160
5 Direct Searches for New Physics
5.3.2 Single Production
Single production of VLQs occurs through the weak interaction, with a t-channel
exchange of a SM weak vector boson. The incoming quark is preferentially scattered
under small angles, or large absolute values of rapidity, where the resulting jet is
usually referred to as forward jet. In the case of VLQs coupling to third-generation
quarks, the VLQ is produced in association with a b or t quark, which is on average
produced with much smaller p T than the decay products of the VLQ. While there
are diagrams involving the exchange of an H boson, their contribution to the total
cross section is small because of the smallness of the Yukawa coupling to first and
second generation quarks. Therefore models with B(B → Hb) = 1 and B(T →
Ht) = 1 predict vanishing production cross sections, because of the absence of
charged and neutral currents with VLQs. In contrast to pair production, the production
cross section depends on the size of the QqV coupling and therefore on the weak
isospin and the representation of the VLQ. Because of the larger phase space for the
production of a single heavy particle compared to two, single VLQ production can
have larger production cross sections than pair production at high m VLQ . The exact
value of m VLQ where the single production cross section is larger and becomes the
better search channel depends on the unknown QqV coupling, but can be between 1
and 1.5 TeV. The largest allowed cross sections, given the constraints on VLQs from
precision data, are in the range of 20–80 fb for T b j, 7–55 fb for Bb j and about 150 fb
for Y b j production at
√
s = 13 TeV and m VLQ = 1 TeV [837]. The ranges indicate
the cross sections obtained for VLQs in the singlet or doublet representations. VLQ
production in association with a top quark involves initial gluon splitting g → tt
and has a much smaller cross section than b-associated production [896]. The cross
sections for T t j and X t j are of comparable size, about 2 fb for m VLQ = 1 TeV [837,
897]. Note that T t j is the only production process for a T quark with very small
coupling to a W boson. Only for m VLQ > 2 TeV, the cross section for VLQ single
production in association with a top quark can be larger than the pair-production
cross section. In some cases interference effects with SM processes need to be taken
into account, for example in pp → T j → W bj, which results in the same final state
as single top quark production. However, interference effects contribute only in cases
where the width of the VLQ is large, with only a few percent change in the cross
section when including these effects for a relative width of 10% [849]. Due to a certain
degree of model dependence and a dependence on the QqV coupling, searches by
the ATLAS and CMS Collaborations usually are performed without considering the
associated b or t quark. The phenomenology of single VLQ production has been
studied in several analyses, see for example [896, 898–904].
The first searches for singly produced VLQs have been carried out by ATLAS
using 20.3 fb
−1 of 8 TeV data. A search in the dilepton final state targets the channels
T → Zt and B → Zb, where the t and b quarks are reconstructed using b-tagged
small-R jets [863]. The first 8 TeV search using jet substructure techniques targets
the B → W t channel, where either the W or t are reconstructed in a single largeR jet [905]. The large-R jet selection is not very constraining, where only the
5 Direct Searches for New Physics
5.3.2 Single Production
Single production of VLQs occurs through the weak interaction, with a t-channel
exchange of a SM weak vector boson. The incoming quark is preferentially scattered
under small angles, or large absolute values of rapidity, where the resulting jet is
usually referred to as forward jet. In the case of VLQs coupling to third-generation
quarks, the VLQ is produced in association with a b or t quark, which is on average
produced with much smaller p T than the decay products of the VLQ. While there
are diagrams involving the exchange of an H boson, their contribution to the total
cross section is small because of the smallness of the Yukawa coupling to first and
second generation quarks. Therefore models with B(B → Hb) = 1 and B(T →
Ht) = 1 predict vanishing production cross sections, because of the absence of
charged and neutral currents with VLQs. In contrast to pair production, the production
cross section depends on the size of the QqV coupling and therefore on the weak
isospin and the representation of the VLQ. Because of the larger phase space for the
production of a single heavy particle compared to two, single VLQ production can
have larger production cross sections than pair production at high m VLQ . The exact
value of m VLQ where the single production cross section is larger and becomes the
better search channel depends on the unknown QqV coupling, but can be between 1
and 1.5 TeV. The largest allowed cross sections, given the constraints on VLQs from
precision data, are in the range of 20–80 fb for T b j, 7–55 fb for Bb j and about 150 fb
for Y b j production at
√
s = 13 TeV and m VLQ = 1 TeV [837]. The ranges indicate
the cross sections obtained for VLQs in the singlet or doublet representations. VLQ
production in association with a top quark involves initial gluon splitting g → tt
and has a much smaller cross section than b-associated production [896]. The cross
sections for T t j and X t j are of comparable size, about 2 fb for m VLQ = 1 TeV [837,
897]. Note that T t j is the only production process for a T quark with very small
coupling to a W boson. Only for m VLQ > 2 TeV, the cross section for VLQ single
production in association with a top quark can be larger than the pair-production
cross section. In some cases interference effects with SM processes need to be taken
into account, for example in pp → T j → W bj, which results in the same final state
as single top quark production. However, interference effects contribute only in cases
where the width of the VLQ is large, with only a few percent change in the cross
section when including these effects for a relative width of 10% [849]. Due to a certain
degree of model dependence and a dependence on the QqV coupling, searches by
the ATLAS and CMS Collaborations usually are performed without considering the
associated b or t quark. The phenomenology of single VLQ production has been
studied in several analyses, see for example [896, 898–904].
The first searches for singly produced VLQs have been carried out by ATLAS
using 20.3 fb
−1 of 8 TeV data. A search in the dilepton final state targets the channels
T → Zt and B → Zb, where the t and b quarks are reconstructed using b-tagged
small-R jets [863]. The first 8 TeV search using jet substructure techniques targets
the B → W t channel, where either the W or t are reconstructed in a single largeR jet [905]. The large-R jet selection is not very constraining, where only the