5.2 Resonances Coupling to Third Generation Quarks
149
tivity of the +jets analysis for masses above 1.5 TeV. This can be considered a great
achievement, considering the difficulties in the identification of highly boosted t and
b jets.
In several BSM models, there exists a relation between W
and Z
resonances.
An important example are non-universal left-right models, where new right-handed
gauge bosons couple predominantly to the third generation [828, 829]. These models
have been introduced to explain the deviation in the Z → b ¯
b decay asymmetry
observed at LEP [107], and have recently been studied in the context of flavour
anomalies in the b sector [830, 831]. Searches for tt and tb resonances at the LHC
help to constrain the parameter space of these models [832].
5.3 Vector-Like Quarks
Hypothetical quarks, for which the left- and right-handed chiral components transform in the same way under the SM electroweak symmetry group, are often referred
to as vector-like quarks (VLQs). While a fourth chiral generation of quarks has been
excluded by the measurements of H mediated cross sections [833, 834] and by
considerations of the vacuum stability [835], VLQs are a viable possibility for a
fourth generation of quarks [836, 837]. Gauge-invariant mass terms of VLQs can
be included in the SM without generating them through interactions with a scalar
field, such that VLQs evade constrains from H measurements. Electroweak precision measurements and the absence of flavour changing neutral currents constrain
the possible mixings of VLQs with SM quarks, such that a dominant mixing with
third generation quarks emerges [838, 839]. Assuming the SM symmetry groups and
a scalar sector with only SU(2) L doublets, VLQs can only appear in two singlets,
three doublets or two triplets [840, 841]. The four quarks in these multiplets are the
two third-generation partners T and B with electric charges of +2/3 and −1/3, and
two exotic quarks X and Y with electric charges of +5/3 and −4/3, respectively.
Possible decays of the heavy quark mass eigenstates are into t/b and W , Z or H .
The branching fractions depend on the mixing parameters with SM quarks and the
multiplet representation. The three benchmark points for the ratios of branching
fractions of up-type quarks B(T → W b) : B(T → Zt) : B(T → Ht) are 1 : 0 : 0,
2 : 1 : 1 and 0 : 1 : 1. These cover scenarios with minimal and maximal mixings for
all multiplet representations [837]. Conversely, for down-type quarks the benchmark
points for the ratios B(B → W t) : B(B → Zb) : B(B → Hb) are 1 : 0 : 0, 2 : 1 : 1
and 0 : 1 : 1. In experimental analyses, often not only these benchmark points are
considered but constraints are derived for all possible combinations of branching fractions under the assumptions of B(T → W b) + B(T → Zt) + B(T → Ht) = 1 and
B(B → W t) + B(B → Zb) + B(B → Hb) = 1. For quarks with electric charges
of +5/3 and −4/3, the only possible decays are X → W
+ t and Y → W
− b. Since
all VLQs have quark-like triplet colour charges, they can be produced in pairs in
pp collisions through diagrams involving the strong interaction. For a VLQ of mass
m VLQ = 1.2 TeV, the pair-production cross section at the LHC with
√
s = 13 TeV is
149
tivity of the +jets analysis for masses above 1.5 TeV. This can be considered a great
achievement, considering the difficulties in the identification of highly boosted t and
b jets.
In several BSM models, there exists a relation between W
and Z
resonances.
An important example are non-universal left-right models, where new right-handed
gauge bosons couple predominantly to the third generation [828, 829]. These models
have been introduced to explain the deviation in the Z → b ¯
b decay asymmetry
observed at LEP [107], and have recently been studied in the context of flavour
anomalies in the b sector [830, 831]. Searches for tt and tb resonances at the LHC
help to constrain the parameter space of these models [832].
5.3 Vector-Like Quarks
Hypothetical quarks, for which the left- and right-handed chiral components transform in the same way under the SM electroweak symmetry group, are often referred
to as vector-like quarks (VLQs). While a fourth chiral generation of quarks has been
excluded by the measurements of H mediated cross sections [833, 834] and by
considerations of the vacuum stability [835], VLQs are a viable possibility for a
fourth generation of quarks [836, 837]. Gauge-invariant mass terms of VLQs can
be included in the SM without generating them through interactions with a scalar
field, such that VLQs evade constrains from H measurements. Electroweak precision measurements and the absence of flavour changing neutral currents constrain
the possible mixings of VLQs with SM quarks, such that a dominant mixing with
third generation quarks emerges [838, 839]. Assuming the SM symmetry groups and
a scalar sector with only SU(2) L doublets, VLQs can only appear in two singlets,
three doublets or two triplets [840, 841]. The four quarks in these multiplets are the
two third-generation partners T and B with electric charges of +2/3 and −1/3, and
two exotic quarks X and Y with electric charges of +5/3 and −4/3, respectively.
Possible decays of the heavy quark mass eigenstates are into t/b and W , Z or H .
The branching fractions depend on the mixing parameters with SM quarks and the
multiplet representation. The three benchmark points for the ratios of branching
fractions of up-type quarks B(T → W b) : B(T → Zt) : B(T → Ht) are 1 : 0 : 0,
2 : 1 : 1 and 0 : 1 : 1. These cover scenarios with minimal and maximal mixings for
all multiplet representations [837]. Conversely, for down-type quarks the benchmark
points for the ratios B(B → W t) : B(B → Zb) : B(B → Hb) are 1 : 0 : 0, 2 : 1 : 1
and 0 : 1 : 1. In experimental analyses, often not only these benchmark points are
considered but constraints are derived for all possible combinations of branching fractions under the assumptions of B(T → W b) + B(T → Zt) + B(T → Ht) = 1 and
B(B → W t) + B(B → Zb) + B(B → Hb) = 1. For quarks with electric charges
of +5/3 and −4/3, the only possible decays are X → W
+ t and Y → W
− b. Since
all VLQs have quark-like triplet colour charges, they can be produced in pairs in
pp collisions through diagrams involving the strong interaction. For a VLQ of mass
m VLQ = 1.2 TeV, the pair-production cross section at the LHC with
√
s = 13 TeV is