Since the underlying high-energy dynamics in these theories are strongly coupled, there are
no reliable calculation techniques that can be applied to analyze their properties. Instead, mostphenomenological studies depend on the construction of a “low-energy” effective theory describing
additional scalar, fermion, or vector boson degrees of freedom, which incorporates the relevant
symmetries and, when available, dynamical principles. In some cases, motivated by the AdS/CFT
correspondence [60], the strongly-interacting theories described above have been investigated byanalyzing a dual compactified five-dimensional gauge theory. In these cases, the AdS/CFT “dictionary” is used to map the features of the underlying strongly coupled high-energy dynamics onto
the low-energy weakly coupled dual theory [61].
More recently, progress has been made in investigating strongly-coupled models using latticegauge theory [62–65]. These calculations offer the prospect of establishing which strongly coupledtheories of electroweak symmetry breaking have a particle with properties consistent with thoseobserved for the H 0 – and for establishing concrete predictions for these theories at the LHC [66].
92.1.6 Summary
The theoretical ideas and models reviewed here motivate searches for a wide variety of newstates. Heavy vector bound states decaying to dibosons and additional scalar states appear naturally in both technicolor (92.1.1) and composite higgs models (92.1.2). Composite Higgs models
based on collective symmetry breaking naturally include W 0 , Z 0 , and vector-like quark states (especially for the third-generation, and potentially including charge +5/3 custodial top-partner quarks).
Vector-like states are also natural for any theory including partial compositeness in order to address
the flavor problem (92.1.4). Top-condensate and related models (92.1.3) predict the existence ofcolorons which preferentially couple to third-generation quarks. Finally, the new states discussed
here also occur in extra-dimensional models [61], which is understandable given that many stronglycoupled theories can be viewed as compactified five-dimensional gauge theories via duality (92.1.5).
We turn now to a review the status of experimental searches related to dynamical electroweak
symmetry breaking.

92.2 Experimental Searches
As discussed above, the extent to which the couplings of the H 0 conform to the expectations
for a standard model Higgs boson constrains the viability of each of these models. Measurementsof the H 0 couplings, and their interpretation in terms of effective field theory, are summarized in
the H 0 review in this volume. In what follows, we will focus on searches for the additional particles
that might be expected to accompany the singlet scalar: extra scalars, fermions, and vector bosons.
In some cases, detailed model-specific searches have been made for the particles described above
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592. Dynamical Electroweak Symmetry Breaking: Implications of the H 0(though generally not yet taking account of the demonstrated existence of the H 0 boson).
In most cases, however, generic searches (e.g. for extra W 0 or Z 0 particles, extra scalars in thecontext of multi-Higgs models, or for fourth-generation quarks) are quoted that can be used – whenappropriately translated – to derive bounds on a specific model of interest.
The mass scale of the new particles implied by the interpretations of the low mass of H 0
discussed above, and existing studies from the Tevatron and lower-energy colliders, suggests that
only the Large Hadron Collider has any real sensitivity. A number of analyses already carried out
by ATLAS and CMS use relevant final states and might have been expected to observe a deviationfrom standard model expectations – in no case so far has any such deviation been reported. The
detailed implications of these searches in various model frameworks are described below.
Except where otherwise noted, all limits in this section are quoted at a confidence level of 95%.
√
The searches at s = 8 TeV (Run 1) are based on 20.3 fb−1 of data recorded by ATLAS, and an√
integrated luminosity of 19.7 fb−1 analyzed by CMS. The datasets collected at s = 13 TeV during
Run 2 of the LHC since 2015 are based on analyses with varied integrated luminosities ranging from
∼2-140 fb−1 .
92.2.1 Searches for Z 0 or W 0 Bosons
Massive vector bosons or particles with similar decay channels would be expected to arise in
Little Higgs theories, in theories of Technicolor, or models involving a dilaton, adjusted to produce
a light Higgs boson, consistent with the observed H 0 . These particles would be expected to decay
to pairs of vector bosons, or to third generation quarks, or to leptons. The generic searches forW 0 and Z 0 vector bosons listed below can, therefore, be used to constrain models incorporating acomposite Higgs-like boson.
A general review of searches for Z 0 and W 0 bosons is also included in this volume [67, 68]. In
the context of the dynamical electroweak symmetry breaking models, we emphasize their decays
to third generation fermions by including a detailed overview, while also briefly summarizing theother searches.
Z 0 → ``:ATLAS [69] and CMS [70] have both searched for Z 0 production with Z 0 → ee or µµ. No
deviation from the standard model prediction was seen in the dielectron and dimuon invariant
mass spectra, by either the ATLAS or the CMS analysis, and lower limits on possible Z 0 boson
0
masses were set. A ZSSM
with couplings equal to the standard model Z 0 (a “sequential standard
0model” Z ) and a mass below 5.1 TeV was excluded by ATLAS, while CMS set a lower mass limitof 5.15 TeV. The experiments also place limits on the parameters of extra dimension models and in
the case of ATLAS on the parameters of a minimal walking technicolor model [17–21], consistent
with a 125 GeV Higgs boson [71]. For a general review of searches in these channels see the PDG
review of Z prime in this volume [67].
In addition, both experiments have also searched for Z 0 decaying to a ditau final state [72, 73].
An excess in τ + τ − could have interesting implications for models in which lepton universality is
not a requirement and enhanced couplings to the third generation are allowed. This analysis led
0
to lower limits on the mass of a ZSSM
of 2.4 and 2.1 TeV from ATLAS and CMS respectively.
0
Z → qq:The ability to relatively cleanly select tt pairs at the LHC together with the existence of enhanced
couplings to the third generation in many models makes it worthwhile to search for new particles
decaying in this channel. Both ATLAS [74] and CMS [75] have carried out searches for new particles
decaying into tt.
Both ATLAS and CMS searched for tt in the all hadronic mode [76] [77] in both the resolved
and boosted regions. No evidence of resonance production were seen and limits were produced for31st May, 2024

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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0various models including the Z 0 boson in topcolor-assisted technicolor which excludes masses lessthan 3.1 to 3.6 TeV (ATLAS) depending on the details of the model and 3.3, 5.25, and 6.65 TeV
for widths of 1, 10 and 30 percent relative to the mass of the resonance .
ATLAS also presented results on the lepton plus jets final state, where the top quark pair decays
as tt → W bW b with one W boson decaying leptonically and the other hadronically; CMS used final
states where both, one or neither W decays leptonically and then combined the results. The tt̄
invariant mass spectrum was analyzed for any excess, and no evidence for any resonance was seen.
ATLAS excluded a narrow (Γ/m = 1.2%) leptophobic top-color Z 0 boson with masses between
0.7 and 2.1 TeV and with Γ/m = 3% between 0.7 and 3.2 TeV. CMS set limits on leptophobic
Z 0 bosons for three different assumed widths Γ/m = 1.0% , Γ/m = 10.0%, and Γ/m = 30.0% of
3.9 TeV to 4.0 TeV and exclude RS KK gluons up to 3.3 TeV.
Both ATLAS [78] and CMS [79] have also searched for resonances decaying into qq, qg or gg
using the dijet invariant mass spectrum. Excited quarks are excluded up to masses of 6.7 TeV
and model-independent upper limits on cross sections with a Gaussian signal shape were set. CMS
excluded string resonances with masses below 7.9 TeV, scalar diquarks below 7.5 TeV, axigluonsand colorons below 6.6 TeV, excited quarks below 6.3 TeV, color-octet scalars below 3.7 TeV, W 0
bosons below 3.6 TeV, Z 0 bosons with SM-like couplings below 2.9 TeV and between 3.1 TeV and
3.3 TeV, Randall–Sundrum Gravitons below 2.6 TeV.
W 0 → `ν:
Both LHC experiments have also searched for massive charged vector bosons. In this section
we include a summary of the results, with emphasis on final states with third generation fermions,while the details on other decays are discussed in the mini-review of W 0 [68]. ATLAS searched
for a heavy W 0 decaying to eν or µν and found no excess over the standard model expectation.
A sequential standard model (SSM) W 0 boson (assuming zero branching ratio to W Z) with mass√less than 7 TeV was excluded [80] using the 139 fb−1 dataset at s = 13 TeV. Model independent
cross-section limits as a function of mass were also set. Based on a similar dataset, the CMS
experiment excluded a SSM W 0 boson with mass up to 5.7 TeV [81] and presented upper limits
on the production of generic W 0 bosons decaying into this final state using a model-independent
approach.
CMS [82] has carried out a complementary search in the τ ν final state. As noted above, suchsearches place limits on models with enhanced couplings to the third generation. No excess wasobserved and limits between 2.0 and 2.7 TeV were set on the mass of a W 0 decaying preferentiallyto the third generation; a W 0 with universal fermion couplings was also excluded for masses lessthan 2.7 TeV.
W 0 → tb:
Heavy new gauge bosons can couple to left-handed fermions like the SM W boson or to righthanded fermions. W 0 bosons that couple only to right-handed fermions (WR0 ) may not have leptonic
decay modes, depending on the mass of the right-handed neutrino. For these W 0 bosons, the tb(tb + tb) decay mode is especially important because in many models the W 0 boson is expected tohave enhanced couplings to the third generation of quarks relative to those in the first and second
generations. It is also the hadronic decay mode with the best signal-to-background. ATLAS andCMS have performed searches for W 0 bosons via the W 0 → tb decay channel in the lepton+jets
and all-hadronic final state.
The CMS lepton+jets search [83–86], W 0 → tb → W bb → `νbb, proceeded via selecting eventswith an isolated lepton (electron or muon), and at least two jets, one of which is identified tooriginate from a b-quark. The mass of the W 0 boson (Mtb ) was reconstructed using the fourmomentum vectors of the final state objects (bb`ν). The distribution of Mtb is used as the search
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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

g'/g√
discriminant. A search [86] using 35.9 fb−1 of data, collected at s = 13 TeV, led to an exclusionof WR0 bosons with masses below 3.4 TeV (3.6 TeV) if MWR0 >> MνR (MWR0 < MνR ), where MνR
is the mass of the right-handed neutrino.
The CMS search for W 0 → tb decays using the all-hadronic final state focused on W 0 masses
above 1 TeV [85, 87]. In this region, the top quark gets a large Lorentz boost and hence the three
hadronic products from its decay merge into a single large-radius jet. Deep neural network(DNN)
algorithms are used to identify the jet initiated by the bottom quark. Techniques including DNN,
which rely on substructure information of the jets [88] are employed to identify boosted all hadronic
W boson and top quark [87] decays. The W 0 candidate mass was computed from back-to-backboosted top-tagged jet and a low mass b-tagged jet. From this all-hadronic search, W 0 bosons were
excluded for masses up to 3.4 TeV [87].
ATLAS has searched for WR0 bosons in the tb final state both for lepton+jets [89] and allhadronic [90] decays of the top. No significant deviations from the standard model were seen ineither analysis and limits were set on the W 0 → tb cross section times branching ratio and W 0
bosons with with both purely right and purely left-handed coupling and limits are set between 0.5
and 6 TeV for different values of the coupling strength.
In addition, the above studies also provided upper limits on the W 0 effective couplings to rightand left-handed fermions. In Fig. 92.1 (bottom) the upper limits on W 0 couplings normalized to
the SM W boson couplings derived by ATLAS [90] are shown. The top panel of Fig. 92.1 shows
the upper limits for arbitrary combinations of left- and right-handed couplings of the W 0 boson tofermions set using a model independent approach by CMS [86].

ATLAS

1

s = 13 TeV, 139 fb-1
Observed 95% CL limit
Expected 95% CL limit
Expected 95% CL limit ± 1 σ
Expected 95% CL limit ± 2 σ
1-lepton Expected 95% CL limit
0-lepton Expected 95% CL limit

0.1
1000

2000

3000

4000

5000
6000
m(W' ) [GeV]R

Figure 92.1: Left panel: Observed limits on the W 0 boson mass as function of the left-handed(aL ) and right-handed (aR ) couplings. Black lines represent contours of equal W 0 boson mass [86].
Right panel: Observed and expected regions, on the g0 /g vs mass of the W 0 boson plane, thatare excluded at 95% CL, for right-handed W 0 bosons [91] showing the exclusion of the values of ahypothetical right-handed coupling of a heavy W particle.
92.2.2 Searches for Resonances decaying to Vector Bosons and/or Higgs Bosons√
Both ATLAS and CMS have used the data collected at s = 13 TeV to search for resonances
decaying to pairs of bosons. Overall no significant excesses were seen in the full datasets that wereanalyzed and the results are interpreted in models with heavy vector triplets (HVT) [92], models
with strong gravity and extra spatial dimensions, and model independent limits as a function ofmass are set. For a full review of models including extra spatial dimensions and the interpretation
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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0of many of these results in that context please see the review of extra dimensions in this volume [61].
√Utilizing data collected at s = 13 TeV, ATLAS [93] and CMS [94], have both looked for aresonant state decaying into V V (with V = W or Z) , V H ( with H representing the SM Higgs
boson), and HH. ATLAS has searched in the qqqq, ννqq, lνqq , llqq , lνlν, llνν, lνll, llll, qqbb
ννbb, lνbb, and llbb final states and combined the results . While CMS analyzed the qqqq, ννqq,
lνqq , llqq , llνν, ννbb, lνbb, llbb , bbbb, τ τ bb , and qqτ τ final states.
The combined limits are expressed both as limits on the cross-section as a function resonancemass as well as constraints on the coupling of the heavy boson triplet to quarks, leptons, and the
Higgs boson.
X → W Z:ATLAS searched for new heavy resonances decaying into W Z in the channels W Z → qqqq [95] ,lνqq [96], and lνll [97]. In the fully leptonic channel , the invariant mass of the W Z pair is obtainedby considering all possible four lepton permutations in each event. The dominant background isStandard Model continuum W Z production, ZZ production where one lepton is not identified orfalls outside the detector acceptance, and top quark plus vector boson production. No resonant
production is seen in data and lower limits on the mass of a HVT decaying into W Z are set at 2260
(2460) GeV assuming a coupling constant of gV = 1 (gV = 3). In the W Z → lνqq mode, ATLAS
searches in both the cases that the quarks are observed as individual jets (resolved) and where
they merge into one jet in the detector (boosted) which probe the low and high pT regime of the Zboson. No significant excess is seen in either channel and combined lower mass limits are placed at2900 (3000) GeV for gV = 1 (gV = 3) in the HVT model. In the all hadronic decay mode, ATLAS
searched for two high pT hadronically decaying vector bosons looking for a resonant structure. No
excess is seen and limits are placed exclude 1200-3000 (1200-3300) GeV for gV = 1 (gV = 3) in the
HVT model.
CMS searched for new heavy resonances in the ZV final state using the semi-leptonic decay
channels excluding W’ masses up to 1800 GEV in HVT two models with gV = 1 and 3 and in the ZZ
channel excludes gravitons up to 1200 GeV. The CMS collaboration searched for V V → qqqq [98]in the large R dijet search. The W and Z boson are identified through the mass of the large R
jet and substructure variables. No excess is seen and limits are set for charged HVT bosons with
masses lower than 3200 (3800) GeV for gV = 1 (gV = 3). Cross-section limits as function of mass
are reported for the charged spin-1 resonance interpretation and are placed at 44.4 fb at 1.4 TeV
to 0.7 pb at 4 TeV. In the ννqq final state [99], the CMS collaboration searched for a charged spin
1 resonance decaying into a V Z final state with a Z boson decaying into a pair of neutrinos andthe other boson decaying into two collimated quarks reconstructed as a large R-jet. The transverse
mass of the V Z candidate is reconstructed and utilized to search for evidence of resonant V Z
production. No excess is seen and lower mass limits are placed on the charged resonance at 3100
(3400) GeV for gV = 1 (gV = 3) . In the 2l2q final state, the CMS collaboration searched for aheavy resonance decaying into ZV [100] looking for events with one large R-jet consistent with thehadronic decay of a vector boson and a Z boson reconstructed in the charged lepton decay channel(e or µ). Limits are set for a HVT W 0 with a lower mass of 2270 (2330) for gV = 1 (gV = 3) .
X → WW:
ATLAS searched for a new heavy resonance decaying into W W in the channels W W → qqqq[95], lνqq [96], and lνν [101]. In the case where both W s decay leptonically, ATLAS utilized thetransverse mass of the two lepton and two neutrino final state and searched for an excess in this
distribution between 200 GeV and 5 TeV. No excess is seen and a HVT is excluded for masses below
1300 GeV. Vector boson fusion is also considered and cross-section limits as a function of mass are
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992. Dynamical Electroweak Symmetry Breaking: Implications of the H 0placed ranging from 1.3 pb to 0.006 pb at 200 GeV to 3 TeV, respectively. In the lνqq mode ,ATLAS completed a companion analysis to the W Z → analysis discussed above and places lower
mass limits of 2850 (3150) GeV for gV = 1 (gV = 3) in the HVT model. ATLAS also interprets
the all hadronic mode analysis in the hypothesis that W W → qqqq and places limits on a HVTboson decaying into WW in the all hadronic mode between 1200 and 2200 (1200 and 2800) GeV
for gV = 1 (gV = 3).
The CMS collaboration searched for V V → qqqq [98] in the large R dijet search. The W andZ boson are identified through the mass of the large R jet and substructure variables. No excess
is seen and limits are set for charged HVT bosons with masses lower than 2700 (2800) GeV forgV = 1 (gV = 3). Cross-section limits as function of mass are reported for the uncharged spin-1
resonance interpretation and are placed at 41.6 fb at 1.4 TeV to 0.6 pb at 4 TeV.
X → V H:ATLAS searched for a new heavy resonance decaying into W H and ZH in the qqbb (W H andZH) [102], lνbb (W H), ννbb (ZH), llbb(ZH) [103] modes. In the all hadronic mode, ATLAS
searched for boosted V H production looking for two large R jets where the larger invariant masslarge R jet is interpreted as the Higgs boson decay products while the lesser invariant mass jet istaken to be the hadronically decaying vector boson requiring b-tagging on the Higgs boson subjets.
The invariant mass is reconstructed and a search is performed for resonant production of ZH. None
is found and limits from 1100 to 2500 (1300 to 3800 GeV) are placed for gV = 1 (gV = 3). ATLAS
also searched for XH where the W or Z boson decays into νν, lν, and ll . The analysis searched forboth resolved and merged (boosted) b-jets from the decay of the Higgs boson and defines the signal
regions based on the number of reconstructed charged leptons (0,1,or 2). In the dilepton channel theinvariant mass is explicitly reconstructed of the entire diboson system, the single lepton channel
reconstructs the diboson final state constraining the lepton and missing transverse momentum
utilizing the known W boson mass, while the 0 charged lepton channel reconstructs the transverse
mass of the diboson system. No excess is seen in any channel and limits on the production of a
HVT are placed at 2800 GeV (2930) GeV for gV = 1 (gV = 3).
The CMS Collaboration searched for a heavy resonance decaying into V H [104] searching fora resonances decaying into a Higgs boson and a hadronically decaying W or Z boson. The search
identifies events with two large-R jets using substructure variables and requires one large-R jets
is tagged with a pair of b-hadrons clustered in a single jet. The invariant mass of the V H bosons
is reconstructed and evidence for resonance production is sought. No excess is seen and limits
are placed. With gV = 1 (3) a narrow W 0 resonance with mW 0 < 2470(3150) GeV and mZ 0 <1150(1190).
The CMS collaboration searched for a heavy resonance decaying into a pair of boosted Higgs
boson with HH → bb̄W W ∗ in the single and dilepton channel [105]. Events are categorized according to lepton flavor and multiplicity, along with discriminators that characterize the compatibility
with having jets arising from the decay of boosted Higgs and W bosons. The reconstructed Higgs
and diHiggs mass distributions are fit and used as the final discriminators. No significant deviationsin the 12 sub-categories are seen and 95% confidence limits on spin-0 bosons are set from 24.5 fb
at 0.8 TeV to 0.78 fb at 4.5 TeV and spin-2 bosons from 16.7 fb at 0.8 TeV to 0.67 fb at 4.5 TeV. A
similar search in the four b-quark final state is used to set limits range 9.74 to 0.29 fb and 4.94 to
0.19 fb in the spin-0 bosons and spin-2 gravitons (respectively) interpretation with masses of 1-3
TeV [106].
X → Hγ or V γ:
Both CMS and ATLAS search for heavy resonances decaying into either a vector boson and aphoton or a Higgs boson or a photon. These searched are motivated generically by the fact that
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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 01035.9 fb-1 (13 TeV)

σ(W') (pb)2016 diboson combination

Combination
WZ → qqqq (PRD 97(2018)072006)
WZ → ν ν qq (JHEP 07(2018)075)
WZ → lν qq (JHEP 05(2018)088)
WZ → llqq (JHEP 09(2018)101)
WH → qqbb (EPJC 77(2017)636)
WH → lν bb (JHEP 11(2018)172)
WH → qqττ (JHEP 01(2019)051)

CMS
1

10−1

10−2

10−3

10−4

95% CL upper limits
Observed
Median expected
HVT model B

1000

1500

2000

σ(pp → W’ → WZ + WH + lv) [fb]new heavy gauge bosons are generically predicted to decay to either a Higgs boson or a vector bosonand a photon by one loop decays [107]. ATLAS searched for for a heavy resonances decaying intoa Higgs boson and a photon and sets limits from 0.7 to 4 TeV from 11.6 fb to 0.11 fb [108]. ATLAS
searched for a heavy resonance decaying into XH where XH → qqbb [109] and sets limits from 1
to 4 TeV. CMS searched for a heavy resonance decaying into a W boson and a photon [110] setting
limits for narrow resonances range between 0.17 fb at 6.0 TeV and 55 fb at 0.7 TeV.
Summary of Searches with Diboson Final States:
Both ATLAS [93] and CMS [94] provide plots summarizing the various searched results andlimits combining. The results are shown in the context of HVT models and models of strong
gravity with extra spatial dimensions. No excess is seen in any search and limits on the 4.3 (4.5)
TeV (ATLAS) and (CMS). Inclusion of decays directly to fermions increase these limits to 5.3 (5.5)
TeV and 5.0 (5.2) TeV from the ATLAS and CMS combinations, respectively. Both collaborations
also place varying limits on the coupling strength as a function of HVT boson mass as well.

104

3000

3500

4000
4500
mW' (GeV)

Observed 95% CL limits = 13 TeV, 36.1 fb-1

Expected 95% CL limit

310

Expected ± 1σ

102

HVT model A

Expected ± 2σ

10
1
10−1

2500

ATLAS

0.5

DY HVT W’ → WZ + WH + lv

1

1.5

2

2.5

3

3.5

4

4.5

5

m(W’) [TeV]

Figure 92.2: Left panel:Observed limits from W 0 to diboson from CMS [94]. Right panel: Observed limits from W 0 to diboson decays from ATLAS [93].
Searches for Triboson Resonances:
The CMS collaboration presents the first search for massive triboson resonances in both thesemi-leptonic [111] and fully hadronic final state [112]. Three boson final states can be produced
via the decay of Kaluza-Klein (KK) states decaying via WKK → W R with R → W W where R
is a scalar radion. The search utilizes novel deep neural networks to perform optimal separation
between signal and background and searched for evidence of resonance structure in the triboson
invariant mass-spectrum. No excesses above SM backgrounds are seen and WKK and radion masses
are excluded up to MWKK = 3.4 TeV and MR = 1 TeV and up to MWKK = 3.6 TeV and MR =R
< 0.7 in the semi-leptonic final state
0.35 TeV . MWKK below 3 TeV are excluded for 0.6 < MM
WKK
and up to MWKK = 3.0 TeV and MR = 200 GeV and up to MWKK = 1.5 TeV and MR = 1.5 TeV
in the fully hadronic final state.
92.2.3 Vector-like third generation quarks
Vector-like quarks (VLQ) have non-chiral couplings to W bosons, i.e. their left- and righthanded components couple in the same way. They therefore have vectorial couplings to W bosons.
Vector-like quarks arise in Little Higgs theories, top-coloron-models, and theories of a composite
Higgs boson with partial compositeness. In the following, the notation T quark refers to a vector-likequark with charge 2/3 and the notation B quark refers to a vector-like quark with charge −1/3, the31st May, 2024

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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0same charges as the SM top and b quarks respectively. The exotic vector-like quarks X5/3 and Y−4/3
have charges 5/3, and −4/3 respectively. Vector-like quarks couple with SM quarks with Yukawa
interactions and may exist as SU(2) singlets (T , and B), doublets [(X5/3 , T ),(T, B),(B, Y−4/3 )], or
triplets [(X5/3 , T, B), (T, B, Y−4/3 )]. At the LHC, VLQs can be pair produced via the dominant
gluon-gluon fusion process. VLQs can also be produced singly by their electroweak effective couplings to a weak boson and a standard model quark. The single production rate is expected todominate over the rate of pair production at large VLQ masses. T quarks can decay to bW , tZ,
or tH 0 . Weak isospin singlets are expected to decay to all three final states with (asymptotic)
branching fractions of 50%, 25%, 25%, respectively. Weak isospin doublets are expected to decay
exclusively to tZ and to tH 0 [113] with equal branching ratios. Analogously, B quarks can decay
to tW , bZ, or bH 0 . The Y−4/3 and X5/3 quarks decay exclusively to bW and to tW . While these
are taken as the benchmark scenarios, other representations and decays to exotic new particles are
possible [114–116], and hence the final results are interpreted for many allowed branching fraction
combinations.
Given the multiple decay modes of the VLQs, the final state signatures of both pair produced andthe singly produced VLQs are fairly rich with leptons, jets, b-jets, and missing energy. Dependingon the mass of the VLQ, the top quarks and W/Z/H 0 bosons may be Lorentz boosted and identified
using jet substructure techniques. Thus the searches are performed using lepton+jets signatures,
multi-lepton and all-hadronic decays. In addition, T or B quarks with their antiparticles can resultin events with same-sign leptons, for example if the decay T → tH → bW W + W − is present,
followed by leptonic decays of two same-sign W bosons. In the following subsections, while we
describe the searches for each of the decay modes of the VLQs, the same analysis can be reinterpreted to obtain the sensitivity to a combination with varied branching fractions to the different
decay modes.
In the following sections, the results obtained for T (B) quarks assuming 100% branching ratio
to W b (W t) are also applicable to heavy vector-like Y−4/3 (X5/3 ) with charge 4/3 (5/3).
92.2.3.1 Searches for T quarks that decay to W , Z and H 0 bosons
T /Y → bW :CMS has searched for pair production of heavy T quarks that decay exclusively to bW [117–119].
The analysis selected events with exactly one charged lepton, assuming that the W boson fromthe second T quark decays hadronically. Under this hypothesis, a 2-constraint kinematic fit can be
performed to reconstruct the mass of the T quark as a narrow mass peak with a mass resolutionof around 7%. In Refs. [118] and [119], the two-dimensional distribution of reconstructed mass vsST was used to test for the signal, where ST is the scalar sum of the missing pT and the transverse
momenta of the lepton and the leading four jets. This analysis, when combined with the search in
the fully hadronic final state [120] excluded new quarks that decay 100% to bW for masses below
0.89 TeV [119]. At times the hadronically-decaying W boson is produced with a large Lorentz
boost, leading to the W decay products merged into a large-radius jet. Algorithms such as jet
pruning [121] were used to remove contributions from soft, wide angle radiation, from large-radius
jets, leading to better discrimination between jets from multijet events and those arising fromdecays of the heavy particles. If the mass of the boosted jet was compatible with the W boson
mass, then the W boson candidate jet and its subjets were used in the kinematic reconstructionof the T quark. No excess over standard model backgrounds was observed. Upper limits on the
production cross section as a function of the mass of T quarks were measured. By comparing themwith the predicted cross section for vector like quark pair production, the strong pair productionof T quarks was excluded for masses below 1.30 TeV (1.28 TeV expected) [117].
Another “cut-based” search for pair produced T quarks in the all-hadronic final state targeting
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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

the W b decay mode [122], relies on mass reconstruction of two highest pT W b combinations using
boosted W boson candidates with pT > 200 GeV and b-tagged jets. HT is used as the signal
discriminator, with selected events divided into nine categories based on the multiplicity of W andb-jets in the event. From this search T quarks with pure W b decays are excluded for masses below
1.04 TeV (1.07 TeV expected).
An analogous search has been carried out by ATLAS [123,124] for the pair production of heavy
T quarks. It used the lepton+jets final state with an isolated electron or muon and at least four
jets, including a b-jet, and required reconstruction of the T quark mass. Given that the mass range
for the T quark being explored was from a 0.4 TeV to a couple of TeV, the W boson from the T
quark may fall in two categories: those with a high boost leading to merged decay products, andothers where the two jets from the W boson were resolved. In addition, the selection was optimized
to require large angular separation between the high pT W bosons and the b-jets.
The T → W b candidates were constructed from both the leptonically and hadronically decaying W bosons by pairing them with the two highest pT b-tagged jets in the event. The pairing ofb-jets with W bosons which minimizes the difference between the masses of leptonically decaying
T (mlep (T )) and the hadronic T (mhad (T )) was chosen. Finally, mlep (T ) was used as the discriminating variable in a signal region defined by high ST0 (here ST0 is defined as the scalar sum of
the missing pT , the pT of the lepton and jets), and the opening angle between the lepton and the√neutrino (∆ R(e, ν)). With the 36.1 fb−1 data collected during Run 2 at s = 13 TeV, assuming
100% branching ratio to the W b decay, the observed lower limit on the T mass was 1.35 TeV, andin the SU(2) singlet scenario, the lower mass limit was obtained to be 1.17 TeV [123].
A targeted search for a T quark, produced singly in association with a light flavor quark and a b√
quark and decaying into bW , was carried out by CMS at s=13 TeV and a dataset corresponding
to 2.3 fb−1 [125]. The analysis used lepton+jets events, with at least one b-tagged jet with largetransverse momentum, and a jet in the forward η region. Selected events were required to have ST00 >500 GeV, where ST00 is defined as the scalar sum of the transverse momenta of the lepton, the leading
central jet, and the missing transverse momentum. The invariant mass of the T candidate wasused as the discriminating variable and was reconstructed using the four-vectors of the leptonicallydecaying W boson and the leading central jet. No excess over the standard model prediction wasobserved. As the VLQ width is proportional to the square of the coupling, upper limits were seton the production cross section assuming a narrow width VLQ with coupling greater than 0.5.
For Y /T quarks with a coupling of 0.5 and a 100% branching fraction for the decay to bW theexcluded masses were in the range from 0.85 to 1.40 TeV [125]. A similar search [126, 127] wasperformed by ATLAS for a singly produced T or Y−4/3 quark decaying to W b using a dataset
corresponding to 36.1 fb−1 . The search was performed using lepton+jets events with a high pT
b-tagged jet, and at least one forward jet. The reconstructed mass of the T /Y−4/3 quark, was usedas the discriminating variable and showed no excess above the expectation from SM. Interference
effects with the SM background are included in the study. This search led to 95% CL upper limits
on the mixing angle |sin(θL )| (CLW b ) in the range of 0.18–0.35 (0.25–0.49) for singlet T quark mass
between 0.8–1.2 TeV. This search also provided limits as a function of the Y−4/3 quark mass, onW b ) for a (B,Y
the coupling of the Y−4/3 quark to bW , and the mixing parameter |sinθR | (CR−4/3 )W b ) are in
doublet model [126]. For VLQ masses between 0.08–1.8 TeV, the limits on |sin(θR )| (CRthe range 0.17–0.55 (0.24–0.77), and for for Y−4/3 quark mass between 0.9–1.25 TeV, the limits on|sinθR | are around 0.18–0.19 and below the constraints from electroweak precision observables. For
Y−4/3 quark in the triplets (T ,B,Y−4/3 ), limits on |sin(θL )| (CLW b ) are between 0.16–0.39 (0.31–0.78)
for masses between 0.8 GeV–1.6 TeV [126].

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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

T → tH 0 :ATLAS has performed a search for T T production with T → tH 0 [124,128]. Given the dominant
decay mode H 0 → bb, these events are characterized by a large number of jets, many of which areb-jets. Thus the event selection required one isolated electron or muon and high jet multiplicity(including b-tagged jets). The sample is categorized by the jet multiplicity (5 and ≥6 jets in the
1-lepton channel; 6 and ≥7 jets in the 0-lepton channel), b tag multiplicity (2, 3 and ≥4) andmass-tagged jet multiplicity (0, 1 and ≥2). The distributions of meff , defined as the scalar sum of
the lepton and jet pT and the missing pT , for each category were used as the discriminant for the
final signal and background separation. No excess of events was found. Weak isospin doublet T
quarks were excluded below 1.16 TeV.
A search by ATLAS for pair produced VLQs with an all-hadronic final state signature yields
an exclusion of pure decays T → tH 0 upto a T quark mass of 2.3 TeV [129].
The CMS search for T T production, with T → tH 0 decays has been performed in lepton+jets,
multilepton and all-hadronic final states. The lepton+jets analysis [130] emphasizes the presence ofa large number of b-tagged jets, and combines it with other kinematic variables in a Boosted Decision
Tree (BDT) to enhance signal to background discrimination. The multilepton analysis [130] was
optimized for the presence of b-jets and the large hadronic activity. For B(T → tH 0 ) = 1, thecombined lepton+jets and multilepton analyses led to a lower limit on T quark masses of 0.71 TeV.
A search for T → tH 0 in all-hadronic decays [131], optimized for a high mass T quark, and basedon identifying boosted top quark jets has been carried out by CMS. This search aimed to resolve
subjets within the jets arising from boosted top quark decays, including b tagging of the subjets.
A likelihood discriminator was defined based on the distributions of HT and the invariant mass ofthe two b-jets in the events for signal and background. No excess above background expectations
was observed. Assuming 100% branching ratio for T → tH 0 , this analysis led to a lower limit of0.75 TeV on the mass of the T quark.
√
Searches for T quarks at s=13 TeV, based on a 2.6 fb−1 dataset [132] have been performedby CMS using the lepton+jets final state. This search has been optimized for high mass T quarks
by exploiting techniques to identify W or Higgs bosons decaying hadronically with large transverse
momenta. The boosted W channel excluded T quarks decaying only to bW with masses below
0.91 TeV, and the boosted tH channel excluded T quarks decaying only to tH for masses below
0.89 TeV.
A CMS search for T → tH 0 with H 0 → γγ decays has been performed [133] in pair productionof T quarks. To identify the Higgs boson produced in the decay of the heavy T quark, and thesubsequent H 0 → γγ decay, the analysis focused on identification of two photons in events with
one or more high pT lepton+jets or events with no leptons and large hadronic activity. A search
for a resonance in the invariant mass distribution of the two photons in events with large hadronic
activity defined by the HT variable showed no excess above the prediction from standard model
processes. The analysis resulted in exclusion of T quark masses below 0.54 TeV.
A search for electroweak single production of T quarks decaying to tH 0 using boosted topologies
in fully hadronic [134] and lepton+jets [135] in the final states has been performed by CMS.
The electroweak couplings of the T quarks to the SM third generation quarks are highly model
dependent, and hence these couplings determine the rates of the single T quark production. In
both analyses, T quark invariant mass was reconstructed using the boosted Higgs boson jet and the
top quark. Higgs boson jets were identified using jet substructure techniques and subjet b tagging.
For the lepton+jets analysis, the top quark was reconstructed from the leptonically decaying W
and the b jet, while in the all-hadronic analysis the top quark jet was tagged using substructure
analysis. There was no excess of events observed above background. Exclusion limits on the
product of the production cross section and the branching fraction (σ(pp → T qt/b) × B(T → tH 0 )31st May, 2024

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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

were derived for the T quark masses in the range 0.70-1.8 TeV. From the lepton+jets analysis,
for a mass of 1.0 TeV, values of (σ(pp → T qt/b) × B(T → tH 0 ) greater than 0.8 and 0.7 pb were
excluded assuming left- and right-handed coupling of the T quark to standard model fermions,
respectively [135]. For the all-hadronic analysis, upper limits between 0.31 and 0.93 pb wereobtained on (σ(pp → T qt/b) × B(T → tH 0 ) for T quark masses in the range 1.0-1.8 TeV [134].
ATLAS performs a similar search [136] for single production of vector-like T quarks decaying intoa Higgs Boson and a top quark and places limits between 1 and 1.7 TeV depending on the coupling
T → tZ:
Both ATLAS and CMS search for T quarks that decay exclusively into tZ in pp collisions at√
s = 13 TeV. No excesses were found in either search.
ATLAS performes a search [137] optimized for pair production of vector-like top quarks decaying
into tZ where the Z boson subsequently decays into neutrino pairs, utilizing 139 fb−1 of data. The
search selected events with one lepton, multiple jets, and significant missing transverse momentum.
No significant excesses were found and lower limits on the mass of a vector like top quark wereplaced, excluding masses below 1.59 TeV (weak-isospin doublet) and 1.47 TeV ( pure tZ mode).
Another search by ATLAS for pair produced T decaying to tZ has been carried out by reconstructing the high transverse momentum Z boson from a pair of opposite-sign same-flavor leptons,
using events with two or three charged leptons [138]. The final analysis is based on three final
state signatures. In the trilepton events, at least one b-tagged jet is required and HT , formed fromall hadronic jets and charged leptons, is used as the discriminating variable. In events with two
leptons, at least 2 b-jets are requested and events with zero or one high pT top-tagged jet, use
HT as the discriminator. The second dilepton analysis with two top-tagged high pT jets focuseson hadronically decaying heavy resonances and the invariant mass of the Z boson and the highest
pT b-tagged jet is found to be a good discriminating variable. No excess was observed over the
background expectations. Limits were placed at a heavy vector like top of 1.6 TeV.
ATLAS has subsequently carried out a search [139] for singly produced T quarks decaying totZ where the Z boson decays into neutrino pairs. The search is carried out using 36.1 fb−1 ofdata in events with two different final state signatures: one with jets and significant missing pT(0L) and the other with a single lepton, jets and missing pT (1L). Events are divided into signal
and dedicated W +jets and tt background control regions. The sensitivity to the T quark signal is
extracted using distributions of missing pT for the 1L and the distribution of T quark transverse
mass constructed from missing pT and the high pT large-radius top-tagged jet, for 0L analysis.
There is no excess found over the expected background and lower limits on the production of T
singlets are obtained as a function of the left- and right-handed couplings cL,W and cR,W to top
quarks and W bosons, where cW above 0.7 is excluded for T quark mass of 1.4 TeV. The limits on
cW are also recasted into expected and observed 95% CL upper limits for the mixing angle (θL ) ofa singlet T with the top quark.
CMS searched [140] for single production of T quarks decaying into tZ with the Z boson decaying
to pairs of charged leptons (electrons and muons) and the top quark decaying hadronically using
35.9 fb−1 of data. Limits were placed on T quarks with masses between 0.7 and 1.7 TeV excluding
the product of cross section and branching fraction above values of 0.27 to 0.04 pb. Additionally,
limits on the product of cross section and branching fractions for a Z 0 boson decaying into tZ wereset between 0.13 and 0.06 pb for Z 0 boson masses in the range from 1.5 to 2.5 TeV.
Similar searches by ATLAS for singly produced T decaying to Zt have been performed in final
state signatures with two or three charged leptons [141]. The analysis relies on tagging b-jetsand high pT large-radius jets originating from top-quarks. Additional selections are devised toreduce the contributions from pair production of T quarks. For events with dilepton analysis, the31st May, 2024

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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

discriminating variable is the mass of the T quark formed using the invariant mass of the Z boson
candidate and the highest pT top-tagged jet, while for the trilepton analysis, the variable ST is
used to search for an excess of data over the expected SM background. No excess above the SM
expectations is observed. The two final states (dilepton and trileptons) are combined to obtainthe final results. For the coupling parameter κT between 0.1–1.6, the 95% CL upper limits on the
production cross section times branching fraction into Zt is between 0.16–0.18 (0.03–0.05) pb at T
quark mass of 0.7 (2) TeV.
The search by the ATLAS experiment for VLQ pair production optimized to search for T → tZ
decays [142] in a dataset with 139 fb−1 of luminosity has been performed in final states with either
two leptons or three or more leptons. A multi-class boosted object tagger based on DNN techniques
for large-radius jets is used to categorize events according to the number of high pT boosted H 0 ,
Z/W , and top quark jets. The discriminant in the signal to evaluate the sensitivity depends on thenumber of leptons in the event. For the two lepton analysis, the discriminant is the mass of the T
quark which is constructed from b and Z candidates, while for the three or more lepton analysis, HT
computed using the pT of the jets and leptons is used as the discriminant. This analysis excludes
T quark masses up to 1.27 GeV (1.46 GeV), for the singlet (doublet) configuration [142].
Combination of T → tZ/tH 0 :The search performed by the ATLAS experiment for electroweak single production of T quark
decaying to tH 0 and tZ uses a dataset corresponding to 139 fb−1 of integrated luminosity, and eventswith a single lepton with multiple jets and b-jets in the final state [143]. The single production
channel for VLQs probes κ, the universal coupling constant, which also controls the production
cross section and the resonance width of the VLQ. The analysis uses techniques to tag boosted
jets, and categorizes events by numbers of jets and b-jets. The event discriminant is the “effectivemass” (meff) observable, defined as the scalar sum of the pT of all central jets, pT of leptons andthe missing pT in the event. No significant excess is observed. Limits on the mass of T quark
and universal coupling strength (κ) are obtained. For singlet T quarks, values of κ above 0.5 are
excluded for all masses below 1.8 TeV. For T quark mass of 1.6 TeV, κ above 0.41 is excluded [143].
A CMS search published in 2020 concentrates on the electroweak production of the T quarkwith T → tH 0 , and T → tZ decays, where subsequently both H 0 and Z Bosons decay hadronically,leading to fully hadronic final states [144]. The search focuses on evidence of T quarks producedin association with a b quark (qg → T bq 0 ) or a top quark (qg → T tq). The production crosssections are model dependent, as the electroweak production depends on the strength of the T
quark coupling, T bW (T tZ), for the charged-current (neutral-current) process, at the production
vertex. The searches are split into two domains depending on the mass of the T quark and consider
a wide range of widths of the T quark from a few percent to about 30% of the T quark mass. The
event selection relies primarily on the large number of jets and on efficient identification of jetsfrom b-quarks, in addition to double-b jet decays of the H 0 and Z bosons. The final sensitivity isderived by the search for a resonant peak in the tH 0 /Z invariant mass spectrum. For low mass T
quark searches, three independent regions based on the b-tagged jet requirements are examined.
For T quark masses above 1 TeV, which result in highly Lorentz-boosted top quarks and H 0 or
Z boson, large-area jets are used to form the T quark. This search reports upper limits at 95%
confidence level on σB(T → tH) and σB(T → tZ) between 2 pb and 20 fb for T masses between
0.6 to 2.6 TeV in the T bq and T tq production channels. The analysis also reports combined resultsfor T → tH and T → tZ associated production with a bottom quark and provides constraints on
T quarks in the T singlet model for masses below 1.00 TeV. For an expected fractional width of
30%, the expected sensitivity extends to 1.28 TeV, comparable to the most stringent results.

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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

Combination of T → bW/tZ/tH 0 :Most of the analyses described above target an individual decay mode of the T quark, with 100%
branching ratio to either bW , tZ or tH 0 and are optimized accordingly. However, they have varied
sensitivity to all three decay modes and the results can be interpreted as a function of branchingratios to each of the three decay modes, assuming the total adds up to unity (B(tH) + B(tZ) +B(W b) = 1).
Combinations of analyses have been performed by both ATLAS and CMS. The limits set by
ATLAS searches in W (`ν)b + X, H(bb)b + X, Z(νν), Z(``)t/b + X, dileptons with same-sign charge,
trileptons, all-hadronic final states have been combined and the results obtained for various sets ofbranching fractions for T quark decays to bW , tH 0 and tZ are shown in Fig. 92.3 (left). In the
combined analysis, ATLAS sets lower T quarks mass limit of 1.31 TeV for all possible values ofthe branching fractions to the three decay modes [123, 145, 146]. In Fig. 92.3, exclusion is shownin the plane of B(T → Ht) versus B(T → W b), for different values of the T quark mass. The
default branching ratio values for the weak-isospin singlet and doublet cases are also shown in
Fig. 92.3 as yellow circle and star symbols respectively. Assuming a weak isospin (T ,B) doublet
and |VT b | << |VtB |, T quark mass below 1.37 TeV is excluded.
A CMS analysis for pair production of T combines three channels with lepton final states:
single lepton, two leptons with the same sign of the electric charge (SS), or at least three leptons(trilepton) [147]. For various combinations of branching fractions for T quark decays to bW , tH 0
and tZ, the combined results exclude T quarks with masses below 1.14–1.3 TeV and are shown in
Figure 92.3 (right). Single lepton events are classified into 16 signal categories and 6 backgroundcontrol regions based on multiplicity of b-tagged, high pT H 0 and W -tagged jets. The discriminating
variables are HT for H 0 -tagged events and the minimum invariant mass constructed from thelepton and the b jet, min[Mlb ], for zero H 0 -tagged events. For the same-sign dilepton and trilepton
analyses, the non-prompt backgrounds due to misidentified jets and leptons are derived from data
control regions. In the trilepton analysis the ST variable is used as the signal discriminator binned
in four categories based on the lepton flavor combinations (eee, eeµ, eµµ, µµµ). The single lepton
analysis is most sensitive for tHbW and W bW b decay modes, within the SS dilepton analysis tHtH
and tHtZ have the best efficiency and for trileptons the tZtZ and tHtZ decays modes have thehighest efficiency. CMS excludes singlet (doublet) T quark masses below 1.2 (1.28) TeV. Masses
below 800 GeV were excluded in previous searches. For T quark masses in the range 0.8–1.8 TeV,
cross sections smaller than 30.4–9.4 fb (21.2–6.1 fb) are excluded for the singlet (doublet) scenario.
Another inclusive search for pair produced T in the all-hadronic final state [122] has been
performed by CMS using the boosted event shape tagger (BEST NN) neural network technique toclassify jets in six categories W , Z, H 0 , t, b, and light. This search does not focus on a given VLQ
mode, but on various combinations of the boson and quark jets in the final state. Anti-kT jets with adistance parameter of 0.8 are used. The BEST NN algorithm simultaneously classifies jets according
to heavy object type. For each of the six particle hypotheses, it boosts each jet constituent into
corresponding frame along the jet momentum direction, and calculates event shape and angularvariables in the boosted frame, with the expectation that when boosting to the correct rest frame,
jet constituents will be isotropic and show the expected N-prong structure of the decaying object
in its rest frame. A neural network is trained using the event shape and angular variables in the
boosted frame to classify jets according to one of those six possibilities (W , Z, H, t, b, or light).
The analysis bins the events into 126 categories depending on the number of W , Z, H, t, b, orlight jets in the final state with a maximum of four such objects. For each category HTAK8 , thescalar sum of pT of all AK8 jets, is used as the signal discriminator. A scan over a combination ofvarious branching fractions is also performed. This search excludes T quark masses in the range
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Figure 92.3: Left panel: observed limits on the mass of the T quark in the plane of B(T → tH 0 )versus B(T → bW ) from a combination [145] of all ATLAS searches for T T production. The markers
indicate the default branching ratios for the SU(2) singlet and doublet scenarios. Right panel: theobserved lower limits on the T quark mass (in GeV), from CMS searches after combining all lepton
channels [147], for various branching fraction scenarios. B(T → W b) + B(T → tZ) + B(T → tH 0 )= 1 is assumed.
An inclusive search for VLQs has been carried out by CMS targeted at heavy T quarks decaying
to any combination of bW , tZ, or tH 0 as described in [130]. Selected events have at least one isolated
charged lepton. Events were categorized according to number and flavor of the leptons, the numberof jets, and the presence of hadronic vector boson and top quark decays that are merged into a single
jet. The use of jet substructure to identify hadronic decays significantly increases the acceptancefor high T quark masses. No excess above standard model backgrounds was observed. Limits on
the pair production cross section of the new quarks are set, combining all event categories, for all
combinations of branching fractions into the three final states. For T quarks that exclusively decay
to bW/tZ/tH 0 , masses below 0.70/0.78/0.71 TeV are excluded.
92.2.3.2 Searches for B quarks that decay to W , Z and H 0 bosons
ATLAS and CMS have performed searches for pair production of heavy B quarks which subsequently decay to W t, bZ or bH 0 . The searches have been carried out in final states with single
leptons, dileptons (with same charge or opposite charge), multileptons, as well as in fully hadronic
final states.
B → bH 0 :A search for B pair-produced events has been performed by the CMS experiment [123] using
137 fb−1 of data where the B decays into a b/b quark and either a H 0 or a Z boson. This searchconcentrates on final states with fully hadronic signatures, and utilizes different jet multiplicity
categories to account for two resolved jets or merged single jets from H 0 or Z boson to bb decays,
depending on the pT of the H 0 or Z bosons. Nine final state categories are used, depending on
the number of observed jets and the reconstructed event mode. No significant excess over expectedbackground is observed. For the case B(B → bH) = 100%(50%), VLQs with masses below 1.57
(1.45) TeV are ruled out.
Using 36.1 fb−1 of data, ATLAS has performed a search for pair produced VLQs with allhadronic final state signature [148]. While this analysis provides exclusion limits for all third
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generation VLQs, it provides the strongest results for the B → bH 0 decay mode and excludes
decays with B(B → bH0 ) = ∞.0 scenario for B masses upto 1.01 TeV. The limits are also castin a two-dimensional plane of branching ratio values of B → bH 0 vs. B → W b. This analysis
required the presence of high pT jets and multiple b tags. It used a multi-class DNN to classify jets
arising from W , Z, H 0 bosons and top-quarks. In addition, the matrix element method was used
to compute the likelihood for the event to arise from a particular VLQ final state and to construct
the final discriminator. To increase the sensitivity of the analysis, processes with the same numberof top quarks, W /Z bosons, and H 0 Higgs bosons are combined into a single hypothesis.
ATLAS has performed a search for single production of a vector-like B quark decaying to ab-quark and a H 0 boson, with H 0 → bb decay, using a dataset corresponding to an integrated
luminosity of 139 fb−1 [149]. The all hadronic final state signature relies on identification of at least
3 b-jets, with an additional soft forward jet from the spectator quark. Large area jets are used
to identify boosted H 0 decays. This analysis results in excluding the single production of a VLQ(B, Y ) doublet with relative width larger than 5% between masses of 1.0 TeV and 2.0 TeV [150].
The CMS experiment has searched for a single vector-like quark decaying into tH or tZ in the all
hadronic final state with 138 fb−1 of data in the all hadronic channel reconstring both the boson
invariant mass and the tH or tZ invariant mass and searching for a resonance. No signficant excess
is found and cross-section limits are placed between 600 GeV and 1200 GeV between 600 and 68
fb [151].
B → W t:A search for B → tW in B pair produced events has been performed by ATLAS [123] using
lepton+jets events with one hadronically decaying W and one leptonically decaying W utilizing√
36.1 fb−1 of data at s = 13 TeV. The search was optimized for T production decaying into
W b. Since the analysis was optimized for T → W b rather than W t decays the analysis does notreconstruct the full B mass. As discussed earlier, the hadronically and leptonically decaying heavy
quarks were required to have similar reconstructed masses (within 300 GeV). The interpretationof the T → W b in the context of B → tW production led to the exclusion of heavy B like VLQs
for masses less than 1.25 TeV and 1.08 TeV, assuming a 100% branching fraction to tW or SU(2)singlet B scenario, respectively.
√
A similar search by CMS [152], using 19.8 fb−1 of s = 8 TeV data, selected events with one
lepton and four or more jets, with at least one b-tagged jet, significant missing pT , and further
categorized them based on the number of jets tagged as arising from the decay of boosted W , Z
or H 0 bosons. The ST distributions of the events in different categories showed no excess of events
above the expected background and yielded a lower limit on the B quark mass of 0.73 TeV for
BR(B → W t) = 1.
CMS [132] also searched for pair production of both T T and BB with collisions from 2.5 fb−1√of s = 13 TeV data. The analysis seaarched for events with one high pT lepton , multiple jets, andhighly boosted W or Higgs bosons decaying hadronically. The analysis focused on pair production
and selects events with either a boosted W or Higgs candidate and then proceeds to search foranomalous production in excess of standard model production. Seeing no significant excesses CMS
then proceeded to set limits in many different interpretations. The strongest was from the B → W t
interpretation leading to excluding heavy vector-like B quarks with mass less than 0.73 TeV.
The all-hadronic inclusive analysis [122] performed by CMS using the BEST NN techniqueto classify W/Z/H 0 /t/b/light jets also gives exclusion limits on B quark production for various
combinations of branching fractions for decays to tW, bZ, bH 0 . By considering categories basedon various combinations of the boson and quark jets in the final state it excludes B quarks with
masses up to 1230 GeV, for B decays to tW with a 100% branching fraction.
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Electroweak production of single heavy B+b production has been studied by CMS in the decay
to tW with the lepton+jets final state [153]. Single lepton events with hadronic jets, including aforward jet, and missing pT are selected and divided into 10 different categories based on lepton
flavor (e/µ), top-tagged, W -tagged, and 0/1/2 b-tagged jets. The B quark mass mreco is fullyreconstructed from lepton, jets, and missing pT , where the neutrino four-momentum is computed
using the missing pT and the W mass constraint. For events within the top-tagged category, thehigh pT top-tagged hadronic jet and the leptonically decaying W boson are used to compute mreco .
The mreco distribution are used as the signal discriminator. In the absence of an excess over the
expected SM background, the exclusion limits on the production cross section for B quark masses
between 0.7-2 TeV varies between 0.3 to 0.03 pb. In addition, B quarks with left-handed couplings
and a relative width of 10, 20, and 30% are excluded for masses below 1.49, 1.59, and 1.66 TeV
respectively.
B → bZ:As mentioned above, a search for B pair produced events, with final states with fully hadronic
signature, using 137 fb−1 of data, has been performed by the CMS experiment [123]. In this search,
the B decays into a b/b quark and either a H 0 or a Z boson. For the case B(B → bZ) = 100%(50%),
VLQs with masses below 1.39 (1.45) TeV are ruled out by this analysis.
A search by CMS [154] for the pair-production of a heavy B quark and its antiparticle has been
performed, where one of the heavy B quark decays to bZ. Events with a Z boson decaying toe+ e− or µ+ µ− and at least one b jet are selected. The signal from B → bZ decays is expected toappear as a local enhancement in the bZ mass distribution. No such enhancement was found andB quarks that decay 100% into bZ are excluded below 0.70 TeV. This analysis also set upper limits
on the branching fraction for B → bZ decays of 30-100% in the B quark mass range 0.45-0.70 TeV.
A complementary search has been carried out by ATLAS for new heavy quarks decaying into a Z
boson and a b-quark [155]. Selected dilepton events contain a high transverse momentum Z boson
that decays leptonically, together with two b-jets. If the dilepton events have an extra lepton in
addition to those from the Z boson, then only one b-jet is required. No significant excess of events
above the standard model expectation was observed, and mass limits were set depending on theassumed branching ratios, as shown in Fig. 92.4. In a weak-isospin singlet scenario, a B quark withmass lower than 0.65 TeV was excluded, while for a particular weak-isospin doublet scenario, a B
quark with mass lower than 0.73 TeV was ruled out.
The search by ATLAS for B → bZ decays [142] in final states with either two leptons or threeor more leptons using a multi-class boosted object tagger to categorize events according to thenumber of high pT boosted H 0 , Z/W , and top quark jets excludes B quark masses up to 1.20 GeV(1.32 GeV), for the singlet (doublet) configuration [142].
In addition to pair production, ATLAS has also searched for the electroweak production of
single B quarks, which is accompanied by a b-jet and a light jet [155]. The dilepton selection for
double B production was modified for the single B production study by requiring the presence of anadditional energetic jet in the forward region. An upper limit of 200 fb was obtained for the processσ(pp → Bbq) × B(B → Zb) with a heavy B quark mass at 0.70 TeV. This search indicated thatthe electroweak mixing parameter XBb below 0.5 is neither expected nor observed to be excluded
for any values of B quark mass.
Combination of B → tW/bZ/bH 0 :ATLAS has combined the various analyses targeted for specific decay modes to obtain the mostsensitive limit on the pair production of B quarks [123, 124, 145]. Various searches (W (`ν)t +X, Z(``)t/b + X, same sign charge dilepton events, trilepton events, and all-hadronic) are combined
to obtain lower limit on the mass of the B quark [in the plane of BR(B → W t) vs BR(B → bH)].
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20

The searches were optimized for 100% branching fractions and hence are most sensitive at large
BR(B → W t), and also at large BR(B → bH 0 ). For all possible values of branching ratios in thethree decay modes tW , bZ, or bH 0 , the lower limits on the B quark mass was found to be 1.03 TeV
and shown in Fig. 92.4 (left) as a function of the B quark branching ratios.
CMS combined three channels with lepton final states: single lepton, two leptons with the same
sign of the electric charge (SS), or at least three leptons (trilepton) [147]. For various combinationsof branching fractions for B quark decays to tW , bH 0 and bZ, the combined results exclude b
quarks with masses below 0.91–1.24 TeV and are shown in Figure 92.4 (right); the details areprovided earlier in subsection 92.2.3.1. The single lepton analysis is most sensitive for tHbW and
W bW b decay modes, within the SS dilepton analysis tHtH and tHtZ have the best efficiency andfor trileptons the tZtZ and tHtZ decays modes have the highest efficiency. CMS excludes singlet(doublet) B quark masses below 1.17 (0.94) TeV. Masses below 800 GeV were excluded in previous
searches. For B quark masses in the range 0.8–1.8 TeV, cross sections smaller than 40.6-–9.4 fb(101-–49.0 fb) are excluded for the singlet (doublet) scenario.

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Figure 92.4: Observed limits on the mass of the B quark in the plane of BR(B → bH 0 ) versus
BR(B → tW ) from ATLAS searches [145] on the left panel, and CMS searches [147] on the rightpanel, for BB production. B(B → Hb) + B(B → bZ) + B(B → Wt) = 1 is assumed. The yellow
markers indicate the branching ratios for the SU(2) singlet and doublet scenarios.
92.2.3.3 Searches for top-partner quark X5/3
Searches for a heavy top vector-like quark X5/3 , with exotic charge ±5/3, such as that proposed
in Refs. [157, 158], have been performed by both ATLAS and CMS [123, 159].
The analyses assumed pair-production or single production of X5/3 with X5/3 decaying with
100% branching fraction to tW . Searches for X5/3 have been performed using two final statesignatures: same-sign leptons and lepton+jets.
The analysis based on searching for same-sign leptons, from the two W bosons from one of
the X5/3 , has smaller backgrounds compared to the lepton+jets signature. Requiring same-sign
leptons eliminates most of the standard model background processes, leaving those with smaller
cross sections: ttW, ttZ, W W W , and same-sign W W . In addition, backgrounds from instrumental
effects due to charge misidentification were considered. Assuming pair production of X5/3 , theanalysis by CMS using HT as the discriminating variable restrict the X5/3 mass to be higher than
1.16 (1.10) TeV for a right (left) handed chirality particle [159–161]. The limits obtained by ATLAS,
by classifying the signal region by number of b-jets, HT , and missing pT in the event, led to a lower
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Figure 92.5: Observed 95% C.L. limits on σ × B × A for string resonances, excited quarks, axigluons, colorons, E6 diquarks, s8 resonances, W 0 and Z 0 bosons, and Randall–Sundrum Gravitons
gKK from [156].
mass limit on X5/3 of 1.19 TeV [162, 163].
Searches for X5/3 using leptons+jets final state signatures are based on either full or partial
reconstruction of the T mass from the lepton, jets (including b-jets) and missing pT . The CMS
search [159, 164] also utilized jet substructure techniques to identify boosted X5/3 topologies. The
discriminating variable used was the mass constructed from the lepton and b-tagged jet, M(`,b) ,which corresponds to the visible mass of the leptonically decaying top quark. To optimize thesearch sensitivity, the events were further separated into categories based on lepton flavor (e, µ),the number of b-tagged jets, the number of W -tagged jets, and the number of t-tagged jets. In
the absence of a signal, the CMS analysis excluded X5/3 quark masses with right-handed (lefthanded) couplings below 1.32 (1.30) TeV [164]. Combining the lepton+jets with the same-sign
leptons analyses leads to a slight improvement and excludes X5/3 quark masses with right-handed(left-handed) couplings below 1.33 (1.30) TeV.
The ATLAS lepton+jets search for X5/3 utilized events with high pT W bosons and b-jets. The
search described earlier for T pair production, with T → W b decays, can be reinterpreted as asearch for X → tW . This analysis excluded X5/3 with masses below 1.25 TeV [123].
The single X5/3 production cross section depends on the coupling constant λ of the tW X vertex.
ATLAS has performed an analysis of same-sign dileptons which includes both the single and pair
production. This analysis led to a lower limit on the mass of the X5/3 of 0.75 TeV for both valuesof λ = 0.5 and 1.0 [165].
Single heavy X5/3 +t production has been studied by CMS in the decay to tW with the lep31st May, 2024

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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

ton+jets final state [153]. The description of the analysis is provided earlier in the discussion of
B → W tX decays, where the reconstructed mass of X5/3 , mreco distribution is used as the signal
discriminator. In the absence of an excess over the expected SM background, the exclusion limits
on the production cross section for X5/3 quark masses between 0.7–2 TeV varies between 0.3 to 0.03
pb, depending on the width of X5/3 between 1–10%. In addition, X5/3 quarks with left-handed
couplings and a relative width of 10, 20, and 30% are excluded for masses below 0.92, 1.3, and1.45 TeV respectively.
92.2.4 Heavy resonances decaying to VLQ
CMS has performed search for VLQ production in the decay of massive resonances such as Z 0
and W 0 bosons.
Z 0 → tT : Specifically searches are presented by CMS in Refs. [166] and [167] for massive spin-1
0
Z resonances decaying to a top quark and a heavy VLQ top quark partner T . The results ofthis search for a heavy spin-1 resonance are interpreted in the context of two different models. In
the G? model which predicts ten VLQs (T, B, T̃ , B̃, T5/3 , T2/3 , T 0 , B 0 , B−1/3 , B−4/3 ) with the mass
relationship M (T5/3 ) = M (T2/3 ) = M (T )cos(φL ). For the benchmark scenario [168], cos(φL )=0.84
and the branching fractions T → tH 0 , tZ, W b are 0.25, 0.25 and 0.5 respectively. The ρ0 model
predicts a multiplet of four new VLQs T, B, X2/3 , X5/3 , and in the benchmark scenario [169], thebranching fractions T → tH 0 , tZ, W b are 0.5, 0.5 and 0 respectively.
Two of the three decays of the Z 0 → tT with T → tH 0 , tZ, W b are characterized by the presenceof two top quark decays and a boson (H 0 /Z). A search [167] by CMS, optimized for T → tH/Zt
decays was carried out in the lepton+jets final state using a dataset corresponding to an integrated
luminosity of 35.9 fb−1 . Jet substructure techniques are used to identify (or tag) the high pT largeradius jets originating from H 0 , Z bosons and merged top quarks. The mass of the Z 0 boson isused as the signal discriminator and constructed using H 0 or Z-tagged dijet, the hadronic andleptonic top quark four vectors. For the leptonic top quark reconstruction (t → b`ν` ), the neutrino
four vector is obtained from the event missing pT using the W boson mass constraint. While thehigh pT hadronic top quark jets from decays of massive Z 0 bosons are mostly merged and identified
by top tagging techniques, those from T decays maybe resolved. The reconstructed Z 0 candidate
events are classified into six different categories requiring the presence of either a H 0 -tagged jetwith 2 b-tagged subjets or one b-tagged subjet or a Z-tagged boson, each with either zero or one
top-tagged jet. This search does not observe any significant deviation in data over the expectationfrom standard model backgrounds. Within the context of the G? model, for a T mass of 1.2(1.5) TeV, this search excludes G? [168] resonances with masses between 1.5–2.3 (2.0–2.4) TeV.
The search in the all-hadronic final state is based on a 2.6 fb−1 dataset [166], and optimized
for T → W b decays. Jet substructure techniques are deployed for tagging jets from high pT W
boson and top quarks. Events are categorized into two groups based on the presence of b-tagged
subjets in the top-tagged jet. The multijet background estimation is challenging and determinedusing side-bands defined by inverting the b-tagging requirement. Upper limits on the cross sectionfor Z 0 → tT are obtained in the range of 0.13–10 pb.
W 0 → T b/Bt: W 0 bosons are predicted to decay to VLQ third generation partners T, B quarks
within composite Higgs and warped extra dimensional models [170]. In the benchmark scenariosof this framework, W 0 decays to T b or Bt are equally distributed and the subsequent VLQ decays
T → tH and B → bH 0 each are assumed to have a branching fraction of 0.5. The search forW 0 → T b/bH 0 → tbH 0 is performed using a sample of 35.9 fb−1 by CMS [171] in the final statewith all-hadronic decays of both the Higgs boson (H 0 → bb) and the top quark. Both the H 0 bosonand the top quark are expected to be boosted in the decay of a heavy W 0 , and hence jet substructuretechniques, including subjet b-tagging and double b-tagging are deployed to identify the H 0 -tagged
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92. Dynamical Electroweak Symmetry Breaking: Implications of the H 0

and the top-tagged jets. The three particle mass mtbH 0 , is used as the signal discriminant to observe
the W 0 resonance. There is no excess observed in data above the expected SM background. Thissearch excludes W 0 production cross section above 0.01–0.43 pb for masses between 1.5–4.0 TeV.
92.2.5 Colorons and Colored Scalars
These particles are associated with top-condensate and top-seesaw models, which involve anenlarged color gauge group. The new particles decay to dijets, tt̄, and bb̄.
Direct searches for colorons, color-octect scalars and other heavy objects decaying to qq, qg, qq,√
or gg have been performed using LHC data from pp collisions at s =7, 8 and 13 TeV. Based on√
the analysis of dijet events from a data sample corresponding to a luminosity of 19.6 fb−1 , at s =8 TeV, the CMS experiment excluded pair production of colorons with mass between 1.20−3.60 and
3.90 − 4.08 TeV [172]. Analyses of inclusive 8- and 10-jet final states with low missing transverse
momentum by CMS [173], set limits in several benchmark models. Colorons (axigluons) with
masses between 0.6 and 0.75 (up to 1.15) TeV were excluded, and gluinos in R-parity violatingsupersymmetric scenarios were ruled out from 0.6 up to 1.1 TeV.
√A search for pair-produced colorons based on an integrated luminosity of 5.0 fb−1 at s =7 TeV by CMS excluded colorons with masses between 0.25 TeV and 0.74 TeV, assuming colorons
decay 100% into qq [174]. This analysis was based on events with at least four jets and two dijet
combinations with similar dijet mass. Color-octet scalars (s8) with masses between 1.20 − 2.79 TeV
were excluded by CMS [172], and below 2.7 TeV by ATLAS [175].
These studies have now been extended to take advantage of the increased center-of-mass energy√
during Run 2 of the LHC. Using 35.9 fb−1 of data collected at s =13 TeV, searches for narrow
resonances have been performed by CMS. An analysis of the dijet invariant mass spectrum formed
using wide jets [156, 176, 177], separated by ∆ηjj ≤ 1.3, led to limits on new particles decaying toparton pairs (qq, qg, gg). Specific exclusions on the masses of colorons and color-octet scalars wereobtained and are shown in Fig. 92.5. Exclusions have been obtained for axigluons and colorons
below 6.1 TeV, and color-octet scalars below 3.4 TeV.
Additional searches for dijet resonances have been performed by both ATLAS [178–180] andCMS [181, 182], though they have not been interpreted in the context of coloron production.

92.3 Conclusions
As the above analyses have demonstrated, there is already substantial sensitivity to possible
new particles predicted to accompany the H 0 in dynamical frameworks of electroweak symmetry
breaking. No significant hints of any deviations from the standard model have been observed, andlimits typically at the scale of a few hundred GeV to a few TeV are set.
Given the need to better understand the H 0 and to determine in detail how it behaves, suchanalyses continue to be a major theme of Run 3 of the LHC, and we look forward to increased
sensitivity as a result of the higher luminosity at the increased center of mass energy of collisions.
References[1] G. Aad et al. (ATLAS), Phys. Lett. B716, 1 (2012), [arXiv:1207.7214].
[2] S. Chatrchyan et al. (CMS), Phys. Lett. B716, 30 (2012), [arXiv:1207.7235].
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