150
5 Direct Searches for New Physics
of the order of 10 fb. Single production of VLQs is mediated by the weak interaction
through t-channel exchange of a W or Z boson. Since these processes depend on
the size of the T bW and T t Z vertices (with similar vertices for B), the cross sections show a pronounced model dependence, but can be larger than the pair production cross sections for high VLQ masses, m VLQ 1 TeV. Maximum values allowed
by constraints from electroweak precision observables can be as large as 100 fb at
1.2 TeV [837].
The phenomenology of VLQs has been studied extensively [842–849], because
VLQs appear in many extensions of the SM. VLQs are required to induce electroweak
symmetry breaking if the H is a pseudo-Goldstone boson, as in Little Higgs [850] or
Composite Higgs [851–854] models. In partial-compositeness theories of flavour,
VLQs emerge as fermionic resonances [855], in extra-dimension models they appear
as quarks propagating in the bulk of these dimensions [856, 857], and they are also
present in grand unified theories [858]. A model independent framework for studying
the phenomenology of VLQs has been introduced in [859], and many searches are
based on the effective Lagrangian defined there.
5.3.1 Pair Production
Pair production of VLQs in pp collisions occurs through SM-like interactions with
either a gluon or a photon in the s-channel exchange. These diagrams only depend
on the strong and electromagnetic couplings, such that a model-independent production cross section is obtained. However, there are diagrams contributing to pair
production of VLQs, that depend on the VLQ-quark-vector boson (QqV ) vertex,
namely s- or t-channel diagrams with a weak vector boson as mediator. These lead
to a dependence of the cross section on the value of the QqV coupling. If VLQs
couple to first-generation SM quarks, the t-channel diagram can be dominant for
high m VLQ above 1.7 TeV, because it is the only diagram with two valence quarks
in the initial state [861]. If, however, VLQs couple to second- and third-generation
SM quarks preferentially, the relevance of these diagrams decreases and the pair
production cross section is approximately independent of the QqV coupling [862].
Pair production of VLQs results in a plethora of possible final states. Models with
non-negligible mixings with SM quarks result in decay chains involving two thirdgeneration quarks and two SM bosons. For example, the pair production pp → T T
can lead to the intermediate states bW bW , bW t Z, bW t H, t Zt Z, t Zt H, t Ht H and
their charge conjugates, as illustrated in Fig. 5.10. Considering the many possible
decays of the W , Z and H , final states with only jets or one to four leptons are possible. A similar situation is obtained for pp → B B. Since it is not feasible to cover
all possibilities in dedicated experimental analyses, the ATLAS and CMS Collaborations have focussed either on searches assuming 100% branching fraction into one
of the three possible decays, i.e. bW bW , t Zt Z and t Ht H in the case of pp → T T ,
or final states characterised by the number of leptons. In fact, this approach covers
all conceivable possibilities once at least one analysis per decay channel has been
5 Direct Searches for New Physics
of the order of 10 fb. Single production of VLQs is mediated by the weak interaction
through t-channel exchange of a W or Z boson. Since these processes depend on
the size of the T bW and T t Z vertices (with similar vertices for B), the cross sections show a pronounced model dependence, but can be larger than the pair production cross sections for high VLQ masses, m VLQ 1 TeV. Maximum values allowed
by constraints from electroweak precision observables can be as large as 100 fb at
1.2 TeV [837].
The phenomenology of VLQs has been studied extensively [842–849], because
VLQs appear in many extensions of the SM. VLQs are required to induce electroweak
symmetry breaking if the H is a pseudo-Goldstone boson, as in Little Higgs [850] or
Composite Higgs [851–854] models. In partial-compositeness theories of flavour,
VLQs emerge as fermionic resonances [855], in extra-dimension models they appear
as quarks propagating in the bulk of these dimensions [856, 857], and they are also
present in grand unified theories [858]. A model independent framework for studying
the phenomenology of VLQs has been introduced in [859], and many searches are
based on the effective Lagrangian defined there.
5.3.1 Pair Production
Pair production of VLQs in pp collisions occurs through SM-like interactions with
either a gluon or a photon in the s-channel exchange. These diagrams only depend
on the strong and electromagnetic couplings, such that a model-independent production cross section is obtained. However, there are diagrams contributing to pair
production of VLQs, that depend on the VLQ-quark-vector boson (QqV ) vertex,
namely s- or t-channel diagrams with a weak vector boson as mediator. These lead
to a dependence of the cross section on the value of the QqV coupling. If VLQs
couple to first-generation SM quarks, the t-channel diagram can be dominant for
high m VLQ above 1.7 TeV, because it is the only diagram with two valence quarks
in the initial state [861]. If, however, VLQs couple to second- and third-generation
SM quarks preferentially, the relevance of these diagrams decreases and the pair
production cross section is approximately independent of the QqV coupling [862].
Pair production of VLQs results in a plethora of possible final states. Models with
non-negligible mixings with SM quarks result in decay chains involving two thirdgeneration quarks and two SM bosons. For example, the pair production pp → T T
can lead to the intermediate states bW bW , bW t Z, bW t H, t Zt Z, t Zt H, t Ht H and
their charge conjugates, as illustrated in Fig. 5.10. Considering the many possible
decays of the W , Z and H , final states with only jets or one to four leptons are possible. A similar situation is obtained for pp → B B. Since it is not feasible to cover
all possibilities in dedicated experimental analyses, the ATLAS and CMS Collaborations have focussed either on searches assuming 100% branching fraction into one
of the three possible decays, i.e. bW bW , t Zt Z and t Ht H in the case of pp → T T ,
or final states characterised by the number of leptons. In fact, this approach covers
all conceivable possibilities once at least one analysis per decay channel has been